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SCIENCE

FOR THE

SCHOOL AND FAMILY.


PART I.

NATURAL PHILOSOPHY.


BY

WORTHINGTON HOOKER, M.D.,

PROFESSOR OF THE THEORY AND PRACTICE OF MEDICINE IN YALE COLLEGE,
AUTHOR OF "HUMAN PHYSIOLOGY," "CHILD'S BOOK OF NATURE," "NATURAL
HISTORY," &C.


=Illustrated by nearly 300 Engravings.=


NEW YORK:
HARPER & BROTHERS, PUBLISHERS,
FRANKLIN SQUARE.
1873.




By Dr. Worthington Hooker.


=The Child's Book of Nature.= For the Use of Families and Schools;
intended to aid Mothers and Teachers in training Children in the
Observation of Nature. In three Parts. Illustrated by Engravings.
The Three Parts complete in one vol. Small 4to, Cloth, $2 00;
Separately, Cloth, 90 cents each.

  PART I. PLANTS.

  PART II. ANIMALS.

  PART III. AIR, WATER, HEAT, LIGHT, &c.

=First Book in Chemistry.= For the Use of Schools and Families.
Illustrated by Engravings. Square 4to, Cloth, 90 cents.

=Natural History.= For the Use of Schools and Families. Illustrated
by nearly 300 Engravings. 12mo, Cloth, $1 50.

=Science for the School and Family.=

  PART I. NATURAL PHILOSOPHY. Illustrated by nearly 300 Engravings.
  12mo, Cloth, $1 50.

  PART II. CHEMISTRY. Illustrated by numerous Engravings, 12mo,
  Cloth, $1 50.

  PART III. MINERALOGY AND GEOLOGY. Illustrated by numerous
  Engravings. 12mo. Cloth, $1 50.

=Published by HARPER & BROTHERS, Franklin Square, N. Y.=


Any of the above Works sent to any part of the United States,
postage pre-paid, upon receipt of the Price.


Entered, according to Act of Congress, in the year one thousand
eight hundred and sixty-three, by HARPER & BROTHERS, in the Clerk's
Office of the District Court of the Southern District Court of New
York.




PREFACE.


Daniel Webster, in his Autobiography, speaks thus of his entering
upon the study of law: "I was put to study in the old way--that
is, the hardest books first, and lost much time. I read Coke on
Littleton through without understanding a quarter part of it.
Happening to take up Espinasse's Law of Nisi Prius, I found I
could understand it; and arguing that the object of reading was
to understand what was written, I laid down the venerable Coke
_et alios similes reverendos_, and kept company for a time with
Mr. Espinasse and others, the most plain, easy, and intelligible
writers. A boy of twenty, with no previous knowledge on such
subjects, can not understand Coke. It is folly to set him on
such an author. There are propositions in Coke so abstract, and
distinctions so nice, and doctrines embracing so many conditions
and qualifications, that it requires an effort, not only of a
mature mind, but of a mind both strong and mature, to understand
him. Why disgust and discourage a boy by telling him that he must
break into his profession through such a wall as this? I really
often despaired. I thought that I never could make myself a lawyer,
and was almost going back to the business of schoolkeeping. Mr.
Espinasse, however, helped me out of this in the way that I have
mentioned, and I have always felt greatly obliged to him."

Here is most graphically depicted a defect which is now, as it was
then, very prominent in all departments of education. It is even
more so in early education than in that of the college and the
professional school. Even in tender childhood pupils are put to
studying books of which, as was true of Webster with his Coke on
Littleton, they do not understand "a quarter part." If the rule is
not "the hardest books first," there are many things in the books
that it is not only hard but impossible for them to understand.
And the hardest things are often put first. For example, in a
very popular primary geography which lies before me the pupil is
introduced to the world and its grand divisions at the outset,
while he is taught about his own state and country only at the
conclusion of the book. And this unnatural mode is the one very
commonly pursued. Similar criticism can be passed upon most of the
books used in teaching young children. Some of them are wholly
useless. This is true of the grammars for primary schools. The
formal statements, called the rules of grammar, are beyond the
understanding of very young scholars, and therefore are useless
burdens upon their memories. They are as useless to them as the
three-fourths of Coke which Webster could not understand was to him.

If we follow education, upward from the primary school we find
the same defect throughout the whole course. In the books which
are used in teaching natural science it is especially prominent.
Even in the elementary books, or compendiums, so called, formal
propositions and technical terms render the study uninviting, and
to a great extent unintelligible. The pupil is apt to be disgusted
and discouraged, as Webster was with Coke on Littleton, and for the
same reason.

Another defect intimately connected with that of which I have
spoken is the very sparing and late introduction of the physical
sciences. They are generally postponed to the latter part of
the course of education, and then but little time is devoted to
them. Generally, when a pupil designs to go through college, the
study of these sciences is wholly neglected in his preparation,
because a knowledge of them is not required for admission. Then in
the college they are not attended to till the latter part of the
course, and in the short time allotted to them there is so much to
be learned that the teaching of them is a failure. Especially is
this true of Chemistry and Geology.

This defect is a _radical_ one. A thorough change should be
effected in this respect in the whole course of education. The
natural sciences should be made prominent from the beginning to the
end, not only because they are of practical value, but also because
they are as useful in their way for mental discipline as the study
of mathematics and of language. They can be taught to some little
extent to the youngest pupils. There are facts about air, water,
and the various objects that they see around them, which they can
understand if they be presented in the right manner. And the busy
inquiries which they make after the reasons of the facts, and their
appreciation of them if stated simply and without technical terms,
show the appropriateness of such teaching. Children are really very
good philosophers in their way. They have great activity not only
of their perceptive but of their reasoning faculties also, to which
due range should be given in their education.

Beginning thus, not a year should pass during the whole course
when the pupil shall not be engaged in studying some one of the
physical sciences to some extent. This continued attention to such
studies in a reasonable amount, so far from interfering with the
due prosecution of the other studies deemed so essential, _will so
promote the pupil's advance in them as to more than make up for
the time that is taken from them_. It will do this not only by the
genial influence which such studies exert upon the mind, but by
the contributions which they make to the knowledge of language and
mathematics; for language is largely built up from natural objects
and from the acquisitions of science, and there is an abundance
of interesting applications of portions of the mathematics in the
facts which the physical sciences develop to us.

I have said that the teaching of the natural sciences in our
colleges is generally a failure, and it always will be so as long
as the present plan is continued. In order to have it successful
there must be _the same gradation in teaching them that we have
in teaching language and the mathematics_. The college student
needs to be prepared for the lectures which he hears on natural
philosophy, chemistry, etc., and for his study of those branches,
by previous familiarity with the simpler portions of them acquired
in the school-room.

There is another very important reason for the early introduction
of the physical sciences into education. By far the larger portion
of pupils in our schools stop short of the college, or even the
academy and high school. That they should go forth into the world
with no knowledge of the principles that lie at the basis of the
arts in which so many of them are to engage is a shame and a wrong,
if the communication of such knowledge be indeed practicable, as
it undoubtedly is. Even those who are not to engage in these arts
will be greatly benefited by this knowledge, because in addition
to its constant practical applications in the management of life,
it will contribute to their mental power, and, what is no small
consideration, to their enjoyment; and it is in fact requisite to
constitute them well-informed persons.

If the views which I have presented be correct, there should be a
series of books on the natural sciences carefully adapted to the
different periods of the course of study. Those intended for the
young beginner should be exceedingly simple, and should not attempt
to present any thing like a full view of the subjects treated.
They should deal largely with familiar facts or phenomena. The
terminology of science and formal statements of principles, such
as we often see in so-called compendiums, should have no place in
them, but should be gradually introduced as the series advances,
and should be made complete only in the concluding books.

It has been the object of the author to supply a part of such a
series. The first book in the series is the "Child's Book of Common
Things," intended to teach the observation of familiar facts, or,
in other words, the beginnings of philosophy, to children as soon
as they have got well started in reading. Next comes the "Child's
Book of Nature," which in its three parts (Part I., Plants; Part
II., Animals; Part III., Air, Water, Light, Heat, etc.) extends
considerably the knowledge of the philosophy of things which the
child has obtained from the first book in the series. Then follows
the "First Book in Chemistry." On a level with this is my "First
Book in Physiology." The next step in the gradation brings us
to three books under one title, "Science for the School and the
Family;" Part I., Natural Philosophy; Part II., Chemistry; Part
III., Mineralogy and Geology. On a level with these is another
book, "Natural History," and another still is to be written, an
"Introduction to Botany."

The three books, of which the present is one, are intended for the
older scholars in what are commonly called grammar-schools. At the
same time they are suited to scholars who are advanced to a higher
grade who have not gone through the previous books of the series.
The preparation of books especially adapted to high schools and
colleges I have left to others, except in one branch of science,
Physiology, on which I some years ago published a work entitled
"Human Physiology."

All of these books are from the press of Harper and Brothers except
the two works on Physiology, published by Sheldon and Co., New
York, and the "Child's Book of Common Things," published by Peck,
White, and Peck, New Haven.

The general plan and style of these books are very different from
what we see in most of the books for schools on the same subjects.
The order of the subjects and the mode of developing them differ
from the stereotype plan which has so generally been adopted. One
prominent feature is the free use of illustrations from _familiar_
phenomena. This leads the pupil to reason or philosophize about
common things, thus giving an eminently practical character to his
knowledge. At the same time it makes the books suitable for use in
the family as well as the school, between which there should be
more common ground than the present mode of education allows.

The style which I have chosen for all the books I have written
for use in teaching is what may be called the _lecture style_.
There are three other kinds of style which are more commonly
used in school-books. The most common is what I term the _formal
statement style_. In this principles and rules are stated, and
then illustrations are given. This makes a formal and uninviting
book. The bare skeleton of the science is generally for the most
part presented, and the young pupil is apt to learn the statements
by rote without understanding them. It is a style fitted only
for books intended for advanced scholars. Another style is the
_catechetical_. This is an unnatural mode of communicating
knowledge; and besides, it encourages learning by rote as the
formal statement style does. In the third style, the _dramatic_,
conversations are held between the teacher and some learners. The
chief objection to this is that it undertakes to put in permanent
shape what should be extemporized in the recitation. What is needed
in the book is simply clear and concise statement in an interesting
style, and the living teacher and his scholars can best furnish
the conversational element as the recitation goes on.

In the lecture style there may be and should be as much precision
of statement as in the formal statement style, while it is more
interesting, because it is the natural mode of communicating
knowledge. In this style the facts are ordinarily so stated as to
develop principles; while in the other the order is reversed, the
principles being first stated and the facts given afterward. One
of the most successful books ever used in our colleges--"Paley's
Natural Theology"--is in the lecture style, and it is a matter of
surprise that this fact has had so little influence with those who
have prepared books for instruction.

Whatever may be true of advanced scholars, in teaching the young
student in science bare, dry statement should be avoided, and the
subjects should be presented in all their attractive features. I
would not be understood as advocating the dressing up of science
in adventitious charms. This is not necessary. Science possesses
in itself an abundance of charms, which need only to be properly
developed to attract the young mind; and the lecture style
furnishes the best vehicle for such a development.

One grand essential for giving interest to any study is the
presentation of the various points in the _natural order_ in which
they should enter the mind. _They should be so presented that
each portion of a book shall make the following portions more
interesting and more easily understood._ This principle, which is
so commonly transgressed, I have endeavored to observe strictly in
the preparation of these volumes.

Questions are put at the end of this book for those teachers who
desire to use them. There is also an Index.

  W. HOOKER.

  _January_, 1863.




CONTENTS.

  CHAPTER                                        PAGE

     I. MATTER                                     13

    II. PROPERTIES OF MATTER                       19

   III. THE ESSENTIAL PROPERTIES OF MATTER         33

    IV. ATTRACTION                                 38

     V. GRAVITATION                                51

    VI. CENTRE OF GRAVITY                          67

   VII. HYDROSTATICS                               80

  VIII. SPECIFIC GRAVITY                          100

    IX. PNEUMATICS                                110

     X. MOTION                                    133

    XI. THE MECHANICAL POWERS                     174

   XII. SOUND                                     193

  XIII. HEAT                                      207

   XIV. LIGHT                                     258

    XV. ELECTRICITY                               287

   XVI. MAGNETISM                                 308




NATURAL PHILOSOPHY.




CHAPTER I.

MATTER.


1. =Matter and Spirit.=--The distinction between matter and spirit
is almost universally recognized even by those who have given
little thought to such subjects. It is a distinction of which we
are conscious in our own persons. We know instinctively that there
is a something within us that causes the movements of our material
bodies, and that something we call spirit.

2. =Bishop Berkeley's Ideas.=--Some philosophers, in their
speculations, have denied that there is any such thing in
existence as matter. Bishop Berkeley, for example, taught that
the impressions which we suppose that we receive from material
objects do not come from real substances, but are the "effects of
the immediate agency of an ever-present Deity." It is no wonder
that the wisdom and learning of a man who could seriously adopt
such a belief could not save him from being the dupe of quackery.
He believed that tar-water was a sovereign cure for all diseases;
and Dr. Holmes playfully remarks of him, that "he held two very
odd opinions: that tar-water was every thing, and that the whole
material universe was nothing."

3. =Hume's Ideas.=--The infidel Hume went beyond Bishop Berkeley,
denying even the existence of the soul as an individual and
responsible agent. He made every thing to consist of ideas and
impressions, and said that these have no necessary connection,
but are "a bundle of perceptions that succeed each other with
inconceivable rapidity, and that therefore I myself of to-day
am no more the I myself of yesterday or to-morrow than I am
Nebuchadnezzar or Cleopatra." A wag proposed the following epitaph
for his tomb-stone as a suitable illustration of his theory:

      "Under this circular idea, vulgarly called tomb,
      Impressions and ideas rest which constituted Hume."

4. =Origin of the word Spirit.=--The name spirit came originally
from an attenuated form of matter, the air or breath, because the
air, like spiritual existence, is invisible. The formation of
language is to a large extent thus based on analogies.

5. =Spirit not an Object of Sense.=--None of the senses can
perceive spirit itself, though they perceive the effects which
spirit produces on material substances. If, for example, you move
your arm, it is the spirit within you, acting upon the muscles
through the nerves, that causes it to move; and you see here the
effects produced by spirit upon matter, but you do not see the
spirit itself.

6. =Effects of Matter on the Senses.=--Some forms of matter can
be perceived by all the senses; others can be perceived by only a
part of them; some by only one. Air you can not see, nor smell,
nor taste; but you can feel it, and hear the sound of its motion.
Sometimes matter affects only the sense of smell, or that with
the sense of taste. Sea-air smells salt; but the salt in the air
is so finely divided that we can not see it. And yet it is the
salt, entering the nostrils and coming in contact with the extreme
fibres of the nerve of smell, that produces the effect. So when we
smell a flower, matter comes from it in particles so fine that no
microscope can detect them, but they produce sensation when they
strike upon the nerve.

7. =Forms of Matter.=--Matter appears in three forms: solid,
liquid, and gaseous or aeriform--that is, like air. Sometimes
matter is spoken of as having only two forms--solid and fluid.
In this case fluids are divided into two classes, the elastic
and non-elastic. The air and the various gases and vapors are
the elastic fluids; while those which are called liquids are the
non-elastic fluids. A foot-ball bounds because the air in it is
an elastic fluid. If it were filled with a non-elastic fluid; as
water, it would not bound. When water takes the form of steam it is
an elastic fluid. Though it is very common to use the expression
elastic fluids, the division of matter into three forms is the one
usually recognized.

8. =Solids.=--In solid matter the particles can not be moved about
among each other; but each particle generally retains the same
position in relation to those particles which are around it--in
other words, it does not change its neighborhood. This is more
true of some solids than of others. It is absolutely true of such
hard solids as granite and the diamond. In these the particles are
always in the same relative position. But it is not so with gold
or lead. By hammering these you can change greatly the relative
position of their particles. India-rubber is a solid, but the
relative position of its particles can be much altered in various
ways.

9. =Liquids.=--It is the grand characteristic of a liquid that its
particles change their relative position from the slightest causes.
It is in strong contrast with solids in this respect. When you move
any portion of a solid body you move all the other portions of it,
and generally in the same direction. But a body of liquid can not
be moved all together as one body except by confining it; as, for
example, in the case of a water-pipe or a syringe. And then, the
moment that the water can escape, the particles use their liberty
of altering their relative position. As wind and other agents act
continually upon water, no particle stays for any length of time
in the neighborhood of the same particles. "Unstable as water" is,
then, an exceedingly significant expression. Water is never at
rest. A particle of it may at one time be floating on the surface
of the ocean, and at another be in depths beyond the sounding of
man. It flies on the wings of the wind, falls in the rain, runs in
the stream, is exhaled from a leaf, trembles in the dew-drop, flows
in the blood of an animal or in the sap of a plant, and is always
ready to be jostled along in its ever-changing course.

10. =Gases.=--The particles of gaseous or aeriform substances move
among each other even more freely than those of a liquid. Air,
therefore, is more unstable and restless than water. Even when the
air seems to be perfectly still its particles are moving about
among each other. You can see this to be true if you darken a room,
leaving a single shutter a little open. Where the light enters
you will see motes flying about in every direction, which would
not be the case if the air were really at rest. The particles of
air have a greater range of travel than those of water; for the
sea of atmosphere which envelops the earth rises to the height of
about fifty miles. How far water rises in its evaporation we know
not; but it is not at all probable that it rises to the uppermost
regions of the atmosphere.

11. =Filling of Spaces by Liquids and Gases.=--It is the freeness
with which the particles of liquids and gases move among each
other that enables them to insinuate themselves into spaces every
where. They are ever ready to enter into any substances which
have interstices or pores of such size as will admit them. There
are mingled with the grains of the soil not only water, but air
and gases. These are present also in all living substances, both
vegetable and animal. Water is the chief part of sap and of blood,
and air and gases always go with water. Part of the air that we
breathe in enters the blood in the lungs, and courses with it
through the system. The fishes could not live in water if there
were not air mingled with it. This can be proved by experiment. If
you put a fish into a close vessel it will soon die, because it
uses up all the air that is in the water. In an open vessel the
fish is kept alive by the constant accessions of fresh air to the
water.

12. =Solution.=--In solutions of solid substances in water it is
the freedom with which the particles of water move about among each
other that enables them to take in among them the minute particles
of the solid. And when water ascends into air by evaporation it
may be said to be a real solution of water in the air; for the
particles of water mingle with those of the air, just as the
particles of a solid mingle with those of water in a solution.

13. =Relation of Heat to the Forms of Matter.=--Some kinds of
matter are seen in all the three forms. Whether these shall assume
one form or another depends on the amount of heat present. Thus
when water is solid, ice, it is because a part of its heat is gone.
Apply heat, and it becomes a liquid, water. Increase the heat to
the boiling point, and it becomes steam, or an aeriform substance.
Alcohol has only two forms--liquid and aeriform. It has never been
known to be frozen. Iron is usually solid; but in the foundry,
by the application of great heat, it is made liquid. Mercury is
liquid in all ordinary temperatures; but it often becomes solid
in the extreme cold of arctic winters. A mercurial thermometer
is of course useless under such circumstances, and the alcoholic
thermometer is relied upon to denote the degree of cold. The
difference between mercury, water, and iron in regard to the liquid
state is this: It takes but little heat, comparatively, to make
mercury liquid, while more is required for this condition in water,
and much more for it in the case of iron.

14. =The Nature of Matter Unknown.=--What now, let us inquire, do
we know of the nature of matter? Can we say that we know any thing
of it? We may observe its phenomena, and learn its properties; but
with our most searching analyses of it we can no more determine
what matter is than we can what spirit is. Newton supposed
"that God in the beginning formed matter in solid, massy, hard,
impenetrable particles." This he believed to be true of liquids,
and even of gases, as well as solids. In the gas these hard
particles are much farther apart than in the solid. The supposition
is a very probable one; but if it be true it does not let us know
what matter is, for it leaves us in the dark as to the nature of
the particles. Newton farther supposed that these particles have
always remained unaltered amidst all the changes that are taking
place; these changes being occasioned by "the various separations
and new associations and motions of these permanent particles."
When, for example, any thing is burned up, as it is expressed, not
one of these particles is either destroyed or altered, but they
merely take on new arrangements. Though most of the substance has
flown off in the form of gas, the ultimate particles composing
the gas are the same now that they were when making a part of the
solid substance; and they may soon again become a part of some new
solids. Such changes in the forms of matter are every where going
on; and when you become acquainted with Chemistry, in the Second
Part, you will be familiar with them.

15. =Atomic Theory.=--These ultimate particles of matter are so
minute that they have never been seen by man. The smallest particle
that can be seen with the most powerful microscope is probably made
up of very many of them united together. These ultimate particles
we term atoms; and the theory in regard to the composition by them
of different substances is called the atomic theory. The atoms of
different substances are not supposed to be alike, but to differ in
both size and weight. This theory will be more particularly noticed
in the Second Part.

16. =Imponderable Agents.=--There are certain agents--light, heat,
electricity, etc.--which are supposed by some to be forms of
matter. If they are, they are exceedingly attenuated; for their
presence, as has been proved by many experiments, never adds in
the least to the weight of any substance. They have therefore been
styled imponderable agents. Their agency is of great importance
and very active, producing every where constant changes. Two of
them--heat and light--are obviously and immediately essential to
life. What their real nature is remains as yet an entire mystery.




CHAPTER II.

PROPERTIES OF MATTER.


17. =Variety in the Properties of Matter.=--All matter has
properties or qualities. Some of these are different in the
different kinds of matter. Thus its three forms have different
properties, as you saw in Chapter I. There is variety also in the
properties of substances of the same class. Thus liquids are unlike
each other in some respects. Some, for example, are lighter than
others. Oil is lighter than water. Gaseous substances also differ
in this and in other respects. But the variety in the properties
of solids is greater than in those of gases or liquids. This will
appear as I proceed.

18. =Divisibility of Matter.=--Any visible portion of matter can
be divided into parts. Even if it be so small that you can see
it only with a powerful microscope, it could still be divided if
you could have an instrument sufficiently fine for the purpose.
_Divisibility_, then, is said to be a _general_ property of matter;
that is, a property belonging to all kinds of matter.

19. =Examples of Minute Division of Matter.=--There are numerous
examples in which the division of matter is carried far beyond that
which can be effected by any cutting instrument. Some of these I
will notice:

A gold-beater can hammer a grain of gold into a leaf covering a
space of fifty square inches. So thin is it that it would take
282,000 of such leaves, laid upon each other, to make the thickness
of an inch. And yet so even and perfect is this thin layer of
gold, that when it is laid upon any surface in gilding it has the
appearance of solid gold. A fifty millionth part of this grain of
gold thus hammered out can be seen by the aid of a microscope which
magnifies the diameter of an object ten times. But the division of
gold is made even more minute than this in the manufacture of the
wire of gold-lace. It is done in this way: A bar of silver weighing
180 ounces is covered with a layer of gold weighing an ounce. It is
then drawn through a series of holes in a steel plate, diminishing
in diameter, till it at length comes out a very fine wire 4000 feet
long. Each foot of it then has only the one 4000th part of the
ounce of gold, and yet the silver is well covered.

A soap-bubble is a beautiful example of the minute division of
matter. That thin wall which incloses the air which you have blown
into it is composed of particles of the soap and of the water
mingled together. It is supposed to be less than one millionth of
an inch in thickness.

The thread of the silk-worm is so minute that the finest
sewing-silk is formed of many of these threads twisted together.
But the spider spins much more finely than this. The thread by
which you see him letting himself down from any height is made up
of about 6000 threads or filaments, each coming from a separate
hole in his spinning machine. A quarter of an ounce of the thread
of a spider's web would extend 400 miles.

A grain of blue vitriol, dissolved in a gallon of water, will
make the whole blue. Such a diffusion could not be without an
exceedingly minute division of the particles.

Perhaps the most minute division of matter is exemplified in odors.
A grain of musk will scent a room for years, and yet have no
perceptible loss of weight. But all this time the air is filled
with fine particles coming from the musk.

The microscope reveals to us many wonderful examples of the
minuteness of the particles of matter, both in the vegetable and
the animal world.

If you press a common puff-ball a dust flies off like smoke.
Examined with a microscope, each particle of this dust, which is
the seed of the plant, is a perfectly round orange- ball.
This ball is of course made up of very many particles, arranged in
this regular form. Beautiful examples of various arrangements of
the minute particles of matter we have in the pollen of different
plants, as seen with the microscope.

Each particle of the dust which adheres to your fingers as you
catch a moth is a scale with fine lines upon it regularly arranged.
And if you look through the microscope at the wing of the moth, you
will see, where the dust is rubbed off, the attachments by which
the scales were held standing up from the surface of the wing, like
nail-heads on a roof where the shingles have been torn off.

The organization of exceedingly small animals, as revealed by the
microscope, furnishes us with wonderful examples of the minute
division of matter. A little of the dust of guano, examined through
a powerful microscope, is seen to contain multitudes of shells
of various shapes. These shells are the remains of animalcules
that lived in the water, their destiny seeming to be in part to
furnish food to other animals larger than themselves. In the chalk
formations of the earth are seen multitudes of such shells. They
have been discovered even in the glazing of a visiting-card; for
they are so small that the fine grinding up of the chalk does not
wholly destroy them. There are animals, both in the air and in the
water, so small that it would take millions of them to equal in
bulk a gram of sand, and a thousand of them could swim side by side
through the eye of a common-sized needle. Now in all these animals
there are organs, constructed of particles of matter, which are
arranged in them with as much order and symmetry as in the organs
of our bodies. How minute then must these particles be!

How do such facts extend our views of the power of the Deity! The
same power that moulded the earth, sun, moon, and the whole "host
of heaven," gave form, and life, and motion to the millions which
sport in every sunbeam; the same eye that watches the immense
heavenly bodies as they move on in their course, looks upon one and
all of these legions of animals in earth, air, and water, though
they are unseen by human eyes, seeing that every particle shall
take its right position, so that this part of creation may with all
the rest be pronounced very good; and the same bountiful hand that
dispenses the means of life and enjoyment to the millions of the
human race, forgets not to minister to the brief life and enjoyment
of each one of these myriads of animalcules, though they seem to be
almost nothingness itself.

20. =Pores and Spaces in Matter.=--In all matter there are spaces
about the particles. Those bodies which are called porous have
quite large spaces in them. But even in those which are not
commonly considered porous the particles are by no means close
together. A celebrated experiment tried in Florence a long time
ago showed that there are spaces among the particles of so dense
a substance as gold sufficiently large to let water through them.
A hollow golden globe containing water was subjected to great
pressure, and its surface was bedewed with the water that came out
through the pores of the gold. In all substances in which there are
pores visible to the naked eye, or by the aid of the microscope,
there are other spaces or interstices among the particles around
the pores. Indeed, it is supposed that there is space around every
ultimate particle or atom, and that no two of these atoms are
in actual contact. The fact that substances which have no pores
can be compressed into a smaller space than they usually occupy
shows that there are spaces or interstices in them. Solids can be
thus compressed, some more than others. But the most compressible
substances are the gases and vapors. The amount of space between
their particles must be very large to allow of so great compression.

[Illustration: Fig. 1.]

21. =Space in Gaseous Substances.=--We can have some idea of
the great amount of space in a gaseous or aeriform substance by
observing the difference between water in its liquid and in its
aeriform state. A cubic inch of water, when it becomes steam,
occupies 1696 times as much room as it did when it was water. The
difference in proportion is exhibited in Fig. 1, the inner circle
representing the water, and the outer the steam into which it is
converted. Now the water is not altered at all in its nature by
being changed into steam. The particles are simply put farther
apart by the heat, and as soon as the heat is withdrawn they
come together again to form water, or, in other words, the steam
is condensed into water. It is plain, therefore, that the space
between the particles is 1696 times as great in steam as it is in
the water from which the steam is made.

22. =Solutions.=--When any substance, as sugar or salt, is
dissolved in water, its particles are diffused through the spaces
that exist between the particles of the water. So also when water
evaporates (§ 12), the particles of water are diffused through the
spaces between the particles of the air. In like manner are the
particles from an odorous substance diffused in these spaces, and
thus mingled with the particles of the air they are carried into
the nostrils, and strike upon the minute extremities of the nerve
of smell.

23. =Relation of Heat to the Spaces of Matter.=--The variation
in the amount of space between the particles of matter in any
substance generally depends on the variation of the amount of
heat present. Thus heat expands iron; that is, it increases the
spaces between the particles of the iron. So also heat increases
the spaces between the particles of mercury, and thus makes it
occupy more room in the thermometer. This effect of heat will be
considered more fully hereafter.

The general views which I have given of the constitution of
matter will throw light upon the different qualities of different
substances, some of which I will notice.

24. =Density and Rarity.=--The density of a substance depends upon
the quantity of matter it contains in a given space. The more
dense, therefore, a substance is the greater is its weight. A piece
of lead is forty times heavier than a piece of cork of the same
size. Mercury is nearly fourteen times heavier than an equal bulk
of water. You see, then, that density must depend on the nearness
of the atoms to each other. In so dense a substance as gold the
atoms are all very close together; in wood there are spaces, some
of which are so large that you can see them; and in air, steam, and
the gases there is a great deal of space among the particles (§
21), so that we speak of their _rarity_ instead of their density.

25. =Tenacity.=--The power of holding together, termed tenacity,
depends on the degree of attraction between the particles. By
attraction I mean a disposition in particles to come together, this
disposition being manifested in opposition to any force tending
to draw them apart. I shall soon speak of this more particularly.
Tenacity does not exist at all in gaseous substances. The particles
of air and of steam, for example, show no disposition to cling
together; that is, have no tenacity. This property is weak in
liquids. It is only strong enough in water to enable its particles
to hang together in the shape of a drop. It is strong in solids,
enabling their particles not only to hold together in large
quantities, but to hold up also heavy weights suspended to them.
It is stronger in iron than in any other solid. It is stronger in
wrought iron than in cast iron; and strongest of all in steel.

26. =Comparative Tenacity of Substances.=--Various metals and other
substances have been tested in reference to their comparative
tenacity. It was done in this way: Wires were made of the metals,
all of the same size. Weights were suspended to them, and additions
were made to the weights by little and little till the wires broke.
The table underneath was made by placing against each metal the
greatest weight that its wire would hold:

  Cast steel              134 pounds.
  Best wrought iron        70 pounds.
  Cast iron                19 pounds.
  Copper                   19 pounds.
  Silver                   11 pounds.
  Gold                      9 pounds.
  Tin                       5 pounds.
  Lead                      2 pounds.

Oak wood, tried in the same way, was found to hold up 12 pounds,
one more pound than silver. Some animal substances have great
tenacity, as the thread of the silk-worm, hair, wool, and the
ligaments and tendons of our bodies and of other animals.

27. =Value of Tenacious Substances.=--"The gradual discovery," says
Dr. Arnot, "of substances possessed of strong tenacity, and which
man could yet easily mould and apply to his purposes, has been of
great importance to his progress in the arts of life. The place of
the hempen cordage of European navies is still held in China by
twisted canes and strips of bamboo; and even the hempen cable of
Europe, so great an improvement on former usage, is now rapidly
giving way to the more complete and commodious security of the iron
chain--of which the material to our remote ancestors existed only
as useless stone or earth. And what a magnificent spectacle is it,
at the present day, to behold chains of tenacious iron stretched
high across a channel of the ocean, as at the Menai Strait between
Anglesea and England, and supporting an admirable bridge-road of
safety, along which crowded processions may pour, regardless of
the deep below, or of the storm; while ships there, with sails
full-spread, pursue their course unmolesting and unmolested."

28. =Hardness.=--This property seems to depend upon some peculiar
arrangement of the particles of matter. We should suppose that the
densest substances would be the hardest. But it is not so. Iron
is the hardest of the metals, but its particles are not so close
together as those of gold, which is quite a soft metal. And gold
is five times as heavy as the diamond, which is so hard as to cut
glass easily. Common flint is hard enough to scratch glass, but
will not cut it like the diamond.

[Illustration: Fig. 2.]

29. =Flexibility and Brittleness.=--If you bend a flexible body as
a piece of wood, as represented in Fig. 2, it is obvious that the
particles on the upper or convex side must be put a little farther
apart, while those on the under or concave side are brought a
little nearer together. But the wood does not break, because the
particles that are thus moved a little apart still retain their
hold upon each other. This is the explanation of what we call
flexibility. On the other hand, the particles in a rod of glass
can not be put farther apart in this way. They are not actually in
contact any more than the particles of the wood are (§ 20), but
they are in a _fixed_ relative position; that is, a position which
can not be disturbed without a _permanent_ separation of particles.
If you attempt to bend the rod there is no slight separation of
many particles, as in the bent wood, but a full and permanent
separation in some one part of the rod. We call the property on
which this result depends brittleness. Brittle substances are
generally hard. Glass, while the most brittle of all substances,
is hard enough to scratch iron. Brittle substances also have much
tenacity. A rod of glass can hold up a heavy weight, although a
slight blow suddenly given would break it.

30. =Flexible and Brittle Steel.=--There are two kinds of steel,
flexible and brittle. The steel of most cutting instruments is
brittle. The steel of a sword-blade is quite flexible, and that
of a watch-spring is so much so that we can wind it up in a coil.
This difference is owing to a difference in the mode of cooling
the steel. If it be cooled suddenly, it is brittle; if slowly,
it is flexible. The process by which it is cooled slowly is
called _annealing_. The explanation of all this is quite plain.
The steel being expanded by heat--that is, its particles being
put farther apart than they usually are--when they are suddenly
brought together again they have not time to arrange their relative
position properly. Brittleness is therefore the result. But, on the
other hand, when the cooling is effected gradually, time is given
for the arrangement.

31. =Tempering of Steel.=--Steel suddenly hardened is too brittle
for common use. A process called tempering is therefore resorted
to for diminishing the brittleness. The steel is reheated after
the hardening, and is then allowed to cool slowly. The degree in
which the brittleness is lessened depends on the degree of heat to
which the steel is subjected. It can be entirely removed by a red
heat, for then the particles have a full opportunity to readjust
themselves; and the more the heat comes short of this point the
less thorough will be the adjustment, because the less perfectly
are the particles released from their suddenly-taken position. In
lessening the brittleness we lessen hardness also, and therefore
the tempering is varied in different cases according to the degree
of hardness which is desired.

32. =Annealing of Glass.=--Glass is always annealed. If this
were not done our glass vessels and windows would be exceedingly
brittle, and would therefore be constantly breaking. Articles made
of glass are annealed by being passed very slowly indeed through
a long oven which is very hot at one end, the heat gradually
lessening toward the other end.

[Illustration: Fig. 3]

33. =Prince Rupert's Drops.=--We have a striking example of
brittleness induced by sudden cooling in what are called Prince
Rupert's drops. These are made by dropping melted green glass into
cold water, and they are of the shape represented in Fig. 3. If
you break off ever so small a bit of the point of one of these
drops, the whole will at once shiver to pieces. That is, the sudden
arrangement of the particles is so slight and unnatural that the
disturbance of the arrangement in a small part suffices to destroy
the arrangement of the whole, very much as a row of bricks falls
over from the fall of the first in the row. Mr. Farraday says that
these drops were not, as is commonly supposed, invented by Prince
Rupert, but were first brought to England by him in 1660. They
excited much curiosity at that time, and were considered "a kind of
miracle in nature." But you see that this, like many other wonders,
receives with a little thought an easy explanation.

34. =Malleability and Ductility.=--Those metals which can be
hammered into thin plates are called malleable. Gold furnishes us
with the best illustration of this property. Silver, copper, and
tin are quite malleable. Most of the other metals are very little
so, and some of them are not at all, breaking at the first blow.
A substance is said to be _ductile_ when it can be drawn out into
wire. The principal metals that have this quality are platinum,
silver, iron, copper, and gold, and in the order in which I have
named them. Melted glass is very ductile. It can be drawn out in
a very fine thread, and when this thread is cut and arranged in
branches it resembles beautiful white hair. In hammering metals
into plates, or drawing them into wire, there is a considerable
change of relative position in the particles, similar to that which
we have in fluids, though nothing like as free. In this change of
position those particles that do remain in close neighborhood have
a remarkable tenacity or attraction, preventing their separation.
In welding two pieces of iron, which is done by the blacksmith by
hammering them together when red-hot, there must be enough movement
among the particles to have those of one piece mingle somewhat with
those of the other.

35. =Compressibility.=--Porous substances can be considerably
compressed. Force applied to them can bring their particles
nearer together, making them to fill up in part their pores. The
most familiar example you have of this is in sponge. The more
porous wood is the more can it be compressed. But even such dense
substances as the metals can be compressed in some degree; that is,
the interstices between their particles can be made smaller. Medals
and coins have their figures and letters stamped upon them by
pressure, just as impressions are made upon melted sealing-wax. The
heavy and quick pressure required to do this actually compresses
the whole piece of the hard metal, putting all the particles nearer
together, so that it occupies less space than it did before it was
stamped.

36. =Incompressibility of Liquids.=--We should suppose, from the
freeness with which the particles of liquids move among each
other, and from the spaces (§ 22) which exist among them, that
these substances could be easily compressed. But it is not so. The
heaviest pressure is required to compress them even in a slight
degree. Water can be compressed so very little that practically it
is regarded as incompressible.

37. =Influence of Heat on the Bulk of Liquids.=--Although the
interstices between the particles of liquids can not be varied by
mechanical pressure, they can be by variations of temperature.
Liquids are dilated or expanded by heat; that is, their particles
are put farther apart. They are contracted or compressed by cold;
that is, their particles are brought nearer together by the
abstraction of heat. The most familiar example that we have is
in the thermometer. The mercury rises in the tube when the heat
increases the interstices between its particles; and it falls when
the loss of heat allows the particles to come near together. The
same effects are seen when alcohol is used in the thermometer,
as is done in the arctic regions, because mercury may freeze
there. A thermometer with water in it would answer if we wished
only to measure temperatures between the freezing point and the
boiling point of water. The expansive influence of heat will be
particularly treated of hereafter.

38. =Compressibility of Aeriform Substances.=--Aeriform bodies
are more compressible than any other substances, showing that in
their ordinary condition there is a great deal of space among their
particles. While they are thus unlike liquids in compressibility,
they are affected by heat in the same way that liquids are.

[Illustration: Fig. 4. Fig. 5.]

39. =Elasticity.=--Closely allied with the compressibility
of matter is its elasticity. We see this property strikingly
exemplified in India-rubber. It occasions the rebounding of a ball
of this substance when thrown down. Observe flow exactly what
occurs in this case. The ball as it meets the resistance of the
floor is flattened, as represented in Fig. 4. Then, as it assumes
the round shape, as seen in Fig. 5, it pushes downward upon the
floor. It is this sudden pushing downward that makes it rebound.
It is as if there were a compressed spring between the ball and
floor. It may be likened also to jumping. When one jumps he bends
his limbs at the thigh and knee joints, and then, in straightening
himself up, gives a sudden push, like that given by the ball as
it assumes its round shape, and so is thrown forward or upward,
according to the direction in which the pushing force is made.
The same flattening occurs in an ivory ball, though not to the
same degree. You can prove that it does occur by experiment. Let a
marble slab be wet and drop the ball upon it. Quite a spot will be
made dry by the blow of the ball, showing that it touched more of
the marble than it does when it is merely placed upon it.

40. =Elasticity Shown in Other Ways.=--If a stick be bent, as in
Fig. 2, as soon as the bending force is withdrawn the stick becomes
straight again from its elasticity. It is this elastic force of the
bow, straightening it, that speeds the arrow. Observe in this case
that while the particles on the concave side of the bent bow are
brought nearer together or compressed, those on the convex side are
moved apart. This moving apart of the particles is often shown in
India-rubber. You can see how very far apart particles that are in
near neighborhood may be carried, if you will stick two pins close
together in a strip of India-rubber before you stretch it.

41. =Degrees of Elasticity in Different Substances.=--Some
substances have so very little elasticity that they are practically
considered as having none. Lead is one of these. A rod of lead
when bent remains so, and a leaden ball does not rebound. While
aeriform substances are the most compressible of all, they are also
the most elastic. Air compressed returns to its usual condition
the moment that it is relieved from the pressure, and with a force
proportioned to the amount of the pressure. So it is with steam
and the gases. The varied results of this quality of aeriform
substances will claim our attention more particularly in some
other parts of this book.

42. =Definition of Elasticity.=--You see from the illustrations
that have been given that elasticity is _that property of matter
by which its particles, when brought nearer together or carried
farther apart by any force, return to their usual condition when
the force is withdrawn_.

43. =Usefulness of Variety in Properties of Matter.=--The
various properties of matter brought to view in this chapter are
providential adaptations to the necessities of man. Each substance
has those properties which best fit it for his use. Iron, for
example, designed by the Creator to be both the strongest and most
extensively useful servant of man among the metals, is therefore
provided in great abundance, and has those strong, decided, and
various qualities which fit it for the services it is to perform.
Gold and silver, on the other hand, designed for services less
extensive, lighter, and in a great measure ornamental, are provided
in very much less quantity, and have properties admirably adapting
them to the services for which they are so manifestly intended.
The same can be substantially said of all other substances, and
especially of those very abundant ones, air and water. And it
may be remarked also that the ingenuity of man is continually
discovering new modes of bringing the various properties of matter
into his service. I will give but a single illustration--the
tempering of steel. "This discovery," says Dr. Arnot, "is perhaps
second in importance to few discoveries which man has made; for it
has given him all the edge-tools and cutting-instruments by which
he now moulds every other substance to his wishes. A savage will
work for twelve months with fire and sharp stones to fell a great
tree and to give it the shape of a canoe, where a modern carpenter,
with his tools, could accomplish the object in a day or two."




CHAPTER III.

THE ESSENTIAL PROPERTIES OF MATTER.


44. =Extension.=--You can not conceive of any portion of matter,
however small it may be, that has not shape or figure. It may be
so small as to appear only as a point to the naked eye, but seen
through the microscope its shape becomes obvious. Even an atom must
have length, breadth, and thickness, though it be so small that
we can not measure it, nor see its shape with the most powerful
microscopes. _Extension_, which is the term commonly used to
express this idea, is then an _essential_ property of matter; that
is, it is a property of which no form or kind of matter can be
destitute. The distinction, in this respect, between this property
and those which I have before noticed may be made obvious to you
by an example. Hardness is not an essential quality of matter, for
some kinds of matter are destitute of it; but no portion of matter,
hard or soft, can be destitute of extension or shape. The air is
sometimes spoken of in common language as being shapeless. This
is partly because it is invisible, and partly because no portion
or body of air assumes any definite shape. But air is continually
forced into definite shapes by confinement in rooms, boxes, etc.;
and then its extension in different directions can be measured as
accurately as the extension of a solid can be. And besides, the
atoms of which air is composed are undoubtedly solid, and we can
not conceive of their existence without attaching to them the idea
of figure or extension.

45. =Impenetrability.=--In common language one substance is said
to penetrate another. Thus a needle penetrates cloth, a nail
penetrates wood, etc. But this is not strictly true. The needle
does not go into the cloth, but goes between the fibres of it,
pushing them to the one side and the other. So the nail goes
between the fibres of the wood, and not into them. It does not
occupy the same room that the fibres do at the same time. So,
also, no atom of matter can penetrate or go into any other atom.
It can only push it out of the way, and then occupy its place.
Impenetrability is therefore said to be one of the essential
qualities of matter. This means simply that no portion of matter
can occupy the same space with another portion of matter at the
same time.

[Illustration: Fig. 6.]

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

46. =Illustrations.=--Many illustrations might be given of this
property. I will give a few. If you press a tumbler into water with
its open end downward, you can not fill it with water, for the
air confined in the tumbler prevents it from rising. It can not
occupy the same space with the air. It fills, indeed, a portion
of the tumbler, but this is because the air is compressible. If
you introduce a glass funnel, _a_, Fig. 6, into a jar of water,
_b_, with your thumb on its mouth, _c_, the water will not rise
to fill it. But if you take your thumb off, the water will rise
to the level of the water outside of the funnel, pushing up the
air before it. If you have not a funnel, a vial or bottle with its
bottom broken off will answer for the experiment. The following
is a very pretty experiment, illustrating the same point. Place a
lighted taper, _a_, Fig. 7, on a large flat cork in a jar of water.
Put over it an open jar or receiver, _b_, having a stop-cock, _c_.
Closing the stop-cock, press the receiver down into the water,
and you will see the taper sink with it, as represented in the
figure, the air preventing the water from entering the receiver.
If now you open the stop-cock the water rushes in, thrusting the
air upward, and making the taper to appear as if rising out of the
water. The diving-bell offers a good illustration. It consists of a
vessel, _a a_, Fig. 8, shaped like a bell, made sufficiently heavy
to sink in water. It is let down by a chain and cable, as seen in
the figure. The water does not enter the bell any farther than
the compressibility of the air permits it. In order that the men
in it may remain under water for some time fresh air is supplied
by the tube _b_, it being forced down by a forcing-pump. At the
same time the vitiated air can be let off by a valve provided for
that purpose. There are windows in the top of the bell to give the
requisite light for work on the sea's bottom. Treasures are often
recovered by this means which would otherwise be lost. You see a
resemblance between the diving-bell and the arrangement in Fig. 7,
the receiver representing the bell and the lighted taper the men in
it.

47. =Other Illustrations.=--If you drop a bullet into a tumbler
of water it pushes the particles to the one side and the other,
and occupies the room thus made. If you drop in several there is a
manifest rise of the water, and you may drop in enough to make it
overflow. The same thing is true of the finest needle dropped in
the water--it does not penetrate it, but like the bullet displaces
some of its particles and occupies their room; and you can make
the water overflow by dropping in many needles. We can truly say,
then, that water can not be penetrated even by a needle. When any
substance, as sugar, is dissolved in water, its particles do not
penetrate the water, but go into the spaces between its particles.
So, also, when particles from odorous substances are diffused
in the air, they are not really in the air, but are between its
particles.

48. =Inertia.=--Matter has no power to put itself in motion. When
it is moved it is moved by some force which is either outside of
the matter or is communicated to it in some way. When your arm is
moved it is not the matter in your arm that is the cause of its
motion. It is caused by a force in you which I will not dwell upon
here, because the subject belongs to Physiology. When air moves it
is set in motion by some force acting upon it, as when you blow it
from your lungs or move it with a fan. When the wind blows, the air
is set in motion by heat and the attraction of the earth, as will
be explained to you in another part of this book. I might multiply
examples to any extent, showing that matter of itself can not move.
This property of matter is termed _inertia_.

49. =Inertia Shown in the Inability of Matter to Stop its
Motion.=--Matter, when once set in motion, has no power to stop
itself. If it could stop itself it could not be said to be inert.
And as it is inert, it would, when once set in motion, keep on
moving forever unless stopped by some force. When a stone falls
to the ground, it stops simply because the earth stops it. If the
earth were not in the way, the stone would move straight on until
it were stopped by something else. So, also, a stone that is thrown
up in the air would keep on, and soon be out of sight, and never
return to the earth, if it were not made to come down by forces
acting upon it. One of these forces is the resistance of the air,
which, from the moment the stone starts, is destroying its motion.
Another force as constantly operating to <DW44> the stone is the
attraction, or drawing force, exerted by the earth upon it. This
powerful though unseen force will be treated of fully in the next
chapter.

50. =Matter Equally Inclined to Rest and Motion.=--It was formerly
taught by philosophers that matter is more inclined to rest than
to motion; and this is the popular notion now. This is because
the chief causes that stop the motions that we see from day to
day--viz., the air and the attraction of the earth--are not
visible. For this reason, until we investigate the subject,
it seems to us that motion has a natural tendency to stop, or
is _spent_, as it is expressed. When friction has an agency in
arresting motion we see it plainly; but the common idea is, that in
this case the motion is in part spent and in part destroyed. But in
no case is motion spent, but it is always destroyed by obstacles.
The more thoroughly these are removed, the longer will the motion
continue; and if they were wholly removed, the motion would never
cease. Perpetual motion, then, is not in itself impossible; for
matter has no more tendency to stop moving when once put in motion,
than it has to begin motion when it is at rest. All motion would be
perpetual if there were not forces opposing it. If there were only
one body in the universe, and that were set in motion, it would
move forever through empty space in a straight line; for there is
no matter any where to resist its motion or to attract it away from
its onward course.

51. =Divisibility.=--Though divisibility is a general quality of
matter (§ 18), it is not an _essential_ one. For if it be true that
every portion of matter is composed of atoms that remain whole (§
14), this property can not be said to belong to these atoms. It is
only the bodies made up of these atoms that can be divided. When we
come to the atoms themselves the division must stop.

52. =Weight.=--In speaking of the properties of matter I have
said nothing about weight, although in the popular mind this is
thought of as being one of the most prominent of the properties
of some kinds of matter. This will be appropriately treated of
when I come to speak of attraction, for it is a mere result of
attraction. Suffice it to state here, that the weight of a body
is _the pressure occasioned by the attraction existing between it
and another body_. If when a stone is raised from the ground the
attraction between it and the earth could be destroyed, the stone
would remain there. It would not press down, and so would have no
weight. It is plain, therefore, that weight, so far from being an
essential property of matter, is not really a property at all. It
is only an effect of a property--attraction. If there were only one
body in the universe, it would have no weight, for it would press
in no direction because there is nothing to attract it. But as it
is, all matter has weight, for there is other matter to attract it.




CHAPTER IV.

ATTRACTION.


53. =Nature of Attraction.=--If you attempt to break a very
tenacious solid substance, why do you not succeed? It is because
the particles are so strongly fastened together. But how? By some
kind of cement or glue, or by some mechanical contrivances as nails
or hooks? No. They are fastened together by some unseen force. We
know nothing of the nature of this force. We know only that it
exists, and we call it attraction. The name is a proper one, for
it simply expresses the fact that one particle attracts or draws
another particle toward itself.

54. =Newton's Idea of Attraction.=--It was stated in § 20 that the
particles of matter, even in the densest substances, are not in
actual contact, but have spaces around them. Now it was supposed by
Newton that there is some kind of ethereal substance pervading all
these spaces, which causes this attraction between the particles.
He supposed also that this ether was every where in space,
causing attraction between masses of matter. But all this is mere
supposition, and we know not whether there is this sort of ethereal
glue keeping the universe together, or whether it is some property
in the particles themselves that makes them thus attract each
other. But the fact of the attraction we know, and we can observe
the phenomena which it produces, and discover the laws or rules by
which this force is regulated in its action.

55. =Attraction in Solids.=--Attraction is stronger in some solids
than in others. The mason with his trowel easily divides a brick;
but he can not do this with a piece of granite, for its particles
have a greater attraction for each other than those of the brick.
So a rap which would break a glass dish would not injure a copper
one of the same thickness. A weight that would hang securely from
an iron wire would break a lead wire of the same size; that is,
it would tear the particles apart, because they are not strongly
attracted to each other. Attraction has different modes of action
in different solids. It therefore fastens their particles together
in different ways, and thus produces all the various qualities,
already noticed, which are so useful to us--tenacity, hardness,
softness, ductility, flexibility, etc.

56. =Attraction in Liquids.=--In a liquid the attraction between
the particles is very feeble compared with that in solids. The
attraction of the particles of steel is in strength about three
million times that of the particles of water. We make the estimate
in this way: We find that a steel wire will sustain a weight
equal to 39,000 feet of the wire. But a drop of water hanging to
the end of a stick can not be more than one-sixth of an inch in
length; that is, water will hold together by the attraction of its
particles only to this extent, which is a little less than the
three millionth part of the length of steel wire which could hang
without breaking.

57. =Freeness of Movement of the Particles of Liquids.=--There
is one prominent characteristic of liquids which is probably not
entirely owing to the feeble attraction of their particles--I mean
the freeness with which these particles are moved among each other.
This is owing probably in part to some peculiar arrangement of the
atoms in making the particles of a liquid. I will illustrate this
in a coarse way. If the atoms of lead in shot were so arranged as
to make irregular jagged forms, they could not readily be moved
among each other. We suppose the ultimate atoms of a liquid to be
so arranged in the formation of particles as to make them not only
round but very smooth. Hence comes the great ease with which they
circulate among each other.

[Illustration: Fig. 9.]

[Illustration: Fig. 10.]

58. =Globular Shape of Drops of Liquids.=--As the particles of a
liquid move thus freely among each other, their attraction disposes
them to assume a globular or round shape. The reason of this can be
made plain by Figs. 9 and 10. The outside of a perfect sphere is
all at the same distance from the centre. So all the circumference
of a circle is at the same distance from the centre, as represented
in Fig. 9. But this is not true of all parts of the surface of
a cube or of a square: _a_, for example, is farther from the
centre than _b_ is. Now in a drop of liquid all the particles are
attracted toward the centre, for in that line from each particle
lies the largest number of particles to attract it. This can be
made obvious by taking some point in the drop, as represented in
Fig. 10, and drawing lines from it through the centre and in other
directions. If _a_ be the point in the drop, it is plain that the
line from it through the centre is longer than _a b_ or _a c_.
Therefore a particle, _a_, will be attracted toward the centre
rather than in the direction _a b_ or _a c_, because there are more
particles in the direction of the centre, and the more particles
there are the stronger is the attraction. But this is not all. The
particles in the line _a c_, tending to make _a_ go toward _c_, are
balanced by the particles in the line _a e_, tending to make it go
toward _e_. The two lines of particles therefore together tend
to make it go in a middle line between them, that is, toward the
centre, just as two strings pulling equally, the one to _c_ and the
other to _e_, would make a body, _a_, move in a middle line between
these two directions. The same can be shown of the two lines
of particles _a b_ and _a d_, and so of any other two alike in
situation on each side of the line through the centre. The tendency
of every particle is, then, to go toward the centre, and it would
go there if there were not particles between to prevent it. You see
how this would operate in the case of the particles on the surface
of the drop. As these are all striving, as we may say, in obedience
to attraction, to get to the centre, none of them will be raised up
into an angle or a point, as would be the case if the drop were in
the shape of a cube. If this should be done it would show that some
of the particles were not as strongly attracted toward the centre
as others are, which is an impossibility.

59. =The Globular Form in Different Liquids.=--The disposition
to form a sphere is seen more distinctly in mercury than in any
other liquid. If you drop a little of it upon a plate it separates
into globules, which roll about like shot. Why can not the same
thing be done with water? Why do the drops of water hang upon the
window-pane, showing only in an imperfect way their disposition
to the globular arrangement? It is because the particles of water
have a greater attraction for other substances, and less attraction
for each other, than the particles of the quicksilver have. Water
sometimes exhibits its disposition to the globular form in full on
the leaves of some plants, and rolls about in balls like mercury.
This is because there is something on the surface of the leaf which
repels rather than attracts the water. If you put your finger,
however, on one of these drops, it will spoil it, and your finger
will be moistened, because there is an attraction between the
particles of your skin and of the water. Take another illustration
of this difference in attraction. If you drop a little oil upon
the surface of water it will float about in round drops. This is
because the water repels the oil, as the surface of some kinds of
leaves does water. But when oil is spilled upon wood or cloth its
particles have so strong an attraction for their particles that
they unite with them, instead of gathering up into little round
companies as they do on the surface of water.

60. =Manufacture of Shot.=--We have a beautiful example of the
tendency of fluids to the globular arrangement in the manufacture
of shot. The melted lead is poured into a large vessel in the top
of the shot-tower. This vessel has holes in its bottom, from which
the metal falls in drops. Each drop, as it whirls round and round
in its fall, takes the globular form. By the time that it reaches
the end of its journey, about two hundred feet, it becomes so far
cooled as to be solid, and as it is received in a reservoir of
water, its globular form is retained. Bullets can not be made in
this way, because a quantity of melted lead sufficient to make a
bullet will not hold together in a globular form.

61. =Globular Form of the Earth and the Heavenly Bodies.=--It is
supposed that the sun, moon, earth, and all the heavenly bodies
were once in a liquid state, and that they owe their globular
shape to this fact. As they whirled on in this condition in their
course, the different solids were gradually formed, and at length
they acquired their present state. How all the mighty changes could
be effected in our earth, converging it from a liquid into a body
with a solid crust, having such various substances in it, and so
variously arranged, with its depressions containing water, and the
whole covered with its robe of air fifty miles in thickness, we
can not understand. And yet there are some portions of the process
which chemistry and geology have revealed to us, giving us some
glimpses of the wonders which, during the lapse of ages, God
wrought in our earth in preparing it for the habitation of man.

[Illustration: Fig. 11. Fig. 12.]

62. =Crystallization.=--The arrangement of the particles of solid
substances is different from that of liquids. The tendency here
is to straight lines and angles; that is, to crystalline forms.
Alum or common salt, when it becomes solid from a solution, forms
crystals. So also does sugar. The crystals of different substances
are different. In Fig. 11 you have the crystal of common salt, and
in Fig. 12 that of alum. We see this crystalline tendency every
where, even in the rude rocks and common stones. The rocks are
disposed to exhibit regular layers, or columns, or battlements, and
always do so except when interfering circumstances prevent. And
when you examine their composition, or that of the stone under your
feet, you see the same crystalline disposition in detail that you
see in the mass.

63. =Crystallization of Water.=--Water, when it changes into a
solid, shows the same disposition, of which the crystals of the
snow and the frost-work on our windows are familiar examples.
When snow forms, the water of the clouds is suddenly crystallized
by the cold air, the particles taking their regular places more
readily and certainly than if they were guided by intelligence,
because in obedience to an unerring law established by the Creator.
We sometimes have an example of this sudden crystallization of
water under our eye. The water in a pitcher may remain fluid,
although it is cooled down to the freezing point, and even below
it, if it be kept perfectly still. But on taking up the pitcher
the water at once becomes filled with a net-work of ice-crystals.
The explanation is this: The stillness of the water has prevented
its particles from taking on the new arrangement needed for the
formation of ice; but the jostling of them in taking up the pitcher
has served to make them do it thus suddenly.

[Illustration: Fig. 13.]

64. =Frost and Snow.=--The frost-work on our windows is a wonderful
exhibition of the variety of forms that crystallization can
produce. It sometimes presents figures like leaves and flowers,
such as we see chased on vessels of silver, but much more delicate
and beautiful. So varied and fantastic are the forms in which these
water-crystals are arranged, that it is very natural to ascribe
them, as is done universally in the dialect of the nursery, to the
ingenuity of a strange and tricksy spirit. Every snow-flake is a
bundle of little crystals as regular and beautiful as the crystals
which you so much admire in a mineralogical cabinet. And there is
great variety in the grouping of these crystals. You have some
specimens of these groups in Fig. 13 as they appear on examining
them with the microscope. Over six hundred different forms have
been enumerated, and a hundred have been delineated. It is a very
quick operation by which the particles of water in the clouds thus
marshal themselves, as if by magic, in these regular forms. But a
quicker operation is that by which hail is formed--so quick that
the particles have not time to set themselves in the crystalline
arrangement, but are huddled together without order. The brilliant
and glistening whiteness of the snow is owing to the reflection of
light from its minute crystals. In the arctic regions the beauty
of the snow is often much greater than with us. "The snow crystals
of last night," says Captain M'Clintock in his "Discovery of the
Fate of Sir John Franklin," "were extremely beautiful. The largest
kind is an inch in length; its form exactly resembles the end
of a pointed feather. Stellar crystals two-tenths of an inch in
diameter have also fallen; these have six points, and are the most
exquisite things when seen under a microscope. In the sun, or even
in moonlight, all these crystals glisten most brilliantly; and as
our masts and rigging are abundantly covered with them, the _Fox_
never was so gorgeously arrayed as she now appears."

65. =Order in Nature.=--We see in this general tendency to
crystallization a striking illustration of the fact that God
is a God of order. Disorderly arrangement is never seen except
where there is an obvious necessity for it. And even when there
is apparent disorder, a little examination generally shows that
essentially there is order. The rocks that give so much variety
to scenery are not piled up in confusion, and order has evidently
reigned in their construction. Pick up a common stone, and on
breaking it you will see the crystalline arrangement in its
interior. Nay, more, much of the very soil is made up of separated
and broken crystals.

[Illustration: Fig. 14.]

66. =Particles Must be very Near to Each Other to Adhere.=--Why
is it that when you have broken any thing made of glass, however
accurately you may bring the two parts together, you can not make
them unite in one again? It is simply because the particles of
substances will not attract each other enough to be united unless
they are brought very near together. Now it is impossible to bring
the particles on the two surfaces of a broken piece of glass as
near together as they were before it was broken. If you could do
so, no crack would be visible. You can join them by some kind of
cement. This is because the particles of the cement, while it is
soft, can be insinuated among the particles of the glass; and thus,
when it dries, it becomes a bond of union between the particles
on each side of the breach. For the same reason you can make the
cut surfaces of some yielding substances adhere. If you divide
a piece of India-rubber with a clean cut, you can make the two
surfaces adhere by pressing them together firmly. The particles in
this case are not unyielding, as those of the glass are, and some
of them are therefore brought into such near neighborhood as to
attract each other sufficiently to unite together. So, too, if you
cut two bullets so as to have a very smooth flat surface on each,
you can make them adhere quite strongly by pressing them together,
especially if you give a little turning motion at the same time
that you press, for this will cause the particles on the two
surfaces to be somewhat mingled together. If you have quite large
balls of lead with handles, as represented in Fig. 14, it will
require considerable force to separate them when they have been
thus pressed together.

67. =Other Illustrations.=--Silver and gold may be made to adhere
to iron by a very great and sudden pressure. The iron must be made
very smooth, and the silver or gold plate very thin. A powerful
blow brings the particles of the thin plate into such nearness to
those of the iron that union is effected, or, in other words, that
they attract each other sufficiently to be united. So, also, a
sheet of tin and one of lead can be made to adhere so as to form
one sheet by the pressure of the rollers of a flatting-mill. Two
very smooth panes of glass laid one upon another may have so many
particles brought into great nearness as to occasion some adhesion.
It will be slight, however, few comparatively of all the particles
coming near enough to adhere, for the smoothest glass is full of
inequalities, as may be seen by the microscope.

68. =Strength of Adhesion.=--In no case do particles come in actual
contact (§ 20), and their adhesion depends on the nearness of their
neighborhood to each other. The strength of union, then, between
two surfaces depends on the number of particles brought near enough
to adhere together. In the case of the two bullets or the lead
balls, if all the particles of the two surfaces were near enough
to adhere, the lead would be just as strong at the junction as any
where else. The reason that so strong adhesion takes place between
portions of some substances when we soften them by heat is that
the particles of the two softened ends are all brought near enough
together for adhesion. Thus the two ends of a broken stick of
sealing-wax may be firmly united by heating them and then pressing
them together. The same thing can be done with glass. When iron is
welded, as it is termed, some hammering is required to make the
particles of the two softened ends of the iron unite.

[Illustration: Fig. 15.]

69. =Attraction Between Solids and Liquids.=--The attraction which
solids and liquids have for each other furnishes us with many
interesting phenomena. The adhesion of drops of water to glass
and other solids is a familiar example of this attraction. If you
dip your hand into water, it is wet on taking it out, because
your skin has sufficient attraction for the water to retain some
of it. A towel will retain more of it for two reasons--with the
interstices between its fibres it presents much more surface to
the water, and it has none of the oily substance which on your
skin, though being in small quantity, serves somewhat to repel the
water. The attraction of solids and fluids for each other is shown
very prettily in the experiment represented in Fig. 15 (p. 47).
A piece of wood is attached by the string, _a_, to one end of a
balance, and weights just sufficient to balance it are placed in
the opposite scale. If now the wood is brought in contact with the
water in the vessel, _b_, it will require additional weight in the
scale to separate the wood from the water.

[Illustration: Fig. 16. Fig. 17.]

[Illustration: Fig. 18.]

[Illustration: Fig. 19.]

70. =Farther Illustrations.=--When you see stems of plants rising
above the surface of stagnant water you will observe that the water
is considerably raised about them. This is from the attraction
between them and the water. For the same reason water is not as
high in the middle of a tumbler as it is at the sides. If you
immerse a piece of glass in water, the water will rise at its sides
as represented in Fig. 16. If you immerse two together, as in
Fig. 17, the water will rise higher between than outside of them,
because the particles between are attracted by two surfaces, while
those outside are attracted only by one. It is for the same reason
that two men can raise a weight higher than one of them can alone.
And if the pieces of glass be brought quite near together, as in
Fig. 18, the water will be raised higher still, because there is
less to be raised by the two surfaces. It is just as two men can
raise a small weight higher than they can a large one. The same
thing may be beautifully illustrated in this way: Let two pieces
of glass, as represented in Fig. 19, be immersed in  water,
with two of their edges joined together at A B, the opposite edges
at E D C being separated. The height to which the fluid rises will
make a curved line, A F C, it being lowest at the edges which are
separated, and highest at the edges which are joined together.

[Illustration: Fig. 20.]

[Illustration: Fig. 21.]

71. =Rise of Liquids in Tubes.=--For the same reason that water
rises higher between plates of glass than outside, so it will rise
higher in a tube than it will outside of it. The diagram in Fig. 20
will make this clear. I represent in this a transverse section of
a tube, enlarged so that the demonstration may be plain. We will
take a particle on the inside and the outside at equal distances
from the glass. It is clear that the particle _a_ is not as near to
as many particles of the glass as the particle _b_ is. The lines
drawn show this. The longest lines extending from the particles _a_
and _b_ to the glass are equal in length; that is, _a e_ and _a f_
are equal to _b g_ and _b h_. It is clear, therefore, that all the
glass between the lines at _c_ and _d_ is as near to the particle
_b_ as the glass between the lines at e and _f_ is to the particle
_a_. But this is not all. The particle _b_ is near enough to all
the inside of the tube to be attracted by it, while very little
attraction is exerted upon _a_ by any part of the glass beyond
that which is included between _e_ and _f_. The same difference
can be shown with regard to all the particles on the inside of the
tube compared with those outside. The former are nearer to more
particles of the glass than the latter, and therefore are more
strongly attracted. Again, as the nearer the plates of glass are
(§ 70) the higher the water rises between them, so the smaller
the tube is the higher will the water rise in it. You can try
the experiment as represented in Fig. 21. It is obvious that the
particle _b_ (Fig. 20) would not be very strongly attracted by the
part of the tube opposite if the tube were a large one; but it
would be if the tube were very small, for then it would be quite
near to that part.

72. =Capillary Attraction.=--The term capillary (derived from
the Latin word _capilla_, hair) has been commonly applied to the
attraction exhibited under the circumstances just noticed, because
it is most obvious and was first observed in tubes of very fine
bore. The same term is used when the attraction is seen in the
rising or spreading of a liquid in interstices as well as in tubes.
Thus capillary attraction causes the rising of oil or burning fluid
in the wicks of lamps. The liquid goes up in the interstices or
spaces between the fibres, as it does in the spaces of tubes. I
will give some other examples. If you let one end of a towel be
in a bowl of water, the other end lying over upon the table, the
whole towel will become wet from the spreading of the water among
the fibres in obedience to capillary attraction. If you suspend a
piece of sponge so that it merely touch the surface of some water,
or if you lay it in a plate with water in it, the whole sponge will
become wet. So, too, if you dip the end of a lump of sugar in your
tea, and hold it there a little time, the whole will be moistened.
In very damp weather the wood-work in our houses swells from
the spreading of water in the pores of the wood in obedience to
capillary attraction. Especially will this be so in basement rooms,
where the water can go up from the ground in the pores of the
walls, as well as from the damp air. In watering plants in pots,
if the water be poured into the saucers, it will pass up through
the earth by capillary attraction. For the same reason plants and
trees near streams grow luxuriantly, being abundantly supplied
with water, which rises to their roots through the pores of the
soil. The disposition of wood to imbibe moisture in its pores
has sometimes been made use of very effectually in getting out
millstones. First a large block of stone is hewn into a cylindrical
shape. Then grooves are cut into it all around where a separation
is desired, and wooden wedges are driven tightly into them. These
absorb moisture from the dews and rain, and therefore swell so much
as to split the stone in the direction of the grooves. The blotter
which you use furnishes an illustration of capillary attraction,
the ink being taken up among the fibres of the paper. Ordinary
writing paper will not answer as a blotter, because the sizing
fills up the interstices between the fibres.




CHAPTER V.

GRAVITATION.


73. =Attraction Between Masses.=--I have thus far treated of
attraction as existing between the atoms or particles of matter
when they are brought very near together, which is called the
attraction of _cohesion_. But it exists also between any portions
of matter that are separate from each other. Thus if two cork
balls be placed on the surface of water near to each other, their
attraction will soon bring them together. To have the experiment
striking, the balls must be varnished, that they may glide easily
over the water. Bubbles of glass will exhibit the same attraction.
So, also, floating pieces of wood are apt to be found together; and
when a ship is wrecked, as soon as the sea becomes calm the parts
of the wreck are in collections here and there. Now when a stone
falls to the ground, it does so for precisely the same reason that
the two cork balls come together in the water. The idea of all who
have not been informed on such subjects is, that the stone comes
to the ground because the ground is down and the stone is up, and
there is nothing to support the stone in the air. They have no idea
that some power makes the stone come down. There is such a power,
and it is the attraction which the earth and the stone have for
each other. If you hold the stone in your hand, and thus prevent
its falling, you simply resist a power which is pulling it down. If
you could in any way suspend the attraction of the earth and the
stone for each other, you could let go of the stone, and it would
remain just there in the air, and would not come down until the
attraction is restored.

74. =Attraction Mutual.=--The cork balls move toward each other
because their attraction is mutual. So do the earth and stone
really move toward each other for the same reason. As the stone is
drawn toward the earth, so is the earth drawn toward the stone.
But the earth is so large a thing to be drawn that its motion is
exceedingly small--so small that practically it may be considered
as nothing.

75. =Illustration.=--This may be clearly illustrated if we compare
the force of attraction to the force of muscular action. Suppose a
man in a boat pulls on a rope which is made fast to a ship lying
loose at the wharf, and in this way draws his boat toward it. He
does not dream that he moves the ship at all; but he in reality
does, for if instead of one boat a hundred or more pull upon the
ship, they will move it so much as to make the motion apparent. In
the case of the single boat, the ship as really moves as when a
hundred boats are pulling on it, but it is only the one hundredth
part as much. Now let the ship represent the earth, and the little
boat some body, as a stone, attracted by it. The earth and the
stone move toward each other, just as the ship and the boat do. And
if, as we multiplied the number of boats, we should multiply the
bulk of the stone till it is of an immense size, it would have by
its attraction a perceptible influence upon the earth's motion.

Observe in regard to the illustration, that it makes no difference
whether the man be in the boat or in the ship as he pulls. In
either case he exerts an equal force on the ship and the boat,
making them to approach each other. So it is with the attraction
between the earth and the stone. It is a force exerted equally upon
both. Its effect on the earth is not manifest, because it is so
much larger than the stone; just as the effect of the man's pulling
is not manifest upon the ship, because it is so much larger than
the boat.

76. =Proportion of the Mutual Motions of Attraction.=--Let us
pursue the illustration a little farther. If a man stand in a
boat, and pull a rope made fast to another boat of the same size
and weight, both boats, in coming together, will move over the
same space. Just so it is with the attraction between two bodies
having the same quantities of matter or equal weights--they attract
each other equally, and therefore meet each other half way. Let
now one boat be ten times as great and as heavy as the other. The
small boat would move ten times as much as the large one when the
man brings them together by pulling the rope. In like manner, if
a body one-tenth as large as the earth should approach it, they
would attract each other, but in coming together the body would
move ten times as far as the earth would. In the case of falling
bodies, even though they may be of great size, the earth moves so
slightly to meet them that its motion is wholly imperceptible. It
has been calculated that if a ball of earth the tenth part of a
mile in diameter were placed at the distance of a tenth part of a
mile from the earth, as the earth and this body would be moved by
their attraction to meet each other, the motion of the earth would
be only the eighty thousandth of a millionth (1/80,000,000,000) of
an inch.

77. =Attraction Universal.=--The attraction of which I have been
speaking exists between all bodies, however distant they may be
from each other. Sun, earth, moon, and stars attract each other;
and in obedience to this attraction they have a tendency to come
together in one great mass, and would do so if another force acting
in opposition to this did not prevent it. This force will be
treated of in another part of this book.

78. =The Tides.=--One effect of the attraction between the earth
and the moon is quite familiar. I refer to the tides. When the tide
rises it is because the water of the ocean feels the attracting
force of the moon. The moon actually lifts the water toward itself.
The attraction of the sun sometimes increases and sometimes
diminishes the tides, according to its position in relation to the
moon and the earth. If the land were as movable as the water, or,
in other words, if its particles were held together by no stronger
attraction than those of water, there would be the same motion that
there is in the ocean over the surface of the earth, as in its
revolution successive portions of it present themselves toward the
moon.

79. =Meaning of the Word Gravitation.=--The attraction thus
existing between different bodies of matter separated from each
other is called the attraction of gravitation or gravity, in
distinction from the attraction of _cohesion_ treated of in the
previous chapter. This name was given to it because we have such
common examples of its influence in the fall of bodies to the
earth. They are said to _gravitate_ toward the earth. And they are
said to do so by the force or attraction of gravitation or gravity.
The term _terrestrial_ gravitation is sometimes used in speaking
of the earth's attraction, in distinction from the same thing in
operation in other planets.

[Illustration: Fig. 22.]

[Illustration: Fig. 23.]

80. =Attraction Toward the Earth's Centre.=--All bodies are
attracted toward the centre of the earth. This is because the
earth is globular, as may be made clear by Fig. 22. Let the circle
represent the earth, and _a_ a body attracted by it. The lines
drawn from the body to the earth represent the attractive force
exerted by the earth upon the body. It is obvious from these that
there is as much attraction on the one side of the line drawn
from the body to the earth's centre as there is on the other. The
attractive force, then, of the earth as a whole is exerted upon the
body in the direction of this middle line. It tends to draw it,
therefore, toward the centre. If, therefore, a weight be suspended
by a string, the line of the string continued would go to the
centre of the earth. This being so, it is clear that two weights
suspended by two strings do not hang perfectly parallel to each
other. The difference is so slight in an ordinary pair of scales
that it can not in any way be perceived. But if it were possible to
suspend in the heavens a beam so long as to stretch over a large
extent of the earth's circumference, as represented in Fig. 23, the
scales attached to it would be very far from hanging parallel to
each other. Substances suspended in different parts of the globe
are hanging in different directions, and those which are hung up
by our fellow-men on the opposite side of the earth, are hanging
directly toward us.

[Illustration: Fig. 24.]

81. =Up and Down.=--All falling bodies fall toward the centre of
the earth, and the same remarks can be made on this point that I
have made in relation to suspended weights. Up and down are merely
relative terms--_up_ being from the centre of the earth, and _down_
toward it. As the earth moves round on its axis, the same line of
direction which we call upward at one time is downward at another.
This may be illustrated on Fig. 24. Let the circle represent the
circumference of the earth. In the daily revolution we pass over
this whole circle. If we are at D at noon, we are at E at six
o'clock, and at F at midnight. If, therefore, the ball A be dropped
from some height at noon, the line in which it falls will be at
right angles to a line in which it will fall if you drop it from
the same height at six o'clock; for this height will have moved in
this time from A to B. If it be dropped from the same height at
midnight its line of direction will be directly opposite to what
it was twelve hours before; for the height will have moved in that
time to C.

It is not always true that falling bodies tend exactly toward the
centre of the earth. It is nothing in the centre that attracts
them, but it is the substance of the whole earth; and as this is
irregular in its density and form, the attraction will be irregular
also. Thus it is found by accurate experimenting that a plumb line
suspended in the neighborhood of a mountain is so attracted by it
that it will not hang exactly parallel with another suspended at
some distance from the mountain. The difference is not, however,
enough in any case to have any practical bearing.

82. =Weight.=--I have said before (§ 52) that what we call weight
is not a property of matter, but merely the result of a property,
the attraction of gravity. This I will now illustrate. If two
bodies are falling to the earth, and one of them contains ten times
as many particles of matter as the other, ten times as much force
of gravity is required, and is actually exerted, to bring it to the
ground. This will appear plain to you if you bear in mind that a
body does not come to the ground because there is nothing to keep
it there, but because it is drawn down by the force of attraction,
and then compare this force to any other force, as, for example,
that of muscular action. If you draw toward you two weights, one of
which is twenty times as heavy as the other, or, in other words,
has twenty times the quantity of matter that the other has, you
must exert twenty times as much strength on the former as you do on
the latter. Just so it is with the force of attraction. The earth
attracts or draws toward itself a body having twenty times the
quantity of matter that another has with twenty times the amount
of force. And the first body will have twenty times the weight of
the other, for it will make twenty times the pressure upon any
thing that resists the force with which the earth draws it toward
itself. Weight, then, is _the amount of the pressure occasioned by
the attraction existing between the earth and the body weighed_.
If you place a substance in one side of a pair of scales, it goes
down because of the attraction between it and the earth. By placing
weights in the other side until the scales are balanced, you find
how much is needed to counteract the downward pressure caused by
the attraction of the substance and the earth for each other; or,
in other words, you find out how much it weighs. In doing this you
use certain standard weights; that is, certain quantities of matter
which have been agreed upon by mankind, and are called by certain
names, as pounds, ounces, etc. When a spring is used in weighing,
the spring has been tried by these standard weights, and its scale
has been marked accordingly.

83. =Weight not Fixed, but Variable.=--Weight does not depend alone
upon the density of the body weighed, but also upon the density
of the earth. For the attraction causing the pressure which we
call weight is a _mutual_ attraction, and is in proportion to the
quantities of matter in both the body and the earth. If, therefore,
the density of the earth were increased twice, three times, or
four times, the weights of all bodies would be increased in the
same proportion; that is, the force with which the earth would
attract them would be twice, three times, or four times as great
as now. This would not be perceived by any effect on balances,
for the weights and the articles weighed would be alike increased
in weight. But it would be perceived in instruments that indicate
the weights of bodies by their influence on a spring. These would
disagree with scales and steelyards just in proportion to the
increase of the earth's density. It would be perceived also in the
application of muscular and other forces in raising and sustaining
weights. Every stone would require twice, three times, or four
times the muscular effort to raise it that it does now.

84. =Weight Varies with Distance.=--The nearer two bodies are to
each other the greater is their attraction. The nearer a body is
to the earth the greater is the attraction that presses it toward
the earth; in other words, the greater is its weight. The force of
gravity, or weight, is greatest, therefore, just at the surface of
the earth, and it diminishes as we go up from the earth. As we go
from the earth, the force of gravity lessens in such a proportion
that it is always _inversely_ as the square of the distance from
the centre of the earth. I will explain. If the distance from the
centre of the earth to its surface, which is 4000 miles, be called
1, then 4000 miles from the earth would be called 2, or twice as
far from the centre, and 8000 miles from the earth would be 3, and
so on. The squares of these numbers would be 1, 4, 9, 16, etc.
Now as weight lessens so as to be _inversely_ as the square of the
distance, a body weighing a pound on the surface of the earth would
weigh but a quarter of a pound at the distance of 4000 miles, and
but the ninth part of a pound at 8000 miles. A body weighs less on
the summit of a high mountain than it would in the valley below,
because it is farther away from the great bulk of the earth, and
therefore is not so strongly attracted. The difference, however,
is but small. A man weighing two hundred and fifty pounds in the
valley would weigh but half a pound less if on the summit of a
mountain four miles high.

85. =Weight Every Where.=--I have spoken of weight only in relation
to the earth. But there is weight in bodies every where, for there
is attraction wherever there is matter. The weight of substances on
the surface of different heavenly bodies varies according to the
quantities of matter in those bodies. As the moon is much smaller
than the earth, what weighs a pound with us would weigh much less
than a pound in the moon. And as the sun is much larger than the
earth, what is a pound with us would be much more than a pound
there. If we knew the exact densities of the sun and the moon and
the earth, as well as their size, we could estimate exactly the
difference in the weights which any body would have in them; for
the attraction which causes the pressure that we call weight is as
the quantity of matter, and the quantity of matter depends on both
density and size.

86. =Cohesion, Capillary Attraction, and Gravitation the
Same.=--The attraction of cohesion, capillary attraction, and
gravitation are only different modes of action of the same power;
viz., the attraction which matter every where has for matter. At
first thought it would appear that there is something peculiar in
the attraction of particles when they are brought together so as
to adhere. For if we take any substance, a piece of glass, for
example, its particles seem to be held together by an attraction
vastly stronger than that attraction which inclines different
bodies to move toward each other. If you break the glass, however
closely you may press the two pieces together, they will not
unite again. It would seem, at first view, that there must be
some peculiar arrangement of the particles which is destroyed by
breaking the glass. But we can readily account for the facts in
another way. The attraction between bodies of matter is the greater
the nearer we bring them together. The nearer, for example, is the
moon to any portion of the earth, the greater is the attraction
which it exerts, as seen in the tides; and if it were much nearer
to the earth than it is, our tides would prove awfully destructive.
What is true of masses is also true of the particles of which
they are composed. Though their attraction is comparatively
feeble when at a distance from each other, it increases, not in
the arithmetical but the geometrical ratio (§ 84), as they come
nearer together; so that when they are exceedingly near together
the attraction is very powerful. It must be remembered in regard
to the pieces of broken glass that you can not bring the particles
on their surfaces as near as they were before the glass was
broken, for the crack does not disappear. And as the attraction is
inversely as the square of the distance, a little distance must
make a great difference. The particles of some substances you can
bring so near together as to cause adhesion, as you saw in the case
of the two bullets (§ 66). That their adhering together depends
merely upon their particles being brought near to each other
appears from the fact, that the smoother you make the surfaces the
more strongly will they adhere. And the reason that liquids and
semi-liquids adhere so readily to solid substances is, that their
particles, moving freely among each other, have thus the power of
arranging themselves very near to the particles of the solid. Thus,
when a drop of water hangs to glass, all the particles of water
in that part of the drop next to the glass touch, or rather are
exceedingly near to, the particles of the glass.

87. =Variety in the Results of Attraction.=--It is one and the
same force, then, which binds the particles of a pebble together,
and makes it fall to the ground--which "moulds the tear" and "bids
it trickle from its source"--which gives the earth and all the
heavenly bodies their globular shape, and, in connection with
another power hereafter to be noticed, makes them revolve in their
orbits. How sublime the thought that this one simple principle that
gives form to a drop extends its influence through the immensity of
space, and so marshals "the host of heaven" that, without the least
interruption or discord, they all hold on their course from year
to year and from age to age! It is thus that Omnipotence makes the
simplest means to produce the grandest and most multiform results.

88. =Opposition Between the Modes of Attraction.=--Although
cohesion and gravitation are essentially the same thing, we see
them continually acting in opposition to each other. Abundant
illustrations might be given, but, I will cite only a few.

[Illustration: Fig. 25. Fig. 26.]

89. =Why Pitchers have Lips.=--If you pour water out of a tumbler
there is a struggle between the attraction of cohesion and
gravitation for the mastery--the attraction of cohesion tending
to make the water adhere to the tumbler, and run down its side,
as in Fig. 25, and gravitation tending to make it fall straight
down. But when water is poured out of a pitcher, as in Fig. 26,
the lip of the pitcher acts in favor of the attraction of gravity;
for the water would have to turn a very sharp corner to run
down the outside of the pitcher in obedience to the attraction
of cohesion. In pouring water from a tumbler, we can often, by a
quick movement, throw the water, as we may say, into the hands
of gravity before the attraction of cohesion can get a chance to
turn it down the tumbler's side. If you can only make the water
_begin_ to run from the tumbler without going down its side there
will be no difficulty; for there is an attraction of cohesion
between the particles of the water, tending to make them keep
together, which in this case acts against the cohesion between the
water and the glass, and therefore acts in favor of gravitation.
It is cohesion that forms the drop on the lip of a vial as we
drop medicine--cohesion between the particles of the liquid,
and cohesion between these particles and those of the glass. It
is gravitation, on the other hand, that makes the drop fall, it
becoming so large that the force of gravity overcomes the cohesion
between the drop and the vial.

90. =Size Limited by Gravity.=--Were it not for the attraction
of gravity there would be no limit to the size of drops of any
liquid. When the drop reaches a certain size, it falls because it
is so heavy; or, in other words, because with its slight cohesion
the attraction of the earth brings it down. Now if this attraction
could be suspended, and the attraction of cohesion left to act
alone, particles of water might be added to the drop to any extent,
and they would cling there. You can see the struggling between
cohesion and gravitation very prettily illustrated if you watch the
drops of rain on a window-pane. If two drops happen to be quite
near together they unite by attraction, and then, being too large
to allow of its being retained there by cohesion in opposition to
gravitation, the united drop runs down. If it meet with no other
drop it soon stops, because by cohesion some portion of it clings
to the glass all along its track, and so at length lessens it
sufficiently to allow it to remain suspended again. It is from the
influence of the attraction of gravitation that different kinds of
liquids furnish drops of different sizes, the heavier giving small,
and the lighter large ones. Thus you can drop from a vial a larger
drop of alcohol than of water, and a larger one of water than of
nitric acid. You have another illustration of a similar character
in the adhesion of chalk to a black-board or any surface. The chalk
crayon itself can not adhere, for the attraction of the earth does
not permit it. But small quantities of it can adhere for the same
reason that water adheres to surfaces in small quantities. So also
dust clings to sides of furniture, though a lump of dirt would not.

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

91. =Illustrated in Solid Bodies.=--We can illustrate the
limitation of size in solid masses by Figs. 27 and 28. Suppose
that _a_ and _b_, Fig. 27, are two projections of timber from a
post, _b_ being twice as large as _a_. It is evident that _b_
can not support twice as much weight as _a_, for gravitation is
dragging it downward from its attachment to the upright post with
twice the force that it does _a_. The case is still stronger when,
as represented in Fig. 28 (p. 64), the larger timber is twice as
long as the smaller. Here _d_ has four times the bulk of _c_. But
it can not support four times as much weight at its end, not only
because its own weight presses it downward, but because half of
its weight is at a greater distance from the place of attachment
than the smaller beam is. Gravitation here operates in opposition
to cohesion in such a way that the projecting timber, if carried
to a certain size, will fall by its own weight, either breaking in
two, or tearing away from its attachment. This tendency is very
commonly resisted in buildings and other structures by braces, as
represented in Fig. 29. Here the weight of the horizontal timber
at some distance on each side of _a_, is made to press upon the
upright post instead of directly downward.

[Illustration: Fig. 29.]

92. =Farther Illustrations.=--The size of bodies, both animate and
inanimate, is limited by the Creator in obedience to the principles
above developed. This is seen in the fact that there are no animals
on the land to compare in size with the monsters of the deep. A
whale does very well in the water, because he is buoyed up by that
element; but an animal as large as a whale could not well exist on
land, because gravitation would act so strongly in opposition to
cohesion. At least it would be necessary, in order that he might
walk about, or even hold together, that his great bulk should be
made up of very firm and tenacious materials. Whenever any thing
very large or tall is to be supported, its support is always broad
and composed of very cohesive materials. We see this exemplified
in the massive trunks of full-grown trees, as compared with the
slender trunks of trees of the same kinds in the nursery. We see
it exemplified in the fact that the highest mountains are built of
the hardest rocks, while the soft chalk formations are confined
to those of small size. There is a limit to the height even of
the granite mountains in the influence of gravity. If carried
much higher than they are, the attraction of the earth, in its
opposition to cohesion, would tear them apart in their fissures, or
cause the immense weight to crush their foundations. In the moon,
where gravitation is less than on the earth (§ 85), mountains can
be much higher without these results, and accordingly the telescope
shows them to be so. In Jupiter, on the other hand, which is much
larger than the earth, the mountains, if there be any, can not rise
to any great height, and if there be any living beings as large as
we are in that planet they must be made of vastly firmer materials
to prevent their being crushed by their own weight.

93. =The Above Principles Transgressed by Man.=--Man often
transgresses these principles in his structures. For example,
a building settles because the foundation is not strong enough
to bear the superincumbent weight; in other words, the force of
gravitation is not sufficiently taken into the account. When a very
tall building is erected, the lower portions ought to be made of
very cohesive substances. Firm granite is therefore an appropriate
material for the lower story of tall brick buildings. At least
should the walls of the lower stories of such buildings be made
thicker than they ordinarily are, to resist properly the force
of gravitation in the weight above. Stores intended to bear much
weight on their floors are often built without due regard to the
cohesive force required to sustain the weight. Long timbers are
sometimes supported only at the ends, when their own weight, to say
nothing of what may be brought to press upon them, requires that
they should be supported at other points. While in modern buildings
the timbers are often too small, in some old buildings the upper
timbers are so heavy as to lessen rather than increase the strength
of the structure. Especially is this true of the unsightly beams
which in some ancient houses we see extending along the ceilings
above. Many other examples could be given, but these will suffice.

The practiced eye, in looking at a building, instinctively requires
that every part should be _seen_ to be suitably supported. A
gallery in a church, therefore, if without pillars or braces from
the wall, is displeasing to such an eye, even though there may be
really sufficient support provided in the mode of structure. The
same can be said of galleries supported by slender iron pillars,
especially if they be painted of some light color so as to look as
if they were wood rather than iron. For the same reason porticoes
without pillars are unsightly. So, too, the eye instinctively looks
for a sufficient base to every pillar and pilaster. The concealment
of the base in any way, or the substitution of any thing else for
it, is an unpleasant anomaly, and yet such anomalies are sometimes
seen even in expensive buildings.

94. =Attraction of Natural Philosophy and of Chemistry.=--The
attraction of which I have treated in this and the previous
chapters is that which belongs to Natural Philosophy, in
distinction from that of Chemistry. Its effects are only
mechanical, while the attraction of Chemistry goes beyond this,
and affects the _composition_ of substances. For example, the
attraction between the two gases, oxygen and hydrogen, which makes
them unite to form water, belongs to Chemistry; while that which
makes the particles of water cohere is in the province of Natural
Philosophy.




CHAPTER VI.

CENTRE OF GRAVITY.


[Illustration: Fig. 30.]

[Illustration: Fig. 31. Fig. 32.]

95. =Centre of Gravity Illustrated.=--If you balance a ruler on
your finger, as in Fig. 30, it is balanced because there is just
as much weight on one side as on the other. Now just over your
finger, in the middle of the ruler, there is a point that we call
the centre of gravity; or, in other words, the centre of the weight
of the ruler. This point is indicated in the figure. There is as
much of the weight of the ruler on the one side of this point as on
the other, and also as much above it as below it. If your finger
should be a little to the one side or the other of this point, the
ruler would not be balanced, and would fall. When balanced it does
not fall, simply because this central point is supported by being
directly over the end of the finger. The weight of the ruler, then,
may be considered practically as all being in that point, for it is
there that is exerted all the downward pressure of the ruler as it
is balanced. So, also, when the ruler is balanced on the finger,
as represented in Fig. 31 (p. 68), this same centre of gravity is
directly over the point of the finger, and is therefore supported.
If it be to the one side or the other, as in Fig. 32, it is not
supported, and the ruler therefore falls. You see, then, that if
a body be balanced, the centre of gravity is directly _over_ the
point of support. If, on the other hand, a body is suspended, the
centre of gravity is directly _under_ the point of support.

[Illustration: Fig. 33.]

96. =Definition.=--If a plumb-line from the centre of gravity could
be extended into the earth it would go directly to its centre. The
body may be considered as making all its pressure from its centre
of gravity in that direction, in obedience to the attraction of
gravitation. The best definition, then, that we can give of the
centre of gravity is, that it is _that point in a body from which
its pressure as a whole toward the centre of the earth proceeds_.
It is that point, therefore, the support of which insures the
support of the whole body. And in speaking of the weight of a body,
or its downward pressure, we may consider all the matter composing
it as collected or concentrated in that point. The body, therefore,
can be balanced in any position in which this point is supported,
as shown in Figs. 30 and 31. And when a body is suspended, it is at
rest only when the centre of gravity is directly under the point of
support. Thus, if you have a circular plate suspended at E, Fig.
33, it will not be at rest when it is moved to the one side or
the other, as represented by the dotted lines, but only when the
centre of gravity, _c_, is directly under the point E.

[Illustration: Fig. 34. Fig. 35.]

97. =How to Find the Centre of Gravity of a Body.=--If you take
a piece of board, and suspend it at A, Fig. 34, and suspend a
plumb-line from the same point, the centre must be somewhere in
that line. But exactly at what point it is you do not know. How
will you ascertain this? Mark the line A B on the board, and
suspend the board by another point, as in Fig. 35. As the centre
of gravity must be somewhere in the plumb-line as it now hangs, of
course it is at O, where the two lines cross.

[Illustration: Fig. 36. Fig. 37.]

98. =Scales and Steelyards.=--When two bodies are connected by a
rod or bar, the centre of gravity of the whole is somewhere in
the connection. If the two bodies be equal in weight, as in Fig.
36, the centre of gravity is exactly in the middle of the rod, as
marked. But if the bodies are unequal, as in Fig. 37, the centre
of gravity is nearer to the larger body than to the smaller. In
balancing a body in one scale with weights in another, you have a
case parallel to that of Fig. 36. The centre of gravity of the
body weighed, the weights, and the scales, as a whole, is midway
between the scales, at the point of support. In the steelyard you
have the heavy body to be weighed nearer the centre of gravity than
the small weight is on the long arm, and so the case is parallel
with that of Fig. 37.

[Illustration: Fig. 38.]

99. =The Centre of Gravity of a Body not Always in the Body
Itself.=--The centre of gravity of a hollow ball of uniform
thickness is not in the substance of the ball, but it is in the
centre of the space in the ball, for the line of the ball's
downward pressure would be from that point. If the ball had a
frame-work in it, as represented in Fig. 38, the centre of gravity
would obviously be at A, the centre of this frame-work. But if
there were no frame-work, and perpendicular lines were supposed to
be drawn from different points of suspension, C, B, D, and E, these
would intersect at the point A, showing that this is the centre
of gravity, according to the rule for finding it given in § 97.
So, also, the centre of gravity of an empty box, or an empty ship,
would be an imaginary point in the space inside. In a hoop it is
the centre of the hoop's circle.

[Illustration: Fig. 39.]

100. =The Centre of Gravity Seeks the Lowest Point.=--The centre of
gravity always takes the lowest place which the support of the body
will allow. In a suspended body, therefore, it is always directly
under the point of suspension. To get to the one side or the other
of this position it must rise. This the attraction of gravity
forbids, and if by any force it is made to rise, this attraction at
once brings it back. This is manifest in the case of a suspended
ball, Fig. 39. If the ball be moved to _b_, it will, on being let
go, return to its first position, simply because its centre of
gravity, in obedience to the earth's attraction, seeks the lowest
place possible. From inertia (§ 49) it moves beyond this point,
and continues to vibrate back and forth for some time; but when
its motion is stopped, it hangs perpendicularly; that is, in such
a way that its centre of gravity shall have the lowest possible
position. I add a few other illustrations of the same point. When
a rocking-horse is at rest, its centre of gravity is directly
over the point at which it touches the floor, for thus it has its
lowest possible place. If it be rocked, the centre of gravity is
moved to a higher point, and for this reason it rocks back again.
The same is seen in the swing, the cradle, the rocking-chair, etc.
Most interesting illustrations of the same thing are found in the
Laggan, or Loggan Stones as they are called, several of which
are seen on the rugged parts of the British coast. An immense
rock, which has been loosened by some convulsion, rests with a
slightly-rounded base on another rock which is flat, and it is so
nicely balanced that one person alone can produce a perceptible
rocking motion in it. I saw, many years ago, a large rock near
Salem, Massachusetts, thus situated. There is one also in Great
Barrington, Massachusetts.

[Illustration: Fig. 40. Fig. 41.]

[Illustration: Fig. 42.]

[Illustration: Fig. 43.]

101. =Farther Illustrations.=--It is because the centre of gravity
always seeks the lowest place that an egg lies upon its side. When
on its side, the centre of gravity is at its lowest point, as is
manifest from comparing Fig. 40 with Fig. 41 (p. 71). Children
often have a toy, called a witch, which illustrates the same thing
in another way. It is a piece of light substance, as pith, with a
shot fastened in one end. It always stands up on its loaded end,
and can not be made to lie down on its side, because the centre of
gravity would not then be at the lowest point. There is an amusing
Chinese toy of the same kind. It is a figure of a fat old woman,
Fig. 42, loaded with lead at the bottom, so that its centre of
gravity is at _a_. If the figure be thrust over to one side, as
shown by the dotted lines, the centre of gravity is raised, and the
upright position is at once resumed. If the toy were not loaded, it
would lie in the position represented in Fig. 43, just as the egg
lies on its side.

[Illustration: Fig. 44.]

[Illustration: Fig. 45. Fig. 46.]

[Illustration: Fig. 47.]

102. =Curious Experiments.=--You can not hang a pail of water on a
stick laid upon a table, as represented in Fig. 44, for the centre
of gravity is not supported. But if you place another stick a as a
brace, in the manner represented in Fig. 45 (p. 73), so as to push
the pail under the table, it will hang securely, because the centre
of gravity is now under the point of suspension. The explanation of
the following experiment is the same: Run a large needle through
a cork; fasten to the cork a fork, and you can suspend the whole
on the edge of a table, as seen in Fig. 46. Here the centre of
gravity is directly under the point of suspension, which is at the
point of the needle. The same can be said of the very common toy
represented in Fig. 47. The horse, made of very light material,
stands securely, because the centre of gravity of the whole is in
the heavy ball, which is under the point of suspension. If the
horse be made to rock back and forth, the centre of gravity in the
ball moves in a curved line, as in the case of a ball suspended by
a string (Fig. 39). It is at its lowest place only when the horse
is at rest. The hanging of a cane with a hook-shaped handle on the
edge of a table is to be explained in the same way.

[Illustration: Fig. 48. Fig. 49. Fig. 50.]

[Illustration: Fig. 51.]

[Illustration: Fig. 52.]

103. =Stability of Bodies.=--The firmness with which a body
stands depends upon two circumstances--the height of its centre
of gravity, and the extent of its base. The lower the centre of
gravity, and the broader the base, the firmer does the body stand.
A cube, represented in Fig. 48, is more stable, that is, less
easily turned over, than a body shaped as Fig. 49, because it has
a larger base. The contrast is still greater between Figs. 48 and
50. The reason of the stability of a body with a broad base is
found in the fact, that in turning it over the centre of gravity
must be raised more than in turning over one of a narrower base.
The curved lines indicate the paths of the centres of gravity as
the bodies are turned over. In the case of a perfectly round ball,
the base is a mere point, and therefore the least touch turns it
over. Its centre of gravity does not rise at all, but moves in a
horizontal line, as shown in Fig. 51. The pyramid is the firmest
structure in the world, because it possesses in the highest degree
the two elements--a broad base, and a low position of the centre of
gravity. On both these accounts the centre of gravity must ascend
considerably when the body is turned over, as seen in Fig. 52.

[Illustration: Fig. 53.]

[Illustration: Fig. 54.]

104. =Bodies not Upright Unstable.=--When a body does not stand
upright, its stability is diminished simply because only a portion
of the base is concerned in its support. In Fig. 53 the base is
broad, but the body is so far from being upright that the centre
of gravity bears upon the very extremity of the base on one side,
as the perpendicular line from it indicates. The least jostle will
turn it over, because the centre of gravity need not ascend the
least when this is done. You see, then, that the less upright a
body is, the less of the base is of service in its support, because
the farther is the line of direction of the downward pressure of
the centre of gravity from the centre of the base. The famous tower
of Pisa, Fig. 54, one hundred and thirty feet high, overhangs its
base fifteen feet. It was undoubtedly built intentionally in this
way to excite wonder and surprise, for what would otherwise have
been a very unsafe structure is rendered stable and safe by the
arrangement of its materials. Its lower portion is built of very
dense rock, the middle of brick, and the upper of a very light
porous stone. In this way the centre of gravity of the whole
structure is made to have a very low position.

[Illustration: Fig. 55.]

105. =Familiar Illustrations.=--You see now the explanation of
the fact which common experience teaches every one, that the
taller is a body, and the narrower its base, the more easily is
it overturned. This is exemplified in the two loads, Fig. 55. The
base is the space included by the wheels. The centre of gravity
is so high in the tall load that a perpendicular line drawn from
it falls outside of the base if the cart come upon a considerable
lateral inclination of the road. But the smaller load, under the
same circumstances, is perfectly secure from overturning. A high
carriage is more easily overturned than a low one, for the same
reason. A stage, if loaded on its top, is very unsafe on a rough
road. Stability is given to articles of furniture by making their
bases broad and heavy, as you see in tables supported by a central
pillar, candlesticks, lamps, etc. The tall chairs in which children
sit at table would be very insecure if their legs were not widely
separated at the bottom, thus widening the base of support. In
the ladder, so commonly used now in picking fruit, a broad base
is furnished between the foot of the ladder and the two standards
which are spread out to sustain its top.

[Illustration: Fig. 56. Fig. 57.]

106. =Support of the Centre of Gravity in Animals.=--The base of
support which quadrupeds have, viz., the space included between
their four feet, is quite large; and this is one reason that they
walk so soon after birth. A child does well that can walk at the
end of ten or twelve months, for the supporting base is quite small
compared with that of a quadruped. It consists of the feet and the
space between them. It requires skill, therefore, in the child to
manage the centre of gravity in standing and walking, and this is
gradually acquired. If one should grow up without ever standing on
his feet, he would find, as the infant does, that some training
is necessary to enable him to do it. It is on account of the
smallness of the base furnished by the feet that the statue of a
man is always made with a large base or pedestal. Although we exert
considerable skill in walking, by no means so much is requisite
as the Chinese ladies must put in exercise with their small feet.
Still more skill is exercised by one who has two wooden legs, or
one who walks on stilts. The base made by the feet can be varied
much by their position. If the toes be turned out and the heels
brought near to each other, the base will not be as large as when
the feet are straight forward and far apart, as is manifest in
Figs. 56 and 57. It is for this reason that the child, in his first
essays at standing and walking, instinctively manages his feet as
in Fig. 56.

[Illustration: Fig. 58.]

[Illustration: Fig. 59.]

[Illustration: Fig. 60. Fig. 61.]

107. =Motions of the Centre of Gravity in Walking.=--In walking,
the centre of gravity is alternately brought over one foot and the
other, and so moves in a waving line. This is very manifest as you
see people before you going down the aisle out of a church. When
two are walking together, if they keep step the two waving lines
of their centres of gravity run parallel, as in Fig. 58, and they
walk easily; but if they do not keep step these lines run as in
Fig. 59, and the movement is both awkward and embarrassing. The
line of movement of the centre of gravity is always slightly waving
_upward_ also, as seen in Fig. 60 (p. 78). In the case of a man
with wooden legs the line would not be gently waving, but somewhat
angular, as represented in Fig. 61.

[Illustration: Fig. 62. Fig. 63.]

[Illustration: Fig. 64.]

[Illustration: Fig. 65.]

108. =The Centre of Gravity and Attitudes.=--The object of various
attitudes assumed under different circumstances is to keep the
centre of gravity over the base of support. A man with a load on
his back would not assume the position of Fig. 63, but that of Fig.
62, so that the centre of gravity of his load may be directly over
his feet. So a man carrying any thing in front leans backward, as
in Fig. 64. In ascending a hill a man appears to lean forward, and
in descending to lean backward; but in fact he is in both cases
upright in reference to the plain on which the hill stands, as may
be seen in Fig. 65. A perpendicular line drawn from his centre
of gravity strikes the ground midway between the feet, that is,
in the middle of the base, and if prolonged would go straight to
the centre of the earth. When one rises from a chair he draws his
feet backward, and then bends his body forward to bring the centre
of gravity over the feet. Unless this is done, it is impossible
to rise, at least deliberately, as you will find by trying the
experiment. A man standing with his heels close to a wall can not
stoop forward and pick up any thing, for the wall prevents him
from moving any part of his body backward, and therefore when he
stoops forward, the centre of gravity being brought far in advance
of the base, he loses his balance and falls. A man who did not
understand this undertook to stoop in this way to pick up a purse
containing twenty guineas, which he was to have if he succeeded,
the forfeiture in case of failure being ten guineas. Of course his
centre of gravity made him lose his wager.

109. =Rope-Dancers, Tops, etc.=--Great skill is exhibited by the
rope-dancer in supporting the centre of gravity. Similar skill is
seen in feats of balancing, as, for example, in balancing a long
stick upright on the finger. In these cases the centre of gravity
is very little of the time directly over the point of support. It
is kept in constant motion nearly but not quite over this point,
this unstable equilibrium, as it is called, being vastly less
difficult to maintain than stable equilibrium; that is, keeping
the balance in one unvarying position. It is the motion of the top
that makes it to stand upright upon its point--a very beautiful
example of unstable equilibrium. The centre of gravity revolves
around a perpendicular line, at exceedingly little distance from it
at first, but greater and greater as its motion becomes less rapid,
till at length the centre of gravity gets so far from this line
that the top falls. For a similar reason an intoxicated man may not
be able to keep himself up if he undertake to stand still, and yet
may do so if he keep moving. As in the case of the top, his centre
of gravity must be kept in motion, or he must fall.




CHAPTER VII.

HYDROSTATICS.


110. =What Hydrostatics Teaches.=--Hydrostatics is that branch of
Natural Philosophy which treats of the pressure and equilibrium
of liquids. The phenomena which it brings to view all results
_from the influence of the attraction of the earth upon liquids_.
It is for this reason that this subject calls naturally for our
consideration after examining the general subject of attraction,
as we have done in the previous chapters. In order to understand
fully the phenomena of Hydrostatics, you must continually bear in
mind the two grand characteristics of liquids. One is, that the
particles move freely among each other (§ 9). The other is, that a
liquid is almost entirely incompressible (§ 36).

111. =Level Surface of Liquids.=--It is the influence of
gravitation upon liquids that gives them a level surface whenever
they are not agitated by any cause. Observe how this is. A still
body of water you may consider as being made up of layers of
particles. Each layer will have all its particles equally attracted
by the earth, and must therefore be level. If any of the particles
were less attracted than their neighbors they would rise, as is
the case when heat is applied, as you will see hereafter. Whenever
the upper layers of the particles are disturbed by wind or any
other cause, as soon as the disturbance ceases the particles will
again take their places in level layers under the influence of
gravitation.

112. =A Comparison.=--The particles of water may be compared to
shot. If you have shot in a vessel, and they are heaped up in
any portion of the surface, on shaking the vessel those that are
highest will roll down, and the result will be a level surface.
They would do this without agitation if they were as smooth as
the particles of water are. If we could have a microscope strong
enough to distinguish the shape of the particles of water, the
surface would probably appear like the level surface of shot in
a vessel. But the particles of water are so exceedingly minute
that the surface of water, when entirely free from agitation, is
so smooth as to constitute a perfect mirror, often feasting our
eyes with another world of beauty as we look down into its quiet
depths. Water was man's first mirror, and one of the most beautiful
passages in the "Paradise Lost" is the description of Eve's first
waking after her creation at the side of a lake, and seeing her
form reflected in its smooth waters.

[Illustration: Fig. 66.]

113. =Surface of Liquids not Truly Level.=--Strictly speaking,
the surface of a liquid is not level, but rounding. But it is so
little so that it can not be perceived unless we take into view a
very large surface, as the ocean. Here it is very manifest, for
whenever a ship comes into port the first thing seen from the shore
is the topmost sail, the rest of the ship being concealed by the
water rounded up between it and the observer. This is illustrated
in Fig. 66. At _a_ the ship is just in sight, while at _b_ it is
so near shore that the eye sees the whole of it. If the earth had
no elevations of land, or if there was water enough to cover them,
the water would make a perfectly globular covering for the earth,
being held to it by the force of attraction. The reason for this
is precisely the same as was given in § 58 for the disposition of
a drop of liquid to take the globular form. As in that case, so in
this, it can be demonstrated that each particle is attracted toward
a common centre, and that this will produce in the freely-moving
particles a uniformly rounded surface. What could thus be shown to
be true if the earth were wholly covered with water, is true of the
portions of water which now fill up the depressions in the earth's
crust; and it can be perceived, as shown in the first part of this
paragraph, in the case of any extended portion of it.

[Illustration: Fig 67.]

114. =Spirit-Level.=--What we call a perfectly level surface is,
then, one all parts of which are equally distant from the centre
of the earth, and is therefore really a spherical surface. But
the sphere is so large that any very small portion of it may be
considered for all practical purposes a perfect plane. A hoop
surrounding the earth would bend eight inches in every mile. In
cutting a canal, therefore, there is a variation in this proportion
from a straight level line. As the variation is but an inch in
an eighth of a mile, it is of no account in taking the level for
buildings. Levels are ascertained by what is called a spirit-level.
This consists of a closed glass tube, Fig 67, nearly filled with
alcohol. The space not occupied by alcohol is occupied by air. The
tube is placed in a wooden box for convenience and security, there
being an opening in the box at _a_. Now when the box with its glass
tube is perfectly level, the bubble of air will be seen in the
middle at _a_; but if one end be higher than the other, the bubble
will be at or toward that end.

115. =Rivers.=--If a trough be exactly level, the water will be of
the same depth at one end as at the other, for the surface of the
water at both ends will be at the same distance from the centre of
the earth. But raise up one end, and it is now deepest at the other
end. If it were not so, the surface at the two ends would not be at
the same distance from the centre of the earth. Now if, with the
trough thus placed, water run in at the upper end and out at the
lower, you have exemplified what is taking place in all rivers--the
water is in constant motion from the influence of gravitation,
causing it to seek to be on a level. A very slight <DW72> will give
the running motion to water, for the particles are so movable among
each other that in obedience to gravity they flow down the inclined
plane to seek a level. Three inches declivity in every mile in a
smooth straight channel will make a river run at the rate of about
three miles an hour. The Ganges, which receives its waters from
the Himalaya Mountains, in running 1800 miles falls 800 feet. The
Magdalena, in South America, falls only 500 feet in running 1000
miles between two ridges of the Andes.

116. =How some Rivers have been Made.=--Changes are constantly
produced in the earth by the disposition of water to seek a level.
In doing this the water carries solid substances of various kinds
from elevated places into depressed ones, tending to fill up the
latter. New channels are also sometimes made by the water. The
boy who makes a little pond with his mud-dam, and lets the water
overflow from it into another pond on a lower level, as he sees a
channel worked by the water between the two ponds becoming larger
and larger, witnesses a fair representation on a small scale of
some extensive changes which have in ages past taken place in some
parts of the earth. It is supposed, and with good reason, that many
rivers had their origin in the way above indicated. For example,
where the Danube runs its long course there was once a chain of
lakes. These becoming connected together by their overflow, the
channels cut between them by the water continually became larger,
until at length there was one long, deep, and broad channel, the
river, while the lakes became dry, and constituted the fertile
valley through which that noble river runs to empty into the Black
Sea. It is said that a similar process is manifestly going on in
the Lake of Geneva, the outlet of it becoming continually broader,
while the washing from the neighboring hills and mountains is
filling up the lake. Towns that a century ago lay directly upon the
borders of the lake have gardens and fields now between them and
the shore; and Dr. Arnot says, "If the town of Geneva last long
enough, its inhabitants will have to speak of the river in the
neighboring valley, instead of the picturesque lake which now fills
it."

[Illustration: Fig. 68.]

[Illustration: Fig. 69.]

117. =Canals.=--The management of the locks of a canal is in
conformity with the disposition of water to seek a level. A ground
view of a lock and a part of two adjacent locks is given in Fig.
68. The lock, C, has two pair of flood-gates, D D and E E. The
water in A is higher than in C, but the level is the same in C and
B, because the gates, E E, are open. Suppose now that there is a
boat in the lock B that you wish to get into the lock A. It must
be floated into the lock C, and the gates E E must be closed. The
water may now be made to flow from the higher level, A, into C,
till the level is the same in both A and C. But this can not be
done by opening the gates D D, for the pressure of such a height of
water in the lock A would make it difficult, perhaps impossible, to
do this; and besides, if it could be done, the rapid rush of water
into C would flood the boat lying there. The discharge is therefore
effected by openings in the lower part of the gates D D. These
openings are covered by sliding shutters, which are raised by racks
and pinions, as represented in Fig. 69. When the water has become
of the same level in A and C, the gates DD can be easily opened,
and the boat may be floated from C into A. If a boat is to pass
downward in the locks, the process described must be reversed.

Canals are also extensively used for supplying water by side
openings to turn water-wheels for the working of machinery. The
water turns the wheel by the force which gravitation gives it as it
descends from the level of the canal to the level of the river.

[Illustration: Fig. 70.]

[Illustration: Fig. 71.]

118. =Other Illustrations.=--We see the tendency of fluids to be
on the same level in other ways. In a coffee-pot the liquid has
the same level in the spout as in the vessel itself, whatever may
be its position, as seen in Fig. 70 (p. 86). If it be turned up so
far that the level of the fluid in the vessel is higher than the
outlet of the spout, the fluid runs out. If two reservoirs of water
be connected together the water will stand at the same height in
both, whatever the distance between them may be. So, also, in the
aqueduct pipes that extend from a reservoir, the water will rise
as high as the surface of the water in the reservoir itself. If
the outlets of the pipes be lower than this level the water will
run from them, as in the case of the coffee. The cause of these and
similar facts is the same as that of the level surface in vessels
and reservoirs--the action of gravitation. This may be made plain
by Fig. 71. Let the figure represent a vessel with divisions of
different degrees of thickness, these divisions, however, not
extending to the bottom of the vessel. Water in this would stand
at the same level in the different apartments, just as it would if
the vessel had no such divisions, as represented. This is simply
because the attraction of the earth acts upon the water in the
same way with the divisions as without them. And you can see that
it will make no difference whether these divisions be thick or
thin, or whether the apartments be near, as you see here, or far
apart, as they are when branch pipes extend from a reservoir. A
branch pipe may be considered as having the same relation to the
reservoir, as one of the narrow apartments in the figure has to the
rest of the vessel. The result is not at all affected by either
the size or form of the tubes that may be connected with a common
reservoir--a fluid will stand at the same height in all. Thus we
have, in Fig. 72, tubes of various size and shape, _a b c d e_,
connected with a reservoir, _r_, and if water be poured into one
of them it will rise to the same height in all, just as in the
different apartments of the vessel represented in Fig. 71. A man
once thought that he had gained the great desideratum, perpetual
motion, by a vessel constructed as in Fig. 73. He reasoned in this
way: If the vessel contain a pound of water, and the tube only an
ounce, as an ounce can not balance a pound, the water in the vessel
must be constantly forcing that in the tube upward. It therefore
must constantly run out of the outlet of the tube, and as it flows
into the vessel the circulation must go on, and the only hindrance
to its being a perpetual circulation would be the evaporation of
the water. He was confounded when he found, on pouring water into
the vessel, that it stood at precisely the same level in the vessel
and the tube.

[Illustration: Fig. 72.]

[Illustration: Fig. 73.]

119. =Aqueducts.=--The ancients built aqueducts of stone at immense
expense, in some cases spanning valleys at great heights, to
supply their cities with water. At the present day the same object
is effected at comparatively small expense with iron pipes laid
under ground. No matter how much lower than the reservoir a valley
crossed by the pipes may be, the water flowing through them will
rise any where in their branches to the same height as it stands in
the reservoir. It is supposed by some that the ancients were not
aware of this fact; but by others that they were aware of it, and
built their immense aqueducts because they had no material for
constructing large pipes.

[Illustration: Fig. 74.]

120. =Springs and Artesian Wells.=--The principles which I have
developed in the previous paragraphs will explain the phenomena of
springs, common wells, and Artesian wells. The crust of the earth
is largely made up of layers of different materials, as clay, sand,
gravel, chalk, etc. When these were formed they were undoubtedly
horizontal, but they have been thrown up by convulsions of nature
in such a way that they present every variety of arrangement. As
some of these layers are much more pervious to water than others,
the rain which falls and sinks into the ground often makes its way
through one layer lying between two others which are impervious
to water, and so may make its appearance at a great distance from
the place of its entrance, and at a very different height. How
this explains the phenomena of springs, common wells, and Artesian
wells may be made clear by Fig. 74. A A and B B B are designed to
represent porous layers of earth lying between other layers which
are impervious to water. The water in A A will flow out at C,
making what is commonly called a spring. If we dig a well at F,
going down to the porous layer, B B B, the water will rise to G,
because this is on a level with the surface of the ground, H, where
the supply of water enters. From this point it may be raised by
a pump. If the well be dug at D, the water will rise not only to
the surface but to E, because this is on a level with H. Water is
sometimes obtained under such circumstances from very great depths.
In this case the porous stratum containing the water is reached
by boring, and then we have what is termed an Artesian well. The
name comes from Artois, in France, where this operation was first
executed. There is a celebrated well of this sort in Paris over
1800 feet deep, and the water rises 112 feet above the surface.
More than 600 gallons are discharged every minute. "London," says
Dr. Arnot, "stands in a hollow, of which the first or innermost
layer is a basin of clay, placed over chalk, and on boring through
the clay (sometimes of 300 feet in thickness) the water issues, and
in many places rises considerably above the surface of the ground,
showing that there is a higher source or level somewhere--probably
among the Surry hills or those north of London."

121. =Pressure of Liquids in Proportion to Depth.=--The pressure of
a fluid is in exact proportion to its depth. For, as the particles
are all under the influence of gravity, the upper layer of them
must be supported by the second, and these two layers together by
the third, and every layer must bear the weight of all the layers
above it. The increase of pressure at great depths produces the
most striking effects. Thus if an empty corked bottle be let down
very deep at sea, either the cork will be driven in or the bottle
will be crushed in. A gentleman tried the following experiment: He
made a pine-wood cork, so shaped that it projected over the mouth
all around. He then covered this with pitch, and fastened over the
whole several pieces of tarpaulin. The bottle, thus prepared, he
let down to a great depth by attaching to it a weight. On raising
it up he found that it contained about half a pint of water
strongly impregnated with pitch, showing that the pressure of the
water forced water through the several pieces of tarpaulin, the
pitch, and the pores of the wooden cork. When a ship founders near
land, the pieces of the wreck, as it breaks up, float to the shore;
but when the accident happens in deep water, the great pressure
forces water into the pores of the wood, and thus makes it so
heavy that no part of the vessel will ever rise again. When a man
dives very deep he suffers much from the pressure on his chest. If
we watch a bubble of air rising in water it is small at first, but
it grows larger as it approaches the surface, because it sustains
less pressure than when it was deep in the water. The force with
which a fluid is discharged from an opening in a vessel depends on
the height of the fluid above the opening. The difference in this
respect between a full barrel and one nearly empty is very obvious.
Most fishes, probably, can not bear the pressure of great depths,
and so are commonly found on the coast, or on banks, as they are
called, in the midst of the ocean.

[Illustration: Fig. 75.]

122. =Sluice-Gates, Dams, etc.=--The application of the above
principles in the construction of sluice-gates, dams, etc., is
a matter of great practical importance. Let us look at this. As
pressure in a fluid is always in proportion to the height of the
fluid above the point of pressure, the pressure upon any portion
of the side of a vessel containing a fluid must be in proportion
to its distance from the surface; or, in other words, it is the
weight of a column of water extending from this portion to the
surface. Let A B C D (Fig. 75) represent a _section_ of a cubical
vessel, that is, one in which each side is of the same size with
the bottom. The pressure on the point _a_, in the line A B, is
that of a column of particles, A _a_. But A _a_ is equal to _c
b_, and _c b_ is equal to _b a_. Therefore _b a_ may represent
the pressure on _a_. In the same way it can be shown that _e d_
represents the pressure on _d_, _n m_ the pressure on _m_, C B that
on B. Therefore the pressure on all the points in A B will be
represented by lines filling up all the triangular space A B C, and
this is half of A B C D, which represents the pressure on the line
C B. It is clear, then, that as the pressure on a vertical line
in the side is half that on a line at right angles to it in the
bottom, the pressure on the whole side is half that on the whole
bottom.

[Illustration: Fig. 76.]

We see from the above demonstration why it is that a dam is built
in the form represented in Fig. 76. We see, also, why in the
monstrous vats in some of the English breweries (some of them
holding many thousand barrels) the hoops and other securities at
the lower part of them require to be made of very great strength.
It is manifest, also, that if a sluice-gate is to be kept shut by a
single support, this must be applied at one third of the distance
from the bottom, there being as much pressure, as seen by Fig. 75,
on the lower third as on the upper two thirds of the gate.

123. =Lateral Pressure in Fluids.=--The pressure of a liquid on
the side of a vessel, of which I have spoken above, is a _lateral_
pressure, and it is caused by the downward pressure of gravitation
in the liquid. But how? The particles of a fluid are freely movable
among each other, and therefore are ready to escape from pressure
in any direction. The particles at _a_, Fig. 75, pressed upon by
the column of particles extending above them to the surface, are
ready to escape laterally, and would do so if there were an opening
made in the vessel at that point. But if the vessel contained
a block of ice, fitting it as accurately as the body of water,
there would be no escape at the opening, because the particles of
the solid are so held together that the downward pressure of the
earth's attraction occasions no lateral pressure.

[Illustration: Fig. 77.]

[Illustration: Fig. 78.]

The manner in which the downward pressure of the earth's attraction
causes lateral pressure may be made clear by Figs. 77 and 78. We
will suppose that the particles of solids and liquids are alike
round, and that a solid differs from a liquid only in having its
particles firmly united by attraction. Let _a_, _b_, and _c_,
in Fig. 77, represent three particles of a solid. As they are
united firmly they will have a united pressure from the centre of
gravity directly toward the centre of the earth, as represented
by the arrow. Let now _d_, _e_, and _f_, Fig. 78, represent three
particles of water. These being but very slightly coherent, will
make each an independent pressure toward the earth's centre, as
indicated by the arrows. It is plain that _d_ tends to separate _e_
and _f_, and will do so if they are left free to move in a lateral
direction. For example, if _e_ be at the side of a vessel, and an
opening be made there, the downward pressure of _d_ will give _e_ a
lateral movement, forcing it out of the opening.

124. =Another View.=--To return to Fig. 75, observe that the
lateral pressure at any point in the side of a vessel, as _a_,
is occasioned _wholly_ by the downward pressure of a vertical
column of particles extending from that point to the surface. The
neighboring columns of particles have nothing to do with it. The
same thing is true in regard to any other point either in the
line A B or another line drawn on the side of the vessel. It is
therefore true of the whole side, that the pressure upon it is
occasioned alone by the columns of particles that are in close
proximity to the side, and not at all by the other columns of
particles in the vessel. The number of these columns, therefore, in
the vessel, or, in other words, the breadth of the body of water
in it, makes no difference with the pressure on its side. For this
reason two flood-gates so little apart that a few hogsheads or
even pails of water fill up the space between them, are as much
pressed upon as they would be if a lake or an ocean of water lay
between them. It has been objected to the project of digging a ship
canal between the Red Sea and the Mediterranean, that as the water
in the former is twenty feet higher than in the latter, it would
burst through the flood-gates with such force as to produce most
disastrous results. But according to the principle which I have
illustrated, there would be no more danger of this than there would
be if two ponds were united by a canal, in one of which the water
is twenty feet higher than in the other.

[Illustration: Fig. 79.]

125. =Pressure in Liquids Equal in all Directions.=--We are
now prepared to go a step farther. The pressure occasioned by
gravitation in fluids operates equally in all directions when the
fluid is at rest. That is, any particle of a liquid is pressed
equally in all directions. If it were not so it would not remain
at rest, but would be moved in the direction in which the superior
pressure operates. Suppose that _a_, Fig. 79, is a stratum of
particles in a vessel containing water at rest. The upward pressure
on it being equal to the downward pressure, the stratum neither
rises nor falls. If a body of liquid be disturbed by wind or any
other cause, those particles which are raised above the common
level in waves are pressed downward more than upward or laterally
in obedience to the action of gravitation. They therefore move
downward, pushing laterally and upward the neighboring particles,
till the liquid regains its level surface and its state of rest.
So, also, if any particles become heated they are lighter than
their neighboring particles, and the latter being more strongly
attracted than the former, push them upward in order to take their
places. When all the liquid comes to have the same temperature it
is at rest, each particle having an equal pressure upon it in all
directions.

[Illustration: Fig. 80.]

126. =Illustrations.=--If a bladder filled with water be
compressed by the hand, the water is pressed no more immediately
under the hand than in any other part of the bladder, and wherever
an opening be made the water will rush out with equal readiness.
A hose-pipe as readily bursts upward as in any other direction. A
large cork, if sunk in very deep water, will be uniformly reduced
in its dimensions, showing that it has been pressed equally on all
sides. In the experiments with the closed bottles (§ 121), the
result is the same if the bottle be so sunk as to have its mouth
downward. If two tubes, shaped as in Fig. 80, be thrust down into
water, the water will rise with equal facility in both, although in
the straight one the pressure which carries up the water is wholly
upward, while in the bent one it is at the first downward.

[Illustration: Fig. 81.]

127. =Upward Pressure as the Depth.=--It has been shown that the
downward and the lateral pressures are as the depth. The same
is true of the upward pressure, for it is produced by the same
cause--the attraction of the earth. Let us look at this. Why is any
particle of a fluid pressed upward at all? It is from the struggle
on the part of the neighboring particles to get below it. And why
this struggle? It is from the attraction of gravitation, and so
the greater this attraction the greater is the upward as well as
the downward pressure. The upward pressure therefore differs at
different depths as the downward pressure does. Thus, in Fig. 81,
the upward pressure against the layer or stratum of particles, _b_,
is greater than that against _a_, for the same reason that the
downward pressure on _b_ is greater than that on _a_. But the two
pressures at _b_ are equal, and so are they at _a_, and therefore
each stratum remains at rest.

[Illustration: Fig. 82]

128. =Experiments.=--Some very neat experiments can be tried,
showing that the upward pressure varies with the depth. Take a
large glass tube, A B C D, Fig. 82, and let there be fitted to one
end a circular plate of brass, which may be held there by a string,
F. Thus arranged, plunge it quite deep into water, and you will
find that you will not need to hold on to the string, for the brass
disk will be held tight to the tube by the upward pressure of the
water. Now draw up the tube slowly, and at length the disk will
fall from the end of the tube. Why? Because the end of the tube has
come to a point where the upward pressure of the water is less than
the downward pressure of the disk. To have this experiment succeed,
the end of the tube where the disk is applied must be very even and
smooth. Another experiment may be tried in this way. Tie to one end
of a glass tube a piece of thin India-rubber or bladder, and fill
the tube partly with water. The India-rubber will of course bulge
out or be convex from the weight of the water. Press the closed
end down a little way in a vessel of water, so that the level in
the tube shall be above the level in the vessel. The India-rubber
is still somewhat convex, because, as the upward pressure upon it
is in proportion to its distance from the surface of the water
outside of the tube, it is not as great as the pressure downward
of the higher water in the tube. Push the tube now so far down
that the level in the tube is the same with that in the vessel.
The India-rubber is now flat, because the downward and upward
pressures upon it are equal, just as would be the case with a
stratum of water in place of it. But press the tube lower down, and
the India-rubber bulges upward into the tube, because the upward
pressure is now greater than the downward.

129. =Great Effects from Small Quantities of a Fluid.=--You are
now prepared to understand the explanation of some very striking
phenomena in the pressure of liquids. If you take a perfectly
tight cask, and, filling it with water, screw into its top a long
tube, by pouring water into the tube you can burst the cask. To
understand this you must bear in mind two facts--that the fluid
in the cask is not compressible, and that its particles move
freely among each other. Any pressure, therefore, exerted upon it
is felt through the whole of it equally. "If the tube," says Dr.
Arnot, "have an area of a fortieth of an inch, and contain when
filled half a pound of water, this produces a pressure of half a
pound upon every fortieth of an inch all over the interior of the
cask; which is more than a common cask can bear." Suppose a small
reservoir of water exists in the side of a mountain wholly closed
up, and that water from a height above finds its way to it by a
crevice, it may by its pressure even burst open the side of the
mountain. And it matters not how large or small the crevice may be,
for pressure in a liquid is only as the height. If the reservoir be
ten yards square and an inch deep, and the fissure leading to it
be but an inch in diameter and two hundred feet in height, it is
calculated that the pressure of the water in the fissure would be
equal in force to the weight of 5000 tons.

[Illustration: Fig. 83.]

130. =Explanation.=--The manner in which these effects are produced
may be made clear by Fig. 83. Let A be a close vessel filled with
water, and let a tube, _b_, be made fast in it, with a movable
plug or piston at _c_. If the surface of the water be pressed
upon by this piston with the force of a pound, as the water is
incompressible and its particles are freely movable among each
other, the pressure will be extended equally through all the
water, and every portion of the vessel of equal extent with the
tube's opening at _c_ will be pressed upon with the force of a
pound. If another tube, _d_, of the same size were inserted with a
piston, _i_, the force of a pound applied to the piston _c_ would
push upward the piston _i_ with the same force. And if there were
several pistons of the same size, by pushing upon one with the
force of a pound they would all be pressed upward with exactly this
force. Farther, if _e_ be a tube five times as large as _b_, its
piston, _n_, will be forced upward with a pressure of five pounds
by the downward pressure of a pound upon _c_. Suppose now that
a pound of water were substituted for the piston _c_, the other
pistons would be pressed upward as before. And if all the pistons
be removed, the pound of water in _b_ will press the water up the
tube _d_ with the force of a pound, and up the tube _e_ with the
force of five pounds.

[Illustration: Fig. 84.]

To make this still more clear I will present it in a little
different form. Let B, Fig. 84, be a close vessel with two tubes,
one of which is five times as large as the other. If sufficient
water be poured into the vessel to occupy a part of the tubes, it
will stand at the same height in both tubes, as indicated. If there
be a pound of water, then, in the tube _c_, there will be five
pounds in _a_. Now if the five pounds of water in _a_ made any more
pressure on the whole body of water in B than the pound of water in
_c_ does, it would press up the water in _c_ to a greater height.
But this is impossible, as has been shown in § 118. Observe that
the five pounds of pressure in _a_ is spread over five times the
area or extent of surface that the pound's pressure in _c_ is. If
the tube _c_ have an area of an inch square, the water in it will
exert a pressure of a pound on every square inch in the vessel.
The water in _a_ exerts a pressure of five pounds; but it must be
remembered that it does not press with this force on every square
inch, but on every space of five square inches, and that therefore
its pressure on every inch is the same as that in the tube _c_.

131. =Hydrostatic Paradox.=--You see in the phenomena and
explanations given above that a small quantity of a fluid can,
under certain circumstances, exert an enormous pressure. This fact
has been called the Hydrostatic Paradox. It does seem, at first
view, incredible or paradoxical, when one asserts that a few ounces
of water can be made to raise weights of hundreds or even thousands
of pounds. But the explanations which I have given show you that
there is no unexplainable mystery in the fact. The cause of it is
the same as that which gives a level surface to liquids; viz., the
force of gravitation acting upon a substance whose particles are
freely movable among each other.

[Illustration: Fig. 85.]

132. =Hydrostatic Bellows.=--The instrument called the Hydrostatic
Bellows is represented in Fig. 85. It consists of two circular
boards, A and B, united together by strong leather, and having a
tube, C, through which water can be poured into it. The amount of
weight which can be sustained on the bellows without forcing the
water out of the tube depends on the size of the bellows. If the
area of the tube is only one thousandth of that of the top of the
bellows, a pound of water in the tube will balance a thousand
pounds' weight on the bellows. It is for the same reason that in
Fig. 84 one pound of water in the tube _c_ balances five pounds in
_a_. As the weight presses upon the top as a whole, it is the same
as if there was a vessel of the same size with the bellows resting
upon it and containing a thousand pounds of water. The water, in
that case, would stand at the same height in the vessel and the
tube. This shows that the Hydrostatic Paradox is only one of the
exemplifications of the great fact that a fluid, from the influence
of gravitation, seeks to be on a level. It is the water in the
bellows seeking to be on a level with that in the tube that causes
the upward pressure sustaining the weight.

When the weight on the bellows is less than is required to balance
the water in the tube, the weight can be raised continually by
pouring water into the tube. But observe that although the lifting
force be so strong, it is very slow in its operation. If the
comparative areas of the tube and the bellows be as above supposed,
the water must fall in the tube ten inches in raising the weight
the one hundredth part of an inch.

[Illustration: Fig. 86.]

133. =Bramah's Hydrostatic Press.=--The principles which I have
elucidated have been applied by Mr. Bramah in his Hydrostatic
Press. This consists of a small metallic forcing-pump, Fig. 86, in
which the water, _a_, is pumped up by the piston, _s_, worked by
the lever, _c b d_, and forced into a strong and large cylinder,
A. In this cylinder is a stout piston, S, having a flat head, P,
above. Between this plate and another, R, is placed the body, W,
which is to be compressed. It is obvious that the pressure exerted
will be in proportion to the difference between the size of the
pump, _a_, and the cylinder, A, just as in the case of the bellows,
it depended on the difference between the areas of the tube and
of the top of the bellows. In the press the force of a pump is
substituted for the pressure of a very high column of water, simply
because it is more convenient. This press is of great service
in the mechanic arts. It is used in pressing paper, cloth, hay,
cotton, etc. It has also been recently used in raising enormous
weights. The tubes of the celebrated bridge over the Straits of
Menai were raised by a machine constructed on this principle.




CHAPTER VIII.

SPECIFIC GRAVITY.


134. =Nature of the Subject.=--We now come to a very interesting
subject, which is at least intimately connected with Hydrostatics,
if it may not be considered a part of it. The principles which
have been developed in the chapter on Hydrostatics in relation to
liquids are to be applied here to various kinds of substances. And
as we proceed you will see that all the phenomena brought to view
in this chapter are to be referred to the same cause with those of
the previous chapter; viz., the attraction of gravitation.

135. =Specific Gravity Defined.=--Before proceeding with the
investigation I will give you the definition of specific gravity.
The specific gravity of any substance is its weight as compared
with the same bulk or volume of other substances. Water is taken as
a standard, and its specific gravity is for convenience called 1.
Mercury, then, is said to have a specific gravity of 13.5, for it
is thirteen and a half times as heavy as the same volume of water.
It is easy to see how the specific gravities of different fluids
may be ascertained. One mode, and the most obvious one, is to weigh
in a vessel equal quantities of them. In what way the specific
gravities of solids are ascertained will be explained in another
part of this chapter.

136. =Action of Gravity on Solids in a Liquid.=--The reason that
a very heavy substance, as a stone, sinks in water is simply that
the earth attracts it more strongly than it does the water, and so
drags the stone down through it. If the stone lay upon a bladder
filled with water, it would press upon it with the force with which
it is attracted by the earth. But where water is not thus confined,
the stone thrusts its particles to the one side and the other till
it gets to the bottom.

It is the attraction of gravity, also, that makes light substances,
as wood and cork, rise in water. In this case the water is
attracted by the earth more strongly than the wood or cork, and
so gets below it, and in so doing pushes the lighter substance up
above itself.

[Illustration: Fig. 87.]

But you will observe that the wood, on rising in the water, does
not come completely out of it and lie upon the surface, but a
part of it remains immersed in the water. The explanation of this
will furnish you with the key to the understanding of many very
interesting facts. Suppose that half of a block of wood, A, Fig.
87, weighing a pound, is above the surface of water. As it is
attracted to the earth with the force of a pound, it has pushed to
the one side and the other just a pound of water, and taken its
place. It is drawn down toward the earth with the same force with
the pound of water on either side of it, _b_ or _c_. If it were
attracted any more than with the force of a pound, that is, if
it weighed more than a pound, it would displace more than a pound
of water. If it were of just the same weight with the same volume
of water, it would displace a volume of water equal to itself; it
would be wholly immersed, and would stay any where in the water,
wherever you placed it, because it is attracted by the earth with
the same force that the same bulk of water is.

[Illustration: Fig. 88.]

137. =Farther Explanation.=--Suppose water in a vessel divided
into equal portions of a pound each, as represented in Fig. 88.
Now suppose that the portion _a_ should at once change into solid
ice without at all altering its bulk or weight. It would not move
from its position, because it is attracted by the earth precisely
as much as when it was water, and as much as is each of the equal
portions of water around it. But as water on becoming ice does
really increase in bulk, and therefore become lighter, this block
of ice would rise so that a part of it would be above the surface.

[Illustration: Fig. 89.]

The lighter a substance is that is immersed in water, the more
there will be of it above the surface. Take two blocks of wood of
different weights though of the same size. Suppose the heaviest
one, A, Fig. 89, is one third lighter than the same bulk of water.
One third of it will be above the surface. If the other, B, is
half the weight of water, half of it will be above the surface.
We should say, then, that the specific gravity of the wood in
the first block is two thirds of that of water, and the specific
gravity of the wood in the second is one half that of water.

138. =Illustrations.=--There are many interesting facts that
illustrate the principles which I have developed. A stone is lifted
much more easily in water than in air, because of the support
afforded by the upward pressure of the water. A boy will often
wonder why he can lift a very heavy stone to the surface, but
can get it no farther. When a bucket of water is drawn up a well
much less exertion is required to raise it through the water than
through the air after it emerges from the water. While it is in the
water you raise only the bucket itself, the water in it having no
weight, being sustained by the water around it. But when it comes
to the air you have the weight of the water added to that of the
bucket. When a person lies in a bath for some time, on raising his
arm from the water it seems to be very heavy. The reason is, that
it has had for so long a time the support of the water that when
it is lifted into the air the want of this support is sensibly
felt, just as we perceive the difference between raising a bucket
of water through water and raising it through air. It is said that
Archimedes took in the full idea of the principles of specific
gravity as his limbs felt the liquid support of a bath, and so
overjoyed was he with the discovery, that he ran home crying out
all the way, "Εὕρηκα! εὕρηκα!"--I have found it! I have found it!
It was a rational joy, for he had found a principle of immense
value to science and to the world.

139. =Boats and Life-Boats.=--A boat of iron will float with as
much of it out of water as one of wood of the same size, provided
that the iron be made so thin that the boat is not heavier than the
wooden one. For what is it that floats? Not the iron or wood, but a
wooden or iron boat filled with air. If it were filled with water
instead of air it would sink, the specific gravity of the materials
of which it is built being on the whole of greater specific gravity
than water. Life-boats have in their structure either a large
quantity of cork or air-tight vessels of tin or copper, and in this
way they are made so light that they will float even when filled
with water.

As the weight of a body can be estimated from the quantity of water
which it displaces, we can very readily estimate the weight of
the load of a canal-boat, as its form is so simple and regular. In
order to do this we must first know how far the boat sinks in the
water when empty, or, in other words, how much water it displaces.

140. =Specific Gravity of Animals.=--Birds have a much less
specific gravity than animals that walk, in order that they may
mount up easily in the air. Their light feathers increase greatly
their bulk, as you may see whenever a bird is stripped of them.
Besides this, the bones are hollow and communicate with the lungs.
Birds that swim, as ducks, swans, etc., have so small a specific
gravity--that is, are so large in proportion to their weight--that
but a small part of the body is under water, and the motion of
their feet is not required at all to sustain them, but only,
like the action of oars, to propel them along. Insects are of
small specific gravity, those that fly the most swiftly being the
lightest. Fishes are very nearly of the same specific gravity with
water, and hence require but little muscular effort to move about
in their element. They are assisted much in rising and falling
by a contrivance by which they can instantaneously alter their
specific gravity. They have an air-bladder, which they can dilate
or contract at pleasure. When dilated, the bulk of the fish is
increased and his specific gravity lessened, and he rises easily
and at once. By compressing it he as readily sinks.

141. =Specific Gravity of the Human Body.=--The human body, when
the chest is filled with air, is so much lighter than water that it
will float with about half the head above the surface. A knowledge
of this fact, with proper presence of mind, might ordinarily
save persons from drowning; for if the body be put in the proper
position, the feet downward and the head thrown backward, the nose
and mouth will be out of the water. So little is required in the
way of support to keep the whole head out of water, that persons
who can not swim are often saved from drowning by catching hold of
very small pieces of wood. An oar would support half a dozen men,
if they would be satisfied with keeping only the head out of water;
but if each one struggle to get his whole body upon the oar, they
may all be lost.[1] A life-preserver is a great aid in preservation
from drowning, for it diminishes the specific gravity of the body.
It is commonly an air-tight bag fastened round the upper part of
the body, which can be filled by blowing into it through a pipe
which has a valve in it. "On the great rivers of China," says Dr.
Arnot, "where thousands of people find it more convenient to live
in covered boats upon the water than in houses on the shore, the
younger male children have a hollow ball of some light material
attached constantly to their necks, so that in their frequent falls
overboard they are not in danger."

When a person is drowned the body sinks because in the struggle
much of the air in the lungs is lost, just as the fish sinks when
his air-bladder is contracted. It is, however, so little heavier
than water after this is done, that it very readily rises when
any gas is produced in it by putrefaction. It is a common popular
notion that firing cannon over the water will raise the drowned.
But it can produce no effect, unless perhaps the agitation caused
by the concussion may hasten a very little the rising of a body
which from commencing putrefaction is about to rise.

In wading a river the feet press upon the bottom with only a force
equal to the weight of half the person's head, this being the
difference between the weight of the body and the weight of the
same bulk of water. Now this pressure is not sufficient to give a
sure footing against even a moderate current. Many persons have
been drowned from ignorance of this fact. A man carrying a load
may often ford a river safely where without a load to press him
down, and thus give him a sure footing, he would be carried down
the stream. So a man may walk in deep water upon broken glass with
impunity.

[Illustration: Fig. 90.]

142. =How to Ascertain the Specific Gravity of Solids.=--It results
from the upward pressure of water that a body weighs less in water
than in air. Take a piece of gold or any other substance, _a_,
Fig. 90 (p. 107), and weigh it suspended as you see from one of
the scales. Introduce the gold now into a cup of water, and you
will find that a part of the weight must be taken from the opposite
scale to preserve the balance. The weight which you take from the
scale will be the weight of a quantity of water equal in bulk
to the piece of gold; for the immersed body is supported with a
force equal to the weight of the water it displaces (§ 137). By
comparing, therefore, its weight in water with its weight in air
we determine its specific gravity. Thus if a lump of gold weigh
nineteen ounces, and on being weighed in water weighs eighteen, it
will prove that gold is nineteen times as heavy as water. And if
a lump of copper weigh nine ounces in air and eight in water, it
is nine times as heavy as water. Calling water, therefore, 1, the
specific gravity of gold is 19, and of copper 9. It is obvious that
a body of the same specific gravity with water would weigh nothing
when immersed in water, for it would be supported with an upward
pressure precisely equal to its own weight, just as the same bulk
of water is. A pound of water, therefore, will weigh nothing in
water. The experiment can easily be tried. Weigh a glass bottle,
suspended on one arm of the scale-beam, and then put a pound of
water in it. On immersing it in water it will be balanced if you
take out the pound weight in the opposite scale.

143. =Archimedes and the Crown.=--Hiero, King of Syracuse,
stipulated for a crown of pure gold. But suspecting the maker of it
of adulterating the gold, he called upon Archimedes to detect the
imposture. He did it in this way: He procured two lumps of gold and
silver of the same weight with the crown, and observed the quantity
of water which each displaced. He then tried the crown, and found
that it displaced less than the silver and more than the gold, and
therefore concluded that it was an alloy of the two metals. All
this was suggested to him by his experience in the bath, referred
to in § 138.

[Illustration: Fig. 91]

144. =How to Ascertain the Specific Gravity of Liquids.=--There are
several modes of ascertaining the specific gravities of different
liquids. The instrument called a Hydrometer furnishes the most
common mode. This is used chiefly in determining the quality of
spirit. The more alcohol and the less water spirit contains the
less is its specific gravity. The Hydrometer consists of two bulbs
of glass, A B, Fig. 91, with a slender stem, C, which is graduated.
In the lower bulb are a few shot or a little mercury, to give the
instrument its proper weight, and to make its centre of gravity
to be in the lower part. The lighter the fluid to be tested is
the lower will the instrument sink in it. This is a very accurate
instrument, detecting the slightest adulteration in spirits. Dr.
Arnot tells an amusing story of the detection of a Chinese trader
in liquors. He had sold a quantity of liquor to the purser of a
ship, averring it to be of the same quality with a sample which
he had given him. The purser tried it with his Hydrometer, and
found it to be of greater specific gravity than the sample. The
Chinese promptly denied the fraud; but on being told the exact
quantity of water which he had added, he was so much confounded
that he immediately confessed his guilt, and made ample amends.
When the Hydrometer was shown to him he offered a large price for
what appeared to him to be a magical instrument, foreseeing that it
would be of great advantage to him in his business.

In Switzerland and in the north of Italy, where the peasants bring
their milk to a common dairy, and are allowed a quantity of cheese
at the end of the season in proportion to the amount of milk which
they have brought, a Hydrometer is used to test the quality of
the milk. There is a propriety in this, not only as a safe-guard
against adulteration, but because there is a difference of quality
in the milk of different cows, some giving a much more watery milk
than others.

145. =Centre of Gravity in Floating Bodies.=--The same principles
which apply to the centre of gravity in bodies standing on a firm
basis apply also to floating bodies. That the centre of gravity
may be low in a loaded vessel the heavy part of the cargo is put
underneath, and generally ballast of stone or iron is necessary
for the same purpose. In large flat-boats, the base of support
being extensive, there is not the same need of taking care that
the centre of gravity be low. If a ship be laden in part with an
article which will dissolve in water, there is much danger, if the
ship should leak, that this portion of the cargo shall be dissolved
and be pumped out with the bilge-water, thus altering the trim of
the vessel, or removing the centre of gravity from over the middle
line, and bringing it too far forward, or carrying it too far back,
making the ship wholly unmanageable. Four large English ships, in
part loaded with saltpetre, were supposed to be lost from this
cause in 1809 off the Isle of France. The immense ice-islands,
or icebergs, which float about in summer in the polar regions,
by melting irregularly often change the place of their centre
of gravity, and in turning over present one of the most sublime
spectacles in nature. A mountain of ice, extending high in the air
and deep in the sea, suddenly turns over, and produces a rolling of
the ocean which is often felt at the distance of many leagues.




CHAPTER IX.

PNEUMATICS.


146. =What Pneumatics Teaches.=--As Hydrostatics treats of the
pressure and equilibrium of liquids, Pneumatics treats of the same
in air and the gases, or aeriform substances. The name comes from
the Greek word πνευμα, meaning air, breath, spirit.

[Illustration: Fig. 92.]

147. =Air Material and has Weight.=--That air is a material
substance has been already proved to you, for it was shown in § 46
that it has impenetrability, one of the essential properties of
matter. It has extension also, for bodies of air can be obtained in
various shapes confined in vessels, so that we can speak of cubes
and spheres of air; and besides, the ultimate atoms (§ 15) of air
must have shape or extension. That air has weight can be proved by
weighing it as you would any other substance. Let a hollow globe,
A, Fig. 92, having a neck with a stop-cock, B, be emptied of air
and weighed. If now you open the stop-cock, and so let in the air,
the other beam of the scale will rise, because the globe is heavier
than it was before. The additional weight required to make the
scales balance will indicate the weight of the air which the globe
contains. It is one eight hundredth (1/800) of the weight of the
same volume of water. How the globe can be emptied of the air will
be shown in another part of this chapter.

148. =Air Attracted by the Earth.=--The weight of the air is
simply the result of the attraction of the earth (§ 52). Air is
attracted by the earth just as water is; and the water takes its
place below air because it is attracted more strongly than the
air. It is from the attraction of the earth that air descends into
any hollow spot in the earth when water is removed from it. It
takes the place of the removed water because from the influence of
attraction it gets as near to the earth as possible. If you put
into a vial mercury, water, and oil, the mercury will be at the
bottom, because it is more strongly attracted by the earth than the
other fluids. The water will be next, then the oil, and lastly,
over all, there is air, that being less attracted than any of the
other substances. It is this attraction of the air by the earth
that gives us the chief phenomena of Pneumatics.

149. =Why Some Things Fall and Others Rise in Air.=--Most
substances fall in air for the same reason that very heavy
substances sink in water. They fall because the earth attracts
them more strongly than it does the air. The reason that some
substances rise in air is precisely the same as that given in § 136
for the rising of substances in water. The air being attracted more
strongly than they are pushes them up to get below them, as cork
or wood is pushed up by water. Thus a balloon filled with hydrogen
gas rises in air for the same reason that a bladder filled with air
rises in water. So, also, smoke rises in air, just as oil rises in
water.

[Illustration: Fig. 93.]

150. =Thickness of the Earth's Air-Covering.=--The air makes
a covering for the earth about fifty miles deep. If the earth
were represented by a globe a foot in diameter, the air might
be represented by a covering a tenth of an inch in thickness.
The line _a_, Fig. 93, gives us the curve of the surface of
such a globe, and the space between _a_ and _b_ represents the
comparative thickness of the covering of air. This is ascertained
by calculation from the pressure of the air upon the earth. It is
just as the depth of water may be calculated from the pressure
which it makes. We do not take this mode of ascertaining the depth
of water, because we can measure it from the surface by sounding.
But we should be obliged to adopt it if we lived at the bottom of
water, as we do at the bottom of the sea of air.

151. =How the Air-Covering Adheres to the Earth.=--The earth
flies on in its yearly journey around the sun at the rate of 1100
miles per minute, and yet it holds on to this loose airy robe by
its attractive force, so that not an atom of it escapes into the
surrounding ether. Of itself it is disposed to escape; and it
would do so, and be diffused through space, if the attraction of
the earth for it were suspended. For, unlike liquids, the air has
no disposition to keep together; that is, there is no attraction
between its particles. On the other hand, there is a repulsion, so
that they are disposed to keep far apart, and are kept together
only by pressure. It is the pressure of the earth's attraction that
keeps them together to the extent of fifty miles all around it.

152. =Compressibility of Air.=--In looking at the influence of
gravitation upon air, it must be remembered that air is very
compressible, while water is very nearly incompressible. While,
therefore, in a body of water the particles are very little nearer
together at the bottom than at the surface, the particles of the
air are much nearer together close to the earth than they are far
away from it. For as all the particles of the air are attracted or
drawn toward the earth, those below are pressed together by the
weight of those above. The air is therefore thinner as we go up
from the surface of the earth, and in the outer regions of the sea
of air it is too thin to support life. Even at the tops of very
high mountains, or the heights sometimes reached by balloons,
disagreeable effects are often experienced from the thinness of the
air. The air has been compared, in regard to its varying density
at different heights, to a heap of loose compressible substance;
as, for example, cotton-wool, which is quite light at the top, but
is pressed more and more together as you go toward the bottom.
Hydrogen gas has only one fifteenth the weight of air at the
surface of the earth; and therefore the hydrogen balloon rises till
it reaches a height where the air is so thin that the balloon is of
the same weight with an equal bulk of air, and there it stops.

153. =In what Aeriform Substances and Liquids are Alike.=--You have
seen in § 36 and § 38 how the air and gases differ from liquids.
But in one very important respect they are alike, viz., the
movability of their particles. Hence pressure is in air, as well as
in water, equal in all directions, so that in the experiment with
the bladder, in § 126, it makes no difference in the result whether
there be water or air in it. For the same reason pressure is as
the depth in aeriform substances as in liquids, and the laws of
specific gravity apply to the one as well as to the other.

You are now prepared to understand the results of _the action
of gravitation upon air and the gases_; or, in other words, the
principal phenomenon of Pneumatics.

154. =Pressure of the Atmosphere.=--The amount of the pressure of
the atmosphere is very readily estimated, the mode of doing which
I will speak of in another part of this chapter. It presses with
a weight of fifteen pounds on every square inch. Suppose that you
extend your outspread hand horizontally in the air. You feel no
pressure upon it, but there is a pressure of some two or three
hundred pounds of air upon it. If your hand be five inches long
and three broad it presents a surface of fifteen square inches,
on every one of which the atmosphere is pressing with the weight
of fifteen pounds. That is, there is a pressure on the upper
surface of your hand of a column of air weighing 225 pounds. So,
also, on the lid of a box only thirty inches square, there is a
pressure of 13,500 pounds. The whole pressure on the body of a man
of common size is about fifteen tons. But why is it that the lid
of the box is not broken in, your hand not borne down, and your
body not crushed? It is simply from the fact, shown in the previous
chapter in regard to liquids, and in this in regard to aeriform
substances, that the pressure is equal in all directions. The lid
and the outspread hand are therefore balanced by an upward pressure
equal to the downward, and the body has the pressure on all sides
the same. If the air could be removed from within the box the lid
would be crushed in; if from under the hand, that would be borne
down; and if from one side of the body, the body would be forced
violently in that direction till it met with an opposing pressure.

But besides this equal pressure of the air on all sides, there is
air within the pores and interstices of all bodies that are not
very dense, and its particles are subject to the same laws as are
those on the outside.

All this can be made clear to you by the air-pump.

[Illustration: Fig. 94.]

155. =Air-pump.=--In Fig. 94 you have a representation of an
air-pump as commonly arranged. At _a a_ are two pump-barrels, the
pistons in which are worked by means of the handle, _b_. Those
pumps are very nicely made, and the frame-work, _d e d e_, to which
they are attached, is very strong and firm, so that the pumps may
work evenly. There is a large, smooth, metallic plate, _f_. At _c_
is a bell-shaped glass vessel, close at the top, but open at the
bottom, the edge of which is ground very true, so that it may fit
exactly on the metallic plate. In the middle of the plate is an
opening which leads to the pump-barrels, and it is through this
that the air is pumped out of the glass receiver, _c_. If we wish
to let the air in after we have pumped it out we loosen the screw
at _g_, for from the opening here is a passage to the opening in
the middle of the plate.

[Illustration: Fig. 95.]

The operation of the air-pump can be made clear by the plan in Fig.
95. But one pump-barrel, _a_, is represented, with a piston, _c_,
working in it. In the piston there is a valve, _i_, opening upward,
and also one at _b_, in the beginning of the passage leading to the
centre of the plate where is the receiver, _d_. The working of the
instrument is thus: If the piston be forced down, the air under it,
being compressed, will close the valve at _b_, and will rush upward
through the valve _i_ in the piston. Let the piston now be raised;
the resistance of the air above it will close the valve _i_, while
the valve _b_ will be opened by the air rushing from the receiver,
_d_, through the passage, _e_, to fill the space between the piston
and _b_. You see, then, that every time that the piston is drawn
up air passes out of the receiver through the valve _b_ into the
space between this valve and the piston. None of this air which has
passed out can go back again, for the moment that you press upon
it by forcing downward the piston the valve _b_ is shut down, and
the air escapes from the pressure by passing out through the valve
_i_. Each time, therefore, that you work the piston up and down
you pump out some of the air from the receiver; and if you pump
for some time there will be exceedingly little air left in it, and
that will of course be diffused throughout the receiver. It will be
thin, like that in the upper regions of the atmosphere.

[Illustration: Fig. 96. Fig. 97.]

[Illustration: Fig. 98.]

156. =Experiments.=--When the receiver is full of air it can be
moved about on the plate easily, and can be lifted from it. But
work the pumps a few strokes and you will find that the receiver is
firmly fastened to the plate, for the air within, being made thin,
presses with little force compared with the air outside. If the
pumps be worked for some time no force could release the receiver
from the pressure without breaking it. But loosen the screw, _g_,
and thus let the air in, and the equality of the pressure on the
outside and inside is at once restored. Take off now this large
receiver, and place a small glass jar, open at both ends, on the
plate, with the hand covering the upper opening, as represented in
Fig. 96. On exhausting the air the hand is so firmly pressed into
the glass that it requires considerable force to disengage it from
the pressure. If we tie a piece of bladder or India rubber over
this jar, as in Fig. 97, and then pump out the air, the bladder at
first is pressed in as represented, and if we pump on it at length
bursts with a loud report. It would make no difference in the
result of the experiment if the jar were shaped as in Fig. 98, for
the pressure is the same in all directions. The resemblance between
air and liquids in this respect may be illustrated thus: Suppose
that a flat fish covers with one of its sides the end of the tube
of a pump. He feels no uncomfortable pressure, because the water
in the pump and that below it press equally upon him. If, now, the
pressure of the water in the pump could be suddenly taken off by
the piston, the fish would be pressed upward into the tube, as the
bladder is pressed upward in Fig. 98, or downward in Fig. 97, or as
the hand is pressed downward in Fig. 96. The Magdeburg Hemispheres,
Fig. 99, illustrate very impressively the pressure of the
atmosphere. They consist of two hemispheres whose edges at A fit
very accurately upon each other. The air is exhausted through the
stem where you see the stop-cock, and then the handle B is screwed
on. The force required to pull these hemispheres apart depends upon
the extent of their surface. In the famous experiment at Magdeburg,
in 1654, by Otto von Guericke, the inventor of the air-pump, two
strong hemispheres of brass of a foot in diameter were employed,
and it required the force of thirty horses to separate them. In
Fig. 100 you see a receiver with an opening at the top. Cemented
in this opening is a wooden cup, _a_, terminating in a cylindrical
piece, _b_. If mercury be poured into the cup, on exhausting the
air from the receiver the mercury will be forced through the pores
of the wood by the external air, and will fall in a silver shower.
A tall jar, _c_, is placed there to receive it, to prevent any of
it from going down into the opening in the metallic plate.

[Illustration: Fig. 99.]

[Illustration: Fig. 100.]

[Illustration: Fig. 101.]

157. =The Sucker.=--The boy's sucker illustrates the pressure of
the air. It is simply a circular piece of leather with a string
fastened to its centre, as seen in Fig. 101. When the leather is
moistened and pressed upon a smooth stone, on pulling the string
a vacuum is made between the middle of the leather and the
stone, and the leather adheres by its edges to the stone, just
as the receiver adheres to the plate of the air-pump when the
air is pumped out. There are many animals that have contrivances
of a similar character. The gecko and the cuttle-fish furnish
interesting examples, as noticed in my Natural History, pages
198 and 320. Snails, limpets, etc., adhere to rocks by a like
arrangement. Some fishes do the same. There is one fish called the
remora, that attaches itself by suckers to the side of some large
fish or a ship, and thus enjoys a fine ride through the water,
without any exertion on his part. In all such cases it is water
instead of air that makes the pressure, but the principle is the
same. Flies and some other insects can walk up a smooth pane of
glass, or along the ceiling over-head, because their feet have
contrivances akin to the boy's sucker. The hind-feet of the walrus
are constructed somewhat like the feet of the fly, enabling this
huge animal to go up smooth walls of ice.

[Illustration: Fig. 102]

[Illustration: Fig. 103.]

[Illustration: Fig. 104.]

158. =Density of the Air Dependent upon Pressure.=--The fact that
the degree of the density of the air is dependent on pressure has
been already shown in § 152. The same thing can be shown in various
ways with the air-pump. If a small bladder, partly filled with
air, Fig. 102, and loaded with a weight so as to sink in water, be
placed in a jar of water, and the whole be set under the receiver
of the air-pump, on exhausting the air the bladder will swell out
with the expanded air in it, and will rise as seen in the figure.
The reason is, that the pressure being taken off the surface of the
water, the bladder bears only the pressure of the water, and not
that of the air with the water, and so the air in it expands and
becomes less dense. If an India-rubber bag be partly filled with
air, Fig. 103 (p. 119), and put under the receiver, on exhausting
the air, the surrounding pressure being thus taken off from the
bag, the air in it becomes expanded, that is, rarefied. For the
same reason, if a vessel with soap-bubbles in it be placed under
the receiver, on pumping out the air the bubbles will become much
enlarged. A very pretty experiment illustrating the same thing may
be tried in this way. Let an egg with a hole made in its small
end be suspended in a receiver, as represented in Fig. 104, a
wine-glass being beneath it. On exhausting the air the egg will all
run out of the shell into the wine-glass, and then, on admitting
the air, it will run back again into the shell. The explanation
is this: There is air in the large end of the egg. As soon as the
pressure of air is taken off from all about the egg the air in the
egg expands, forcing out the contents; but when the air is admitted
into the receiver the air in the egg is at once condensed into its
former small bulk by the surrounding pressure.

[Illustration: Fig. 105.]

[Illustration: Fig. 106.]

159. =Hydrostatic Balloon.=--The philosophical toy represented in
Fig. 105 illustrates very beautifully the influence of pressure
upon the density of the air. The balloon in the jar of water is of
glass, with a small orifice at its lower part. Care must be taken
in putting water in the balloon to have just enough to make it of a
little less specific gravity than water. In that case it will be at
the top of the jar, with a very little of its top above the surface
of the water. Now tie a piece of India-rubber cloth over the top of
the jar, and the apparatus is complete. On pressing upon the India
rubber the balloon will go down in the jar, and on taking off the
pressure it will rise. The explanation is this: The pressure upon
the India rubber is felt through the whole body of the water in
the jar, and forces a little more water into the orifice of the
balloon, condensing the air that is there. The balloon consequently
becomes heavier, and has a greater specific gravity than water,
and sinks in it. But when the pressure is taken off, the condensed
air in the balloon, by its elasticity, returns to its former bulk,
expelling the surplus water just introduced, and the balloon,
becoming therefore as light as before, rises. Grotesque figures of
glass may be managed in the same way. The Cartesian image, Fig.
106, is an example. This has air in its upper part, _a_, and water
up to _c d_. When pressure is made on the India rubber more water
is forced into the image through the tail, _b_, and it goes down
like the balloon, to rise again when the pressure is taken off.

[Illustration: Fig. 107.]

[Illustration: Fig. 108.]

160. =Air in Substances.=--I have said that there is air in the
pores and interstices of wood, flesh, and a great variety of
substances. In all these cases the presence of the air can be made
manifest by taking off the pressure of the surrounding air, and
thus allowing the air in these substances to expand. If an egg
be placed in a jar of water, Fig. 107, under the receiver of an
air-pump, on exhaustion being made air-bubbles will constantly rise
in the water from the egg. So, too, a glass of porter, Fig. 108,
will have its surface covered with foam, the carbonic acid gas in
it escaping freely when the pressure of the air upon it is taken
off. The same thing may be seen to some extent even in water, for
it always contains some air. For the same reason a shriveled apple,
with the pressure of the air taken from it, will become plump and
fair, but will shrink at once to its shriveled state when the air
is admitted into the receiver.

161. =Elasticity of the Air.=--All the phenomena cited in § 158,
§ 159, and § 160 exhibit the elasticity of the air. It is from
this property that it is always disposed to expand. It will do
so whenever pressure is taken from it, or when it can overcome
pressure to which it is subjected. This property is most strikingly
exhibited when the air is much condensed by pressure. And the
greater the condensation the stronger is the expansive or elastic
force.

[Illustration: Fig. 109.]

162. =The Condenser.=--In Fig. 109 you have the plan of an
instrument called the Condenser. In A B, a cylinder, moves the
piston, P. Air is admitted to the cylinder at F, and into the
receiver, V, at G. The valve at F prevents any air from escaping
from the cylinder, and the valve at G prevents it from escaping
from the receiver. The operation of the instrument is this: If
the piston be pressed downward, the pressed air in the cylinder
shuts the valve F and opens G, and so enters the receiver V. If
now the piston be raised, air rushes in at F to fill the space
in the cylinder. It can not come from V, because the valve G is
shut by its pressure. By working the piston for some time you can
get a body of air into V of very great density. You see that this
instrument is the very opposite of the air-pump. In the receiver,
V, you have condensed air, while in the receiver of the air-pump
you have rarefied air. If you compare the two instruments you will
see that the opposite results are owing to different arrangement of
the valves.

[Illustration: Fig. 110.]

163. =The Gasometer.=--Gas is distributed in pipes from the
gasometer at the gas factory by the agency of the elasticity
occasioned by condensation under pressure. The apparatus, Fig. 110
(p. 122), consists of a large round vessel, G, open below, and sunk
in a larger vessel of water, _w_. We will suppose the vessel, G,
to be full of water. Gas is introduced into it through the pipe, _p
r_, the gasometer rising as it fills with the gas. P is a weight
balancing the gasometer, and so permitting it to rise as the gas
enters. The gasometer being filled, the gas is to be distributed.
For this purpose weights are put upon the gasometer, so that the
gas may be compressed. Under this pressure it by its elasticity
seeks for more room, and obtains it by escaping through the pipe
_o b c_. As the pressure on the gas needs to be regulated, there
is sometimes a gauge, _h i_, attached, which shows the amount of
the pressure. It is a bent tube with water in the bend. You see at
once that the greater the pressure upon the gas the higher will the
water be in the branch, _h_, of the gauge.

[Illustration: Fig. 111.]

164. =Air-Guns and Pop-Guns.=--These illustrate the elasticity of
condensed air. The air-gun is constructed in this way: A receiver,
like V, Fig. 109, is made so that you can screw it on and off
from the instrument. After being charged with condensed air it is
screwed upon the gun, its stem communicating with the barrel. In
order to discharge the gun there is a contrivance connected with
the trigger for raising the valve, G, so that some of the condensed
air may enter the barrel. On doing so, it by its sudden expansion
rapidly forces out the contents. The principle on which the common
pop-gun operates is the same. There is air confined between the two
corks, _a_ and _b_, Fig. 111 (p. 123). As the rod, R, is pushed
quickly in, the cork _b_ is carried nearer to _a_, so that the air
between them is condensed. With the condensation the expansive
force is increased; and when it becomes so great that the cork _a_
can no longer resist it, it throws the cork out, and so quickly as
to occasion the popping sound.

165. =Powder and Steam.=--The explosion of powder furnishes a
good illustration of the expansive force of condensed air or
gases. These gases are produced so suddenly from the powder that
at the instant they are in a very condensed state, and therefore
expand powerfully. So, also, steam has power in proportion to
its condensation. When formed under the confinement of a boiler,
on being allowed to escape it expands with great force. The
application of the expansive power of steam will be treated of
particularly in another part of this book.

[Illustration: Fig. 112.]

166. =Retardation by Condensed Air in Gunnery.=--When a ball is
fired it is constantly retarded in its flight by the resistance of
the air, for it has to push the air away on every side in order
to make its way through it. Of course, then, the more condensed
the air is the greater is the resistance. Now it is condensed air
that the ball is obliged to remove; for as it goes forward it,
by its rapid pressure, condenses the air directly before it. And
the more rapid is its flight the greater is the condensation, and
therefore the greater the resistance. Besides, the retarding effect
is increased by the tendency to a vacuum behind the ball. All this
can be made clear by Fig. 112. Let B be a ball going very rapidly
in the direction indicated by the arrow, the cloud representing
the condensed air before it, and the space included in the two
lines the vacuum behind it. It is obvious that the more rapidly the
ball goes the less readily is the air pressed out of the way, and
therefore the more it is condensed in front of the ball. At the
same time the more rapid is the ball the less readily does the air
close up behind it, and therefore the greater is the tendency to a
vacuum there. For these reasons there is more retarding influence
exerted by the air upon a ball in the first part of its course than
in its latter part.

[Illustration: Fig. 113.]

[Illustration: Fig. 114.]

[Illustration: Fig. 115.]

167. =Pressure of the Air on Liquids.=--If you plunge a tumbler
into a vessel of water, and turning it over hold it so that its
open part is just under the surface, it will remain full. The
reason is that the weight of the air pressing upon the surface of
the water in the vessel prevents the water in the tumbler from
passing downward. Now if you introduce a bent tube under the
tumbler, as in Fig. 113, and blow through it, the air that you
force up into the tumbler presses the water down, taking its place.
That is, the pressure of the air acts in opposition to the pressure
of the air outside upon the surface of the water in the vessel.
You take a jar, _a_, Fig. 114, and filling it with water, turn it
over with its open end downward, the water will remain in the jar.
You have here a representation of the pneumatic trough used by the
chemist in collecting gases. To fill the jar _a_ with gas he puts
the mouth of the retort from which the gas issues under the jar
_a_, and the gas passing upward expels the water, as the water is
expelled by the breath from the tumbler in Fig. 113. In Fig. 115
(p. 125) is represented an experiment which shows not only that
the pressure of the air sustains the column of water in the cases
cited above, but also that it makes no difference in what direction
this pressure is exerted. Take a large tube, _a_, closed at one end
and open at the other, and fill it even full with water. Place,
now, a piece of writing-paper over its mouth, and carefully invert
the tube, as seen in the figure. The paper will remain, and the
water will not run out. It is the pressure of the air that sustains
the water, and the paper only serves to maintain the surface of the
water unbroken. If the paper were not there the particles of the
air would insinuate themselves among those of the water, and pass
upward in the tube. You can try this experiment with a wine-glass,
and may even succeed with a tumbler. We see in these experiments
the reason that a liquid will not run from a barrel when it is
tapped, if there be no vent-hole above, unless there be so large
an opening made as to let the air work its way in bubbles among
portions of the liquid. It is this entrance of the air that causes
the gurgling sound in pouring a liquid from a bottle.

168. =Amount of Atmospheric Pressure.=--If, instead of the jar _a_,
in Fig. 114, you have a tube thirty-four feet high, and closed at
the top, situated as the jar _a_ is, it will remain full of water.
If the tube be longer the water will stand only at thirty-four
feet, leaving a vacuum above it. It makes no difference what the
size of the tube is; the result will be the same in all cases.[2]
That is, a column of water thirty-four feet high can be sustained
by the pressure of the atmosphere. It is easy, therefore, to
estimate the weight or pressure of the air. The pressure of the
column of water is found to be fifteen pounds to the square inch of
its base, and this, of course, is the amount of pressure or weight
of the atmosphere which it balances. Mercury is thirteen and a
half times as heavy as water, and therefore the air will sustain a
column of it only about thirty inches in height.

[Illustration: Fig 116.]

169. =Barometer.=--The weight of the atmosphere varies to some
extent at different times, and the barometer is an instrument
for measuring these variations. It is constructed on the
principles developed in the previous paragraphs. In Fig. 116 is
a representation of the instrument. A B is a glass tube about 34
or 35 inches long, closed at one end. It has been filled with
mercury, and then inverted in a cup of the same liquid, C. The
vacuum above the mercury is called the Torricellian vacuum, from
Torricelli, an Italian, who first developed the principles of the
instrument. The mercury generally, as stated in § 168, stands at
about the height of thirty inches. But it varies from this with the
weather. When the weather is bright and clear the air is heavier
than this, and, pressing upon the mercury in the vessel, forces it
up higher in the tube. But when a storm is coming the air is apt
to be lighter, and therefore pressing less strongly on the mercury
in the vessel, the mercury in the tube falls. The barometer is of
great service, especially at sea, in affording the sailor warning
of an approaching storm. An incident is related by Dr. Arnot which
strikingly illustrates its value in this respect. He was at sea in
a Southern latitude. As the sun set after a beautiful afternoon
the captain foresaw danger, although the weather was perfectly
calm, for the mercury in the barometer had suddenly fallen to a
remarkable degree. He gave hurried orders to the wondering sailors
to prepare the ship for a storm. Scarcely had the preparations been
made when a tremendous hurricane burst upon the ship, tearing the
furled sails to tatters, and disabling the masts and yards. If the
barometer had not been observed the ship would have been wholly
unprepared, and shipwreck, with the loss of all on board, would
have been the result.

A water-barometer could be made, but it would be an unwieldy thing,
for the tube must be over 34 feet long. Besides, it would not
answer in very cold weather, as the water would freeze. So short
a column of the heavy fluid, mercury, balances the weight of the
atmosphere that a barometer made with this is of very convenient
size; and then there is no danger of the mercury's freezing, except
in the extreme cold of the Arctic regions.

170. =Barometer a Measurer of Heights.=--The atmosphere, as stated
in § 152, diminishes regularly in density as we go upward. The rate
of this diminution has been accurately ascertained, and therefore
we can estimate heights by the amount of pressure on the mercury in
the barometer. At a height of 500 feet the barometer will be half
an inch lower than in the valley below. At the summit of Mont Blanc
it stands but half as high as at its foot, indicating a height of
15,000 feet. Du Luc, in his famous balloon ascension from Paris,
saw the barometer at one time standing at about twelve inches,
showing an elevation of 21,000 feet.

171. =Relation of the Air's Pressure to the Boiling Point.=--Water
heated to 212 degrees of Fahrenheit boils, that is, it becomes
vapor. Now if water be heated on the summit of a high mountain it
boils before it arrives at this degree of temperature. On the top
of Mont Blanc it boils at 180 degrees, that is, 32 degrees below
the boiling point of water at the foot of the mountain. This is
because the pressure of the air acts in opposition to the change of
water in to vapor, and the less the pressure is the less heat will
be required to vaporize the water. We may illustrate this influence
of the pressure of air upon boiling by the following experiment.
Let a cup of ether (which boils at 98 degrees) be placed under the
receiver of an air-pump. On rarefying the air by the pump the
ether will boil. The general effect of pressure upon boiling may
be prettily illustrated by another experiment. Boil some water in
a thin flask over a spirit-lamp. Blow out the lamp, and, corking
the flask tightly, let the boiling cease. If, now, you pour some
cold water over the flask the boiling will commence again with
considerable force. Why? Because you condense the steam which is
over the water by the application of cold, and thus take off the
pressure. Then, again, if, while the water is boiling, you pour hot
water over the flask, the boiling ceases, because the heat favors
the accumulation of steam, and therefore renews the pressure on the
surface of the water.

You can see from what has been stated that most liquids have the
liquid form because of the pressure of the atmosphere upon them.
If there were no atmosphere, ether, alcohol, the volatile oils,
and even water, would fly off in vapor; and the earth would be
enveloped in a vaporous robe, for the particles of the vapors would
be held to the earth by attraction, just as the particles of the
air are now, § 151.

[Illustration: Fig. 117.]

172. =Syphon.=--The pressure of air upon fluids is beautifully
exemplified in the operation of the syphon. This instrument is
simply a bent tube having one branch longer than the other. Its
operation is shown in Fig. 117. The tube having been first filled
with the liquid, has its shorter branch in the liquid of the vessel
A, which is to be emptied, and the other in the vessel B, which is
to receive the liquid. As you see it here, the opening of the long
branch is below the surface of the liquid in B. It is manifest,
therefore, that the air presses equally upon the surfaces in both
vessels, tending to support the fluid in the tube, just as the
fluid is supported in the jar in Fig. 114. But, notwithstanding
these equal pressures, the liquid runs up the tube from A, and down
its longer branch into B. Why is this? As the pressure of a column
of fluid is as its height, there is greater pressure or weight in
the longer branch than in the other; and it is this difference in
weight that causes the flow from A into B through the syphon. The
difference in the columns in the two branches is not the difference
in length of these branches, but the distance between the levels of
the fluid in A and B, that is, the distance from _a_ to _b_. The
operation, then, of the instrument is this. There is a constant
tendency to a vacuum at C, the bend of the tube, from the influence
of gravitation on the excess of fluid in the long branch over that
in the short one. This tendency is constantly counteracted by
the rise of fluid in the short branch, it being forced up by the
pressure of the air upon the surface of the fluid in A.

[Illustration: Fig. 118.]

If the syphon were so placed that the surface of the liquid in A is
precisely on a level with that in B, as represented in Fig. 118,
the liquid would remain at rest, for as pressure is as the height,
§ 121, and the pressures on the two surfaces are equal, there would
be an exact balance. But let the surface in B be in the least lower
than in A and the flow will begin. And the greater the distance
between the two levels the more rapid will be the flow, for the
greater will be the influence of gravitation in the long branch.

[Illustration: Fig. 119.]

Again, if the end of the long branch of the syphon be free, as in
Fig. 119 (p. 130), the syphon will operate in the same way, for
the air, pressing in all directions equally, tends to support the
column of fluid in the long branch by a direct upward pressure,
but is prevented from doing so by the excess of fluid in it above
what is in the shorter one. The operation of the syphon is commonly
represented in this way; but I have given first the arrangement in
Fig. 117, in order that you might more clearly see the principle of
the instrument.

[Illustration: Fig. 120.]

173. =Uses of the Syphon.=--The syphon is used chiefly for
discharging liquids from one barrel or vessel into another. For
convenience, it is often constructed after the plan of Fig. 120.
To the long branch, B C, is attached the tube ED. It is used in
this way: The end of the short branch, A, being introduced into the
liquid to be drawn off, you place your finger upon C, and after
filling the syphon by suction at E, you remove the finger and let
the liquid run. The syphon has sometimes been used to drain pits
and mines. It of course can never be used where the elevation over
which the tube is to bend is over 34 feet from the surface of the
water to be discharged, for then the air would not press the water
up to the bend of the syphon.

[Illustration: Fig. 121.]

174. =Cup of Tantalus.=--This cup, Fig. 121, has a syphon in it,
the short branch, _b_, opening into the cup, and the long branch,
_d_, having its outlet in the bottom. As you pour water into the
cup it will remain there until you pour enough in to cover the bend
of the syphon. As soon as this is done, the syphon being filled,
the water suddenly flows out from the outlet, _a_, of the long
branch.

[Illustration: Fig. 122.]

175. =Intermitting Springs.=--The operation of an intermitting
spring is essentially the same with that of the cup of Tantalus.
You have a representation of such a spring in Fig. 122. There is a
cavity in a hill, supplied with water from a passage above. There
is also a passage from it which takes a bend upward like a syphon.
Now when the water in the cavity is low it will not run out from
the syphon-like channel; but when the cavity becomes filled above
the level of the bend the water will at once flow out, just as it
does from the cup of Tantalus as soon as the bend of its syphon is
covered.

[Illustration: Fig. 123.]

176. =Pumps.=--In Fig. 123 you have a plan of a common pump. A
tube, C D, extends down into the well, W. Above this is the barrel
of the pump, A B, in which the piston works up and down. There is
a valve, F, in the piston, and another, E, at the bottom of the
barrel. Both of them open upward. We will suppose that the pump is
entirely empty of water. If, now, the piston descend, the valve E
shuts down, and F opens, letting the pressed air between the piston
and E pass upward. See what will happen when the piston rises.
The air above the piston can not get below, for its pressure will
shut the valve F. But there will be a tendency to a vacuum below
the piston as it rises, and the air will go up through the valve E
to fill up the space. But why does the air rise? Because of the
pressure of the air upon the surface of the water in the well.
This forces up in the pump the water and the air above it, just in
proportion as the downward pressure in the pump is lessened. If the
pumping be continued all the air will soon be expelled, the water
following it and flowing out at the opening, G. It is obvious that
the pump will be useless if the valve E be over 34 feet above the
surface of the water in the well, as the pressure of the atmosphere
will not sustain a column of water higher than this.

177. =Suction.=--In common language, the operation of the pump is
attributed to what is called a principle of suction, as if there
was a drawing up of the water. But the water, you see, is not
drawn, but forced, up. So it is with all operations of a similar
character. In sucking up a fluid through a tube the fluid is forced
up, because the pressure downward in the tube is removed. But how
is it removed? It is done by a movement of the tongue downward from
the roof of the mouth, thus causing a tendency to a vacuum, as the
upward movement of the piston in the pump causes this tendency
under it. To fill the space made by the movement of the tongue the
air is forced up the tube, the liquid following; and, as in the
case of the pump when the air is all expelled, the liquid will
begin to discharge into the mouth.

[Illustration: Fig. 124.]

178. =Forcing-Pump.=--The forcing-pump is constructed differently
from the common pump. Its plan is given in Fig. 124. It has a pipe,
C D, and a barrel, A B, like the common pump. It has also the valve
E at the bottom of the barrel. But it has no valve in the piston.
Connected with the barrel is another pipe, F G, from which the
water issues. This has a valve, H, opening upward. The operation of
the pump is obvious. As the piston is drawn up E opens and H shuts,
and when it is forced down E shuts and H opens.

[Illustration: Fig. 125.]

179. =Fire-Engine.=--The fire-engine has commonly two
forcing-pumps, with a contrivance for making the water issue in
a uniform stream. This contrivance can be explained in Fig. 125.
The discharging pipe, L M, extends down into a large vessel, I K,
which is filled with air. The uniformity of the stream depends upon
the elastic force of compressed air, as you will see if I explain
the operation of the machine. When the water is forced through the
opening H, it rises to the level N O, compressing the air in I K,
for the tube L M is too small to allow all the water to escape that
comes from the larger tube, H E. Now the moment that the piston
ceases to force the water through H, the elastic force of the
compressed air operates, shutting down the valve H, and forcing the
water up L M. The result, you see, is a continuous forcing up of
the water through this tube, and therefore a uniform stream.




CHAPTER X.

MOTION.


180. =Universality of Motion.=--The world is full of motion. The
rising and setting of the sun, the changes of the seasons, the
falling of the rain, the running of rivers into the ocean, the
ascent of water into the air by evaporation, the wind moving
in silence or rushing on in its might, are familiar examples of
motion constant and every where present. But with all this motion,
sometimes in conflict and often variable, order and regularity
reign. The causes of motion, though various in their operation, are
kept by the Creator from producing confusion and disorganization by
a few simple laws, which regulate the movements both of atoms and
of worlds. The principal of these causes I will now briefly notice.

181. =Causes of Motion.=--Attraction is the most universal of the
causes of motion in the universe. While it binds atom to atom, it
also binds system to system throughout the immensity of space; and
while it makes the stone fall to the ground, it moves the countless
orbs forever onward in their courses. It is this which causes
the tides to flow and the rivers to run down their <DW72>s to the
ocean, and thus by keeping up the never-ending motion of water all
over the earth in seas, lakes, rivers, and the millions of little
streamlets, diffuses life and beauty over the vegetable world,
and gives to man the vast resources which we see developed in the
numberless applications of water-power and navigation.

Heat pervades all matter, and is every where uniting its influence
with the other causes of motion. It is heat that produces all the
motions of the air, termed winds. It is heat that causes the rise
of the water all over the earth in evaporation, so that it may be
collected in clouds, again to descend to moisten the earth and keep
the ever-flowing rivers full. Heat applied to water gives to man
one of his best means of producing motion in machinery.

The agencies which Chemistry reveals to us are ever at work causing
motion among the particles of matter; and though they generally
work in silence, they sometimes show themselves in tremendous
explosions, and in convulsions of nature.

Busy life is every where producing motion, more especially in the
animal world. It gives to the myriads of animals, great and small,
that swarm the earth not only the power of moving themselves, but
also the power, to some extent, of moving the material world around
them.

182. =Action and Reaction Equal.=--When any of the causes of motion
act, the action is met by an opposite and equal reaction. If, for
example, a blow be given, an equal blow is received in return. For
this reason, if one in running hits his head against the head of
another both are equally hurt. When a child knocks his head against
a table, there is sound philosophy in the common saying that he has
given the table as good a blow as he has received, though it may
afford him no comfort. Many very interesting illustrations of this
law of motion suggest themselves, of which I will give a few.

[Illustration: Fig. 126.]

A swimmer, pressing the water downward and backward with his hands
and feet, is carried along forward and upward by the reaction
of the water. And in this case, as in every other, the greater
the action the greater is the reaction; in other words, the more
strongly he presses with his hands and feet, the more rapidly is he
borne along by the reaction of the water against the pressure. A
boat advances in proportion to the force with which the oars press
against the water. So the rapidity of a steamboat depends on the
force with which the paddles drive the water astern. Birds rise in
the air by the reaction of the air against their wings as they are
pressed downward. A sky-rocket pursues its rapid flight because
a large quantity of gaseous matter issues from its lower end,
and, being resisted by the reaction of the air, by its pressure
throws the rocket upward. So if a ship fire guns from the stern
its advance will be accelerated, but if from the bow it will be
retarded. When a broadside is fired the ship inclines to the other
side. In Fig. 126 (p. 136) is represented the plan of Barker's
mill. It consists of a cylinder, _c_, arranged in a frame in such
a way that it can revolve on the point upon which it rests. Water
runs into it by a tube, _p_, and escapes by the branches, _a_ and
_d_. These are so arranged that the reaction upon the issuing
water makes the cylinder revolve rapidly, causing the ends of the
branches to whirl around as indicated by the dotted lines and
arrows.

If a spring be compressed between two equal bodies, it will throw
them off with equal velocities. If they are unequal, the velocity
of the smaller body will be greater than that of the larger, and in
proportion to its smallness. For this reason, when a ball issues
from a cannon, though the cannon and the ball are equally acted
upon by the elastic or expansive force of the gases set free by
lighting the powder, the gun is moved but very little because the
force is diffused through so large a mass, while the ball being
so much smaller moves with great velocity. When a volcano throws
stones from its crater the earth may be compared to the cannon, the
stones to the ball, and the explosive materials throwing the stones
to the exploding powder projecting the ball. As the cannon is moved
as much as the ball, so is the earth moved as much as the stones,
the only reason that it does not move as far and as rapidly as the
stones being that the force is diffused through so large a bulk.
These examples illustrate very well the relation of action and
reaction, for whenever there is an action of one body upon another
it is as if a spring were between the two bodies, acting equally
upon both. When a man jumps from the ground it is as if a spring
were compressed between him and the earth, and this expanding moves
the earth exactly as much as it does the man. He really kicks the
earth away from him. The motion of the earth is not obvious because
it is diffused through so large a mass. The case is parallel to
that of the ball and cannon. The same force is exerted upon the
man and the earth, but the man, like the ball, moves the most, and
in proportion to his comparative smallness. So when a bird hops
from the ground, the earth moves as really as the bird. If the
bird hop from a twig, you perceive that the twig is moved by the
pressing down of the bird as it rises. When it starts from the
ground it exerts the same downward pressure, and moves the earth as
really as in the other case it did the twig.

183. =Inertia Shown in the Communication of Motion.=--What is meant
by the inertia of matter you have already learned in § 48. This
property is exemplified in the communication of motion to any body,
or, in other words, in setting it in motion. Of this I will give
some illustrations. When the sails of a vessel are first spread to
the wind the vessel does not move swiftly at once, for some time
is required for the force applied to overcome the inertia of so
large a mass, and to put it in rapid motion. Horses make a greater
effort to start a load than they do to keep it in motion after it
is started. If one be standing up in a carriage, and the horses
start off suddenly, he falls backward, because his body, from its
inertia, does not readily and at once partake of the motion of the
carriage. If a person start forward quickly with a waiter filled
with glasses in his hands, the glasses will slide backward. So if a
person start quickly from his chair with a cup of tea in his hand,
the tea will be thrown backward upon him.

You see from the foregoing illustrations that it requires some time
to communicate motion to any body. I will give some illustrations
of this fact of a more striking character. If a ball be thrown
against an open door it will move the whole door, and perhaps
shut it; but the same ball if fired will pass through the door
without moving it perceptibly. In the latter case its velocity is
so great that there is not time enough to communicate motion to
the whole door, and it moves only that part of it with which it
comes in contact. A bullet thrown with but little force against a
window will crack a whole pane of glass; but if shot from a pistol
it merely makes a round hole. So, also, a cannon-ball having a
great velocity may pass through the side of a ship, doing perhaps
comparatively little damage, while one moving with much less
velocity may do vastly more damage by splintering the wood to a
considerable extent. For the same reason a rapid ball hitting a
person may occasion less suffering and do less harm than a slow
ball; for a rapid ball kills merely the parts which it touches,
leaving the flesh around in a sound state, while the slow ball
bruises over a large space. If a large pitcher filled with some
heavy liquid be quickly taken up the handle will break, leaving
the pitcher behind. Large dishes are often broken in this way when
heavily loaded.

184. =Inertia Shown in the Disposition of Motion to Continue.=--Of
this I will cite some illustrations. As in the case of the ship,
in the first illustration in § 183, it takes time to communicate
motion to the whole ship, or, in other words, to overcome its
inertia, so when the ship is once in rapid motion it does not stop
suddenly when the sails are taken down, but its inertia tending
to keep it moving is gradually overcome by the resistance of the
water. If one be standing up in a carriage in motion, and the
horses suddenly stop, he will be thrown forward, for his body has
a motion in common with the carriage, and from inertia is disposed
to go on when the carriage stops. When you strike your foot against
any thing to get the snow off, you give the foot and the snow a
common motion together, then arresting the motion of the foot,
the snow from inertia passes on. The same thing is illustrated in
striking a book against any thing to get the dust off. If a ship
strike upon a rock every thing on board which is loose is dashed
forward. The earth as it revolves on its axis has a velocity at
the equator of about 1000 miles an hour. If this revolution should
be suddenly arrested every thing loose on its surface, having
acquired the motion of the earth, would be at once thrown eastward,
just as the furniture, etc., on board ship are dashed forward
when it is stopped by running against a rock. All the houses,
and monuments, and structures of every kind would fall prostrate
eastward. All the cities on our Atlantic coast would be plunged
into the ocean; and while the waters would leave the western shores
of the Atlantic, they would overflow its eastern shores, and deluge
the continent of Europe, as water in a vessel on board a ship that
had struck an obstacle would be thrown forward over its side.

[Illustration: Fig. 127.]

185. =An Equestrian Feat.=--In the feat represented in Fig. 127 the
only exertion made by the rider is to raise himself sufficiently to
pass over the cord, and he comes down again upon the horse's back,
simply because of the motion which he has in common with the horse,
his feet going in the path represented by the dotted line. If he
should attempt to throw himself forward, as in leaping from the
ground, he would go too far, and perhaps strike upon the horse's
neck instead of his back. Skill in jumping from a moving carriage
consists in making proper allowance for the forward motion which is
had in common with the carriage. Most persons are apt to overdo the
matter, and so come to the ground prostrate, and with more violence
than is necessary.

186. =A Case in Court.=--A dashing young man driving a light
phaeton ran against a heavy carriage. His father was induced by his
son's representations to prosecute the driver of the carriage for
driving too fast. A knowledge of motal inertia very readily decided
the case. The son and his servant both declared that the shock of
the carriage was so great against the phaeton that they were thrown
over the horses' heads. They thus proved themselves guilty of the
fast driving, for it was their own rapid motion that threw them
out when the phaeton was stopped by running against the carriage.
The following case is a parallel one. If two boats, the one of
large size sailing slowly up stream, the other a small one sailing
rapidly down, run against each other, a man standing in the bow of
the one going down will be thrown much farther forward than one
standing in the bow of the other.

187. =Course of Bodies Thrown into the Air.=--It results from the
principle that I have illustrated that when any body, as a stone,
is thrown, as we say, straight upward, it does not, in reality, go
up or come down perpendicularly. If it did it would come down at
a great distance from us. Suppose it takes two seconds for it to
go up and to reach the ground. If we are at the equator, in that
two seconds we move from the point where we threw up the stone
nearly 3000 feet eastward, and therefore if the stone rose and
fell perpendicularly it would fall 3000 feet westward of us. Why,
instead of this, does it fall at our feet? Because that when thrown
into the air it has not only the upward motion given by the hand,
but also the forward motion of the earth. It is a case similar to
that of the rider in Fig. 127, the horse representing the surface
of the earth and the rider the stone. For the same reason a man on
board of a steamboat, though it move fifteen miles an hour, tosses
up his ball or orange and catches it as well as if he were on land.
This he could not do if both he and his orange did not have the
same forward motion that the boat does. So, also, if a man fall
from a mast-head he reaches the deck at the foot of the mast when
the vessel is sailing rapidly, just as he would if it were lying
still at the wharf. If he did not by inertia retain the forward
motion which he had in common with the vessel he would fall at
some distance behind the mast.

188. =The Earth and the Atmosphere.=--The air being held to the
earth by attraction, § 151, it has a motion in common with the
earth. It revolves with the earth just as the tire of a wheel
revolves with the wheel. This being so, our winds are nothing
but slight variations of this constant rapid whirl of the aerial
coating of the earth. If the atmosphere were suddenly to stop
whirling round with the earth we should move through it with a
velocity of 1500 feet a second; and the destructive effect upon
us would be the same as it would were the earth standing still
while the air moved over its surface with this fearful velocity. A
wise man, not reflecting that the atmosphere moved with the earth,
proposed rising in a balloon, and waiting till the country to which
he wished to go should be passing under him.

189. =Motion and Rest.=--Though we use the term rest in opposition
to motion, it is obvious from some of the illustrations given that
rest is merely a relative term, for not a particle of matter in
the universe is at rest. Though when we are sitting still we call
ourselves at rest, we are moving every hour, by the revolution of
the earth on its axis, 1000 miles eastward, and 68,000 miles in
our annual journey round the sun. Why, then, are we so insensible
to these rapid motions? It is partly because the motions are so
uniform, but chiefly because all things around us, our houses,
trees, and even the atmosphere, are moving along with us. If we
were moving along alone, even at a slow rate, while all these
objects were standing still, we should be conscious of our motion,
as we are when, as we ride along in a carriage, we see the objects
at the road-side not moving along with us.

190. =A Comparison.=--The above can be made more clear and
impressive by a familiar comparison. A man on board of a steamboat,
by confining his attention to things within the boat, may, after a
while, be almost unconscious of the boat's moving, if the water be
smooth, though the boat may be going at the rate of fifteen miles
an hour. If he be reading in the cabin he will think as little of
his motion as he would were he reading in his parlor at home. If
he should be blindfolded, and turned around a few times, it would
be impossible for him to tell the direction in which the boat is
going. Now it is with a man on the earth as it is with the man in
the boat. He is unconscious of the motion of the earth for the
same reason that the man in the boat is unconscious of the boat's
motion. All objects around him are moving along with him, as the
objects around the man in the cabin of the boat are moving along
with him. We can carry the parallel farther. While the man sits in
the cabin he knows not how fast the boat moves, nor even whether
it moves at all. He must look out to decide this, and even then
he may not be able to tell whether the boat moves, or whether he
merely sees the water running by it. We often are actually deceived
in this respect. A steamboat struggling against wind and wave
may appear to those on board to be advancing when it is really
stationary, or even when it is losing ground. So when we look at
the sun we know not whether it is the sun or the earth that is
moving. Mere vision, without reasoning on the subject, leads one
to think that it is the sun that moves. For the same reason, if a
child should be placed in a carriage for the first time without
seeing the horses, but with its eyes fixed on objects at the
road-side, he would probably think that all the fences and trees
and rocks and houses are in motion.

191. =Absolute and Relative Motion.=--The motion of a body is said
to be _absolute_ when it is considered without relation to the
position of any other body. Its motion is said to be _relative_
when it is moving with respect to some other body. Absolute rest
is unknown, for no body in the universe is known to be without
motion. But a body may be relatively at rest, that is, in a fixed
relative position to other bodies. Every body is in a state of
absolute motion, and yet it may be in a state of relative rest. All
objects that appear to us to be at rest have a very rapid absolute
motion. They appear to be at rest merely because they have the same
rapidity and direction of absolute motion that we have ourselves.
And all the motions which are apparent to the eye are only slight
differences in the common absolute motions, of which, though they
are so exceedingly rapid, we are entirely unconscious. Thus, if
I stand still, and another at my side walks at the rate of three
miles an hour eastward, we both of us have a common absolute motion
of 1000 miles in every hour, and he merely adds three miles to his
thousand--I move 1000 miles, and he 1003. So if I sit still in my
parlor, and my friend travels eastward at the rate of 20 miles an
hour, I move every hour 1000 miles, and he 1020. And if he travel
westward at this rate he really travels slower than I do--he has an
absolute motion eastward of 980 miles, and I of 1000. At the same
time we are both whirling on in our annual journey around the sun
at the rate of 68,000 miles an hour.

192. =Obstacles to Motion.=--As motion is naturally disposed
to continue (§ 49 and § 184), whenever it is stopped it does
not spend itself, but is stopped by obstacles. The principal of
these obstacles are: gravitation; the resistance of opposing
substances--solids, liquids, and gases; and friction. When a stone
is thrown into the air its upward motion is gradually destroyed by
the attraction of the earth and the resistance of the air. Observe,
now, why it descends. It is from the action of one of the causes
which arrested its upward flight--the attraction of the earth. In
its descent it is retarded by the resistance of the air, as it was
in its ascent. This retardation is very obvious in the case of
substances which present a large surface to the air, as a feather.
A small piece of lead will outweigh many feathers, and therefore,
as its quantity of matter is so much greater in proportion to its
surface than that of a feather, it will fall to the ground much
more quickly. That this is owing wholly to the resistance of the
air can be proved with the air-pump.

[Illustration: Fig. 128.]

Suppose that you have a tall receiver, Fig. 128, on the air-pump,
and a piece of lead and a feather are placed at its upper part
in such a way that they can be made to fall at the same instant.
Exhaust the air, and then let them fall. They will go down side
by side, as represented by the figure, and reach the bottom of
the receiver at the same time, because there is no air there to
resist the progress of the feather. The toy called the water-hammer
illustrates the same thing. When water falls through the air the
resistance of the air tends to separate its particles, as we see in
the falling of water thrown up by a fountain. In the water-hammer,
which is a closed tube containing a little water and no air, when
the water is made to fall from one end to the other, as there is
no air to divide it, it falls as one mass, and gives a sharp sound
like the blow of a hammer. An instrument essentially like this can
be made with a thin glass flask. Put a little water in it, and,
after heating it to boiling over a spirit-lamp, cork the flask
tightly, and then leave the water to cool. As all the space above
the water was filled with steam when the flask was corked, it is a
vacuum now that the steam is condensed.

[Illustration: Fig. 129.]

193. =Relation of Bulk to the Resistance of Liquids and
Gases.=--You have already seen, in § 192, that the more surface a
body has in proportion to its weight the greater is the resistance
of the air to its motion. This truth, which applies to liquids
as well as to airs or gaseous substances, explains the fact that
small bodies meet with proportionately more resistance than large
ones. The body B, Fig. 129, you see is made up of eight cubes of
the size of the cube _a_, that is, it has eight times the quantity
of matter that _a_ has. Now if B were moving through air or water,
any of its sides pushing the water before it would meet with only
four times the resistance that a side of _a_ would, for its surface
is only four times as large, and yet the body is eight times as
large as _a_. And the greater the difference of size the greater
is the difference of resistance. If B were a cube twenty-seven
times as large as _a_ it would meet with only nine times as much
resistance. You see here the reason that shells and cannon-balls
can be thrown much farther than bullets and small shot. The
sportsman does not throw away his shot by foolishly aiming at birds
at great distances, and yet shells and large cannon-balls can be
thrown the distance of several miles. The difference is not in the
degree of velocity which the powder produces, but in the resistance
of the air. It is for the same reason that rain falls with greater
rapidity than drizzling mist.

As liquids and aeriform substances resist solids in motion in
proportion to the amount of surface which the solids present to
them, so also when they strike against solids they cause motion
in them in proportion to the amount of surface acted upon. Thus a
violent wind could not move a lump of tin, but could blow along
a sheet of it, or tear up a roofing of it if it got beneath. So
clouds of sand are raised into the air in the deserts of Africa,
although the particles are of the same material as stones, and
therefore have the same specific gravity. For the same reason dust,
feathers, the down and pollen of flowers, etc., are blown about,
although they are heavier than the air. A pebble is moved more
easily by a current of water than a stone, because it has a larger
surface, in proportion to its weight, to be acted upon by the
water. For the same reason sand is moved more easily than pebbles,
and fine mud than sand, though stones, pebbles, sand, and mud
may all be of the same material. This explains why you will find
mud where the current is slow, sand where it is faster, pebbles
and stones where it is still faster, and where the current is
exceedingly rapid you find nothing but large rocks--sand, pebbles,
and stones not being able to resist its force. For the same reason,
in the process of winnowing, the chaff is carried away by the wind;
while the grain, presenting less surface in proportion to its
weight to be acted upon by the air, falls to the floor.

In all the above cases the moving water or air may be considered
as acting in opposition to the attraction of the earth, the latter
pulling the substance down to the earth, and the former pushing
it away from the earth. Of course, the more surface the water
or air has to push upon the greater is the effect; and it is to
be remembered that the attraction of gravity is as the quantity
of matter, without any regard to amount of surface in the body
attracted.

[Illustration: A B C D
Fig. 130.]

194. =Relation of Force to Velocity.=--It would seem at first
thought that the motion produced in any body must be in exact
proportion to the force producing it; that is, that twice the force
which produces a given velocity would double that velocity, and
three times would treble it, etc. This is true where there are no
obstacles to motion, as in the case of the heavenly bodies moving
in their orbits. But in all motions here upon the earth there are
obstacles; and as reaction is always equal to action, the greater
the velocity the greater is the reaction of the obstacle. If,
therefore, you increase the velocity of any body, you not only have
to communicate more motion to it, but you must overcome also the
increased reaction. The rate of increase of force for increased
velocities has been very accurately ascertained. This I will
explain. A boat moving from B to A, Fig. 130, we will suppose,
displaces a quantity of water represented by the space between
the two lines extending from B to A. Now if it move from B to C,
it displaces twice the bulk of water B C; and as it is displaced
in the same time that B A was, each particle is displaced with
twice the velocity. Double the force is required to displace a
double portion of water, and to do this with double the velocity
the force must be doubled again. So if the boat is made to move
three times as far in the same time, that is from B to D, three
times the quantity of water is displaced, and each of these three
portions, B A, A C, and C D, is displaced with three times the
velocity. The force required, then, to do this is nine times that
required to carry the boat from B to A in the same time. It is
plain, therefore, that with velocities represented by the numbers
1, 2, 3, 4, etc., the forces requisite to produce these velocities
must be as the squares of these numbers; viz., 1, 4, 9, 16, etc.
This law is a very important one in a practical point of view. For
example, it shows us how much larger a quantity of coal is required
to produce in steamboats a high velocity than a moderate one. Its
application too to the science of gunnery is important.

195. =Relation of Shape to Velocity.=--The resistance of air or
water to a flat surface is greater than to a convex one, because
the latter readily turns the particles to the one side and the
other. So, also, a concave surface is resisted much more than a
flat one, because the particles of the air or water can not so
easily escape sideways. Fishes are of a spindle-like and slender
shape, that they may have as little resistance as possible from the
water. It is for this reason that a fish has no neck, for if it
had one the upper portion of its body would, from the resistance
of the water striking against it, prove a serious impediment to
rapidity of motion. Mankind have in some measure imitated the shape
of fishes in their boats and ships. Boats which are intended to
bear light burdens and go swiftly are made very long and narrow.
The webbed feet of water-fowls, when they are moved forward, are
folded up so as to meet with as little resistance as possible; but
when they are moved backward they are spread out so as to press
against the water a broad concave surface. For the same reason the
wings of a bird are made convex upward and concave downward; and
when it moves its wing upward it makes it cut the air somewhat
edgewise, but in moving it downward it presses directly with the
whole concave surface.

196. =Friction.=--Friction is generally an obstacle to motion.
When we roll a ball, the more rough is the surface on which we
roll it the greater is the friction and the sooner is the ball
stopped. Friction lessens the rapidity of motion in machinery, and
to prevent this as far as possible oiling and other expedients are
employed. But sometimes friction is a cause of motion, as, for
example, the friction of the driving-wheels of a locomotive upon
the rails. In this case the wheel pushes backward on the rail at
each successive point of contact. To make this clear, suppose a
common wheel is deprived of its rim and is made to revolve on the
ends of its spokes. The end of each spoke gives a backward push
as it strikes the ground. Now the rim of a wheel makes the same
pushes, but they are more numerous--they are continuous, being made
by all the successive points in the rim. Sometimes the rails of a
railroad are too smooth from frost or some other cause, and then
sand is thrown upon them to give the locomotive a start. The sand
serves to prevent the wheels from sliding by enabling them to get
some hold upon the rails in their backward pushes.

197. =Friction of Liquids in Tubes.=--So easily does water flow
along that we should not at first view suppose that it would be
delayed much from friction as it passes through pipes or along
channels. But the retarding influence is considerable. An inch tube
200 feet long, lying horizontally connected with a reservoir, will
discharge water not one quarter as fast as an inch orifice in the
side of the reservoir. Sudden turns in a pipe should be avoided,
because they occasion so much friction against the sides of the
pipe and among the particles of water by disturbing the regularity
of the current. In the entrance of the arteries into the brain,
in order to prevent the blood from flowing too rapidly into this
organ, there are sudden turns in the arteries to <DW44> the blood;
and in grazing animals, as there is special danger that the blood
will flow too freely to the brain as the head is held down in
eating, there is a special provision to prevent this in a net-work
of arteries. If the arteries of the brain in such animals were
straight tubes they would continually be dying of congestion of the
brain or of apoplexy.

[Illustration: Fig. 131.]

Friction in a small pipe is greater in proportion to its size than
in a large pipe. In a pipe an inch in diameter water will not move
more than one-fifth as fast as in a tube two inches in diameter.
This may be made clear by Fig. 131, in which is represented the
area of a small tube inside of the area of a tube of twice its
diameter. Suppose the effect of the friction in the large tube to
extend in to _a_. In the small one it will extend in as far, that
is, to _b_. But _e a_ is about five times as long as _e b_, so that
there is full five times as much water clear of friction in the
large tube as there is in the smaller one.

[Illustration: Fig. 132.]

198. =Friction in Streams.=--The retarding effect of friction is
very obvious in brooks and rivers. The water in the middle of a
stream runs much more rapidly than it does near its banks. When a
river is very shallow at its sides the water there scarcely moves,
though in the middle the water may be running at a rapid rate. A
tide, therefore, flowing up a river, moves more freely near its
banks than it does in the middle of the stream, because it meets
with less resistance there from the downward current. Water moves
less rapidly at the bottom of a river than it does at the surface.
For this reason, if a stick be so loaded at one end as to stand
upright in water, in the current of a river its upper end will be
carried along faster than its lower end, and therefore it will
incline forward, as in Fig. 132. As the sea rolls in over a beach,
each wave at length pours over its crest and breaks, because the
lower part of the wave is retarded by friction on the beach. Were
it not for the constant retardation of friction at the sides and
bottom of rivers, and at their bends, those rivers which have their
rise at a considerable height above the level of the sea would
acquire an immense velocity. Thus the Rhone, drawing its waters
from 1000 feet above the level of the ocean, would pour them forth
with the velocity of water which had fallen perpendicularly the
same height, that is, at the rate of 170 miles an hour, did not
friction continually diminish the velocity.

199. =Waves.=--Waves are generally formed by the friction of air
upon water. Observe how they are formed. As soon as any portion
of water is raised above the general surface it tends by gravity
to fall to a level with the water around it, and in doing so the
portion next to it is forced upward, forming another wave; and
so one wave produces another, each one being smaller than the
preceding, till at length the motion is wholly lost. This is always
the process when the cause of the motion is a single impulse, as
when a stone is dropped into the water. But when the waves are
produced by a succession of impulses, as when wind makes them,
they are mostly of the same size. It is quite a common notion that
the water moves as rapidly as the waves appear to do; but the water
really remains nearly stationary, rising and falling, while merely
the form of the wave advances. The same wave is made up continually
of a succession of different portions of water, or rather it is a
succession of different waves. This is very well illustrated by the
waving of a rope or carpet. In an open sea a wave <DW72>s regularly
on either side; but when it comes near the shore, for the reason
given in § 198, it grows more and more nearly perpendicular on the
side toward the shore, till at length it falls over, and if it be
very large the roar thus caused by its breaking is heard to a great
distance.

200. =Height of Waves.=--"So awful," says Dr. Arnot, "is the
spectacle of a storm at sea that it is generally viewed through a
medium which biases the judgment; and lofty as waves really are,
imagination pictures them loftier still. Now no wave rises more
than ten feet above the ordinary sea-level, which, with the ten
feet that its surface afterward descends below this, gives twenty
feet for the whole height from the bottom of any water-valley to an
adjoining summit. This proposition is easily verified by a person
who tries at what height on a ship's mast the horizon remains
always in sight over the top of the waves--allowance being made for
accidental inclinations of the vessel, and for her sinking in the
water to much below her water-line, at the time when she reaches
the bottom of the hollow between two waves. The spray of the sea,
driven along by the violence of the wind, is of course much higher
than the summit of the liquid wave; and a wave, coming against an
obstacle, may dash to a great elevation above it. At the Eddystone
Light-house, when a surge breaks which has been growing under a
storm all the way across the Atlantic, it dashes even over the
lantern at the summit."

201. =Momentum.=--The momentum of a body is its force when in
motion. In estimating the momentum of any body two things must be
considered--its velocity, and its quantity of matter or weight.
A bullet fired from a gun has a vastly greater force, or power
of overcoming obstacles, than one thrown by the hand, from its
greater velocity. Now suppose the weight or quantity of matter to
be increased ten times, and that it moves with the same velocity
as before, it will have ten times as much force as before, and
will overcome ten times as great an obstacle. For this reason a
small stone dropping upon a man's head may do but little harm,
while one ten times as large, falling from the same height, may
stun and perhaps kill him. But if the large stone could fall with
only one-tenth of the velocity of the small one, the effect of
both would be the same. Let this example illustrate the rule for
calculating the momentum of moving bodies, viz., multiply the
quantity of matter into the velocity: Let the weight of the small
stone be 1 ounce, and that of the large one 10 ounces. If they fall
from a height of 16 feet the force with which the large one will
strike will be expressed by 160 (16×10), that of the small one by
16 (1×16). Suppose, now, that by some force in addition to gravity
the small one could be made to move ten times as fast as the large
one, the force with which it would strike would be equal to that of
the large one, and would be expressed by the number 160.

[Illustration: Fig. 133.]

I will illustrate this in another way. Let _a_ and _b_, Fig. 133,
be two balls of clay of equal size hanging over a graduated arc.
Now if _b_ be let fall from the top of the arc, 6, on striking
against _a_ it gives half of its motion to _a_, and they both move
on together. But how far will they go? To 3, on the other side of
the arc. Why? Let the quantity of matter in each ball be called
1, and the motion of _b_ 6. The momentum will therefore be 6.
Now the momentum of the two together will be the same after the
blow as that of _b_ was before it. But the quantity of matter is
twice as great, and must be called 2. Therefore the motion must be
represented as 3, to make the momentum 6 (2×3). But suppose that
_b_ is twice as large as _a_. Falling from 6, its momentum would
be represented by 12 (2×6). After it has struck _a_, the momentum
of the two together would be the same as that of _b_ before the
stroke; but the quantity being 3, the motion would be represented
by 4. They would therefore move to 4 on the arc.

202. =Examples.=--A few examples illustrating momentum as being
compounded of quantity of matter and velocity will suffice. If a
musket-ball of an ounce weight were so much spent as to move with
only a velocity of a foot in a second, its force would be so small
that if it hit any one it would do no harm. But a cannon-ball
weighing a thousand ounces moving at this slow rate would have
a very great force--equal, in fact, to the momentum of an ounce
ball moving 1000 feet in a second.--If a plank push a man's foot
against a wharf he will scarcely feel it; but if the plank, instead
of being alone, is one of a thousand planks fastened together in
a raft, and the whole move with the same velocity, the force will
be increased a thousand-fold, and the plank will crush the foot.
So, also, if the one plank when alone should move a thousand times
as fast as the whole raft, the same result would follow.--So soft
a substance as a candle can be fired through a board from the
momentum given to it by an immense velocity.--Perhaps there is
no better example of the great force given to a substance by an
enormous velocity than we have in the wind. So light a thing is
air that people think of it as almost nothing. But let it be set
in rapid motion, and the velocity gives to it a force, a momentum,
which will drive ships upon the shore, throw over buildings,
and tear up trees by the roots. In this last example we see
beautifully illustrated the meaning of the expression quantity of
motion. In the moving air each particle does its share of the work
in the destructive effects mentioned. Each particle, therefore,
may be considered as a _reservoir_ of motion, and the quantity of
motion in any case depends upon the quantity which each particle
has and the number of the particles.

203. =Production of Great Velocities.=--When there are no obstacles
to motion great velocities may be produced by a single impulse.
Thus at the beginning the Creator gave a single impulse to each of
the heavenly bodies, producing enormous velocities, which continue
unaltered year after year and age after age, because these bodies
fly in their orbits through space where there is no resistance of
any thing like air to <DW44> the motion. But in all the motions
that we see around us there are obstacles continually retarding
them; and therefore no very rapid motion is produced by any single
impulse, but a succession of impulses is required to accumulate
sufficient momentum so to overcome the obstacles as to secure a
great velocity. I will give a few examples in illustration. One
of the best examples we have in the fall of bodies to the earth.
You know that the greater the elevation from which a body falls
the greater is its velocity, and therefore the greater the force
with which it strikes. Why is this? If it fell because of a single
impulse making it go toward the earth, this would not be the case,
and if there were no air in the way the velocity would be uniform;
but the resistance of the air would <DW44> the velocity, so that
if a number of bodies should receive the same impulse at different
elevations, the one the farthest off would be the most retarded,
and therefore come down slower than all the rest. In this case,
the higher the elevation from which a man should fall the less
would be the injury. But a body does not come to the ground by
a single impulse, but by a succession of impulses, or rather a
continued impulse. Every moment that the body is coming down it
is drawn upon by the attraction of the earth, and this continued
action of the cause of the motion makes it continually increase in
rapidity. It is on the same principle of continued action that a
man lifts his hammer high when he wishes to inflict a heavy blow.
In this case both gravitation and the muscular power of the arm
exert their force on the hammer through the whole space. A horse
in kicking does the same thing, and by the great length of the leg
the velocity given to the foot by this continued action of the
muscles is very great. An arrow is not shot by a single momentary
impulse of the bow-string, but the string, by following it through
a considerable space, gives it a continued impulse. The action
of gunpowder upon a bullet issuing from a gun is apparently an
instantaneous and single impulse, but it is not really so. The
great velocity given to the bullet is given to it by the continued
impulse of the expansive force of gases produced from the powder,
and it therefore depends much on the length of the barrel. If this
be short, the force of the powder is not confined long enough to
the bullet to give it a great velocity.

204. =Arrest of Great Velocities.=--As a continued force is
required to produce great velocities, so a continued resistance
is necessary to arrest them. It is by the gradual or continued
resistance of the air that the motion of a cannon-ball is
destroyed. Now if instead of this gradual resistance any hard
substance, as a block of granite, were opposed to the progress
of the ball, it would be at once broken asunder. We see then the
reason that a hard substance of moderate thickness does not offer
so effectual a resistance to a body moving very rapidly as some
substance of a more yielding kind and of greater bulk. For example,
a bale of cotton will arrest a ball which would pass through a
plank, for the cotton yielding easily permits the force of the
ball to be felt and resisted by a larger bulk, while the wood,
not yielding, opposes but a small portion of its whole bulk to
the force of the ball, and therefore does not arrest it; in other
words, the momentum of the ball is communicated to a much larger
quantity of matter in the cotton than in the wood. These principles
afford a ready explanation of a feat which is sometimes performed.
A man lies upon his back, and, having an anvil carefully placed
upon his chest, allows some one to strike a heavy blow with a
hammer upon the anvil, and no injury is received. Why? Because the
momentum or force of the hammer is diffused throughout the bulk of
the anvil, and then again through the bulk of the yielding chest.
The man takes good care to have his lungs well filled with air at
the moment of the blow, for this increases the bulk and elasticity
of the chest, and thus promotes the diffusion of the momentum. If
the blow of the hammer were received directly upon the chest great
injury would be done, for the force would now be spent upon one
small spot alone.--The principles above elucidated are applied
by men instinctively in their common labors and efforts. You see
a man catching bricks that are tossed to him. As he receives
the bricks into his hands he lets his hands and the bricks move
together a little way, so that he may gradually arrest the motion
of the bricks. To do it suddenly would give him a painful lesson
on momentum. So when a man jumps from a height he does not come
to the ground in a straight position. This would cause a sudden
and therefore a painful arrest of the motion of the whole body.
To avoid this he comes to his feet with all the great joints of
his body bent, so that the different portions approach the ground
successively, his head having its motion arrested last.

[Illustration: Fig. 134.]

205. =Communication of Motion in Elastic Bodies.=--Momentum is
transferred from one body to another very differently in elastic
from what it is in non-elastic bodies. As you saw in § 201, when
one non-elastic body strikes upon another the momentum is divided
between them, and both move on together. Now if _a_ and _b_, Fig.
133, were elastic bodies, as ivory balls, and _b_ should be let
fall against _a_, it would give all its momentum to _a_. Therefore
_b_ would stop, and _a_ would move on to the same height from
which _b_ came. The reason is, that the velocity lost by _b_ and
received by _a_ is just double what it would be if the balls were
non-elastic. For the same reason, if _a_ and _b_, being elastic,
meet each other from equal heights on the arc, they will both
rebound, and return to the same heights from which they came. But
if non-elastic they simply destroy each other's momentum and stop.
The effect produced in the former case is just twice as great as
in the latter, as you may see by reckoning on the arc. For the
same reason, too, if you have a row of elastic balls, as in Fig.
134, and let _a_ fall from the point _i_ upon _b_, it will stop
there; and communicating all its momentum to _b_, this momentum
will pass from _b_ to _c_, and so on through all the row of balls
to _e_, the last one, which will fly off to the point _h_, at the
same height with _i_, the point from which _a_ fell. If _b_ be held
still, and _a_ be let fall upon it, _a_ will rebound to the height
from which it fell, for then the compressed elastic spring (§ 39)
of each ball, as _b_ is immovable, communicates all the motion to
_a_. It is for this reason that an elastic ball, on being thrown
against any thing fixed, rebounds. If what it is thrown against be
perfectly elastic it rebounds with a force equal to that with which
it is thrown.

[Illustration: Fig. 135.]

206. =Reflection of Motion.=--If an elastic body be thrown
perpendicularly upon a surface it rebounds in the same path in
which it is thrown. But if it hit the surface obliquely it is
thrown off or reflected in a different direction. Thus a ball
thrown from _b_ upon _c_, Fig. 135 (p. 158), will return in the
line drawn to _b_. But if it be thrown from _d_ it will be
reflected in the line _c a_. Now the angle _d c b_, called the
angle of incidence, is always equal to _b c a_, the angle of
reflection. The same, you will find in other parts of this book, is
true of sound and light and heat.

207. =Uniformity of Motion.=--Since motion, when once begun, is
disposed to continue unless arrested by obstacles, it is naturally
uniform both in its velocity and its direction. I will speak now
only of velocity. Suppose a body to be set in motion, and to meet
with no opposition from friction, or the resistance of air, or
attraction, it would move on forever, and with the same velocity
with which it began. Now precisely these circumstances we have in
the motion of the heavenly bodies in their orbits. They are, it
is true, under the influence of attraction, but in such a way,
as you will soon see, as not to interfere with the uniformity of
their motion. Were it not for this uniformity we should have no
regularity of times and seasons. It is only by the uniform motion
of the earth round the sun, and round its own axis, that we can
calculate for to-morrow, or next week, or next year. If these
motions were irregular it would throw confusion into all our
calculations for the future and all our recollections of the past.
We can measure time by nothing else but regular motion, and were
there no regular motion we should have merely the very inaccurate
measure furnished by our sensations. To measure time with accuracy
we take some great and extensive uniform motion as our standard.
Thus, the revolution of the earth around the sun we take as one
division of time, and call it a year. We observe that during this
time it whirls around on its own axis 365 times, and the time
occupied by each of these revolutions we call a day.

208. =The Pendulum.=--Various modes of measuring time have been
adopted by mankind. At first time was inaccurately divided by
merely observing the sun. But after a while man resorted to
various contrivances to measure short periods of time with
accuracy. All of these depend upon the uniformity of motion alone.
The sun-dial measures time by the uniform movement of the shadow on
its face, caused by the uniform movement of the earth in relation
to the sun. The hour-glass measures time by the uniform fall of
sand produced by the attraction of gravity. The best measurement
of time is by the comparatively modern invention of clocks and
watches, in which time is divided into very minute periods by the
uniform motion of the pendulum or the balance-wheel. The pendulum
furnishes an interesting example of motion kept up by the influence
of gravity. It was not till the time of Galileo, less than three
centuries ago, that its operation was understood and appropriated
to the measurement of time. He observed that chandeliers hanging
from lofty ceilings vibrated very long and uniformly after they
were accidentally agitated, and the thought of the philosopher
evolved from this phenomenon the most important results. Though
it had been before men's eyes in some shape or other since the
creation, it was reserved for Galileo to observe its significance,
and the result is that the pendulum has become man's time-keeper
over the whole earth.

[Illustration: Fig. 136.]

209. =Explanation of its Operation.=--A pendulum consists commonly
of a ball or weight at the end of a rod suspended so as to vibrate
with little friction at the point of the suspension. Let _a b_,
Fig. 136, represent such a pendulum. When it is at rest it makes a
plumb-line hanging toward the centre of the earth. If it be raised
to _c_ and be left to fall, the force of gravity will not only
carry it to _b_, but, by the accelerated velocity or accumulated
momentum which it gives it in its descent, it will carry it to
_d_. The same would be true of its return from _d_. And it would
vibrate forever in this way if it could be entirely freed from the
resistance of the air and friction. But, as it is, the pendulum
left to itself gradually loses its motion from these obstacles.
In the common clock the office of the weight is to counteract the
influence of these obstacles, and keep the pendulum vibrating.
In the watch the mainspring performs the same office to the
balance-wheel.

[Illustration: Fig. 137.]

The times of the vibrations of a pendulum are nearly equal whether
the arc it describes be great or small. For when the vibration is
a large one the velocity which the pendulum acquires in falling is
greater than when the vibration is of small extent. The reason is
that the higher it rises the more steep is the beginning of its
descent. Thus _a c_, Fig. 137, is steeper than _c b_.

[Illustration: Fig. 138. Fig. 139.]

210. =Gridiron Pendulum.=--The longer a pendulum is the longer time
does its vibration occupy. It requires a pendulum of the length of
a little over thirty-nine inches to vibrate seconds. Cold weather,
by contracting the pendulum, makes it vibrate quicker than in
summer, and so makes the clock go faster. Various contrivances have
been resorted to in order to counteract the variation of length
in pendulums by heat and cold, but what is called the gridiron
pendulum is the best. In this pendulum an ingenious use is made of
the fact that heat expands brass nearly twice as much as it does
steel. A simple form of this pendulum is given in Fig. 138. The
middle rod is made of brass, and the side rods, _b_ and _c_, of
steel. Suppose that the brass rod expands or increases in length
half an inch. The rod _c_ would be drawn upward by it, and the
rod _b_ downward, each one quarter of an inch; but this effect is
counteracted by the expansion of each steel rod, which is half
that of the brass, that is, one quarter of an inch. The ball _d_,
therefore, always retains the same distance from the point of
suspension, _e_. In Fig. 139 you have a gridiron pendulum of a more
compound character, a part of the bars being steel, and a part
brass.

211. =Motion Disposed to be Straight.=--When a body is set in
motion, if it be left to itself--that is, if nothing interfere
with its motion--it will move in a perfectly straight line. It
requires some interference from some force to bend the motion. You
will readily see from the views which I have given you that there
never is any motion that is, strictly speaking, straight, because
every motion is in some measure compound; that is, each cause of
motion is modified in its action by other causes of motion. But we
can approximate very nearly to straight motion by making one cause
preponderate very much over other causes. This I will illustrate.
If we fire a bullet horizontally from a gun it is acted upon by
three forces: the propulsive force of the powder, the resistance
of the air, and the attraction of the earth. The action of the
second of these is in direct opposition to the first, and therefore
only <DW44>s the motion, and does not tend at all to turn it from
its straight course. This is seen in the fact that the ball is
turned neither to the right hand nor to the left. But the third
force tends to make the ball bend its course toward the ground. It
does this from the instant that the ball leaves the gun throughout
its flight, but so slightly that practically we can consider the
ball as going straight for short distances. When we take a long
range we must make allowance for this bending down of the motion.
Accordingly, for the sake of precision, a double sight is provided
in modern guns, as seen at A and B, Fig. 140. This you see secures
the pointing of the gun a little above the level of the object
aimed at, that level being indicated by the dotted line.

[Illustration: Fig. 140.]

[Illustration: Fig. 141.]

The greater is the propulsive force the more nearly to a straight
line is the path of the propelled body. This may be seen very
clearly in Fig. 141, representing the issuing of water at different
points from a vessel. As pressure in a liquid is as depth, § 121,
the force with which the water is thrust out is greater at C than
at B, and at D than at C. The issuing stream, therefore, is most
nearly straight at the lowest point, D.

The motion of projectiles, thus alluded to, will be more
particularly noticed farther on.

[Illustration: Fig. 142.]

[Illustration: Fig. 143.]

212. =Compound Straight Motion.=--We call that motion compound
which is produced by two or more forces acting upon the body. This
may be straight or curved. I will first speak of the straight. If a
man attempt to row a boat straight across a river, the point which
he will reach will not be directly opposite to that from which
he started, but below. Two forces act upon the boat: the current
tending to carry it straight down the stream, and his rowing
tending to carry it straight across. The boat will go in neither
of these directions, but in a line between them. Let A B, Fig. 142,
represent the bank of the river, from which he starts at A, with
the bow of the boat pointing to C, on the opposite bank. Suppose
now that in the time that it takes him to row across the current
would carry him down to B if he did not row at all. He will in this
time, by the two forces together, reach the point D, opposite to B,
his course being the line A D. So if the wind blow upon a vessel
in such a way as to carry it eastward, and a current is pushing it
southward, the vessel will run in a middle line, viz., southeast.
For the same reason if a boy kick a foot-ball already in motion, it
will not be carried in the direction in which he kicks it, but in a
line between that direction and the direction in which its former
motion was carrying it. In swimming, flying, rowing, etc., we have
examples of compound motion, the middle line between the directions
of the forces always being taken by the body moved.

[Illustration: Fig. 144.]

[Illustration: Fig. 145.]

If we take Fig. 142, illustrating the movement of the boat, and
draw two lines, one from A to C and the other from B to D, we shall
have the parallelogram A C D B, Fig. 143, in which the line A C
represents the force of the rowing, A B the force of the current,
and A D the path of the boat. You see, then, that if we wish to
find in what direction and how far in a given time a body acted
upon by two forces will move, we are to draw two lines in the
direction of these forces, and of a length in proportion to the
distances to which they would move it in that time; then by drawing
two lines parallel to these we shall have a parallelogram, and the
diagonal of this will represent the distance and the course of the
moving body. If a body be acted upon by two equal forces and at
right angles to each other, the figure described will be a square,
as you see in Fig. 144. If they vary from being at right angles to
each other the figure will vary in the same proportion from the
square figure, as seen in Figs. 145 and 146. In the three figures A
B and A D represent the two forces, and A C the resulting motion.
You observe by these diagrams that the nearer the two forces come
to being in the same direction the farther will they move the body.

[Illustration: Fig. 146.]

[Illustration: Fig. 147.]

You see this in the different lengths of the diagonals in Fig. 144
and Fig. 146. The more nearly, therefore, the wind coincides with
the current the more rapidly will a vessel be carried along before
the wind. When, on the other hand, the angle at which two forces
act upon a body is much greater than a right angle, they will
propel it but a small distance. Thus if two forces act on a body in
the directions D A and D C, Fig. 147, they will move it only the
distance represented by the diagonal D B. This diagram represents
the motion of a vessel sailing almost directly against a current by
a wind the force of which is equal to that of the current, while
Fig. 146 represents the motion of a vessel where wind and current
being of equal force very nearly coincide. In the above diagrams
I have supposed the forces to be equal; but the same truth can be
shown in regard to unequal forces as seen in Fig. 143.

213. =Curved Motion.=--No single impulse can produce a curved
motion. Neither can two or more impulses communicated at one time.
In both of these cases the motion would be in a straight line.
Curved motion may be produced by two forces, one of which gives
it a single impulse, and the other acts upon it continuously. A
familiar example you have in a ball whirled around at the end of
a string. You can give it an impulse, and then, holding it in
your hand, let it whirl. Here the impulse you give the ball is
one force, and the tension of the string is the other, the latter
acting continuously. Your hand holding the end of the string is
the centre about which the motion revolves; the impulse which you
have given the ball tends to make it fly away from the centre in
a straight line, and hence is called the _centrifugal_ force; the
tension of the string keeps it from thus flying off, and so is
called the _centripetal_ force. When the earth, at the creation,
was put in motion it would have moved in a perfectly straight
line, were it not constantly drawn toward the sun by attraction,
the continuous action of this latter force being the same as the
tension of the string in the case of the whirling ball. The force
of attraction, then, is the centripetal force of the earth, and the
impulse which was given to it by the Creator in the beginning is
its centrifugal force; and, balanced between these two forces, the
earth and all the heavenly bodies move uniformly onward in their
orbits. The centrifugal force you see in these illustrations is
simply the tendency of motion to a straight line from the inertia
of matter; and this is constantly counteracted by the centripetal
force.

[Illustration: Fig. 148.]

214. =Illustrations of Centrifugal Force.=--When a wet mop is
whirled the water flies off in every direction by its centrifugal
force. On the same principle a dog, coming out of the water,
shakes off the water by a semi-rotary motion.--When a suspended
bucket of water is turned swiftly around the water rises high on
its sides, and leaves a hollow in the middle. It is the tendency
to fly away from the centre of motion that causes this.--Large
wheels, revolving with great velocity, have been broken by the
centrifugal force of its particles, and hence the necessity of
having such wheels made very strong. The immense grindstones used
in gun-factories have sometimes been broken through in the middle,
or have flown into pieces from the same cause.--A man riding
horseback on turning a sharp corner inclines his body toward the
corner, to avoid being thrown off by the centrifugal force. So, in
the feats of the circus, a man standing on a horse running at full
speed around the ring inclines his body strongly inward, as you
see in Fig. 148 (p. 167). The horse also instinctively inclines in
the same direction for the same reason. If the rider finds himself
in danger of falling, by making the horse go a little faster, thus
adding to the centrifugal force, the difficulty is relieved.--The
centrifugal force is made use of in milling. The grain is admitted
between two circular stones by a hole in the centre of the upper
one, and as the stone revolves it constantly moves toward the
circumference, and there escapes as flour.

[Illustration: Fig. 149.]

215. =Bends in Rivers.=--We see the operation of the centrifugal
force in the bends of rivers. When a bend has once commenced in
a river it is apt to increase, for as the water sweeps along the
outer bank of the bend it presses strongly against it, just as the
water in the whirled bucket, § 214, presses against its sides,
by its centrifugal tendency, or, in other words, its tendency to
assume a straight motion. Of course the result is a wearing away of
this outer bank, and in proportion to the looseness of the material
of which it is composed and the velocity of the river's current.
And when one bend is formed another is apt to form below, but in an
opposite direction. The water, by sweeping along the bend _a_, Fig.
149, is directed by it toward the opposite bank at _b_, and makes a
bend there also.

[Illustration: Fig, 150.]

It is in this way that a river, running through a loose soil, the
Mississippi, for example, acquires a very serpentine course. As the
water in the whirled bucket rises around the sides, so in the river
the water will be higher against the bank _a_ than on the opposite
side. Eddies and whirlpools are produced on the same principles,
when water is obliged to turn quickly around some projecting point.
If a current were moving swiftly along the shore _a_ toward the
point _b_, Fig. 150, it would be directed outward by the resistance
of this projection, and so a depression would be left at _c_, just
behind it, and this depression would be surrounded by a revolving
edge of water.

[Illustration: Fig. 151.]

216. =Application of the Centrifugal Force in the Arts.=--Much use
is made of the centrifugal force in the arts, but I will give but
two examples. In the art of pottery the clay is made to revolve
on a whirling table, the workman at the same time giving the clay
such shape as he chooses with his hands and various instruments. In
doing this he constantly has reference to the centrifugal force,
giving the table a velocity proportioned to the amount of this
force which is needed in each stage of the operation. The most
beautiful application of this force that I have ever witnessed is
in the manufacture of common window-glass. The glass-blower gathers
up on the end of his iron tube a quantity of the melted glass, and
blows it out into a large globe. When it is of sufficient size and
thinness he places it on a rest, as you see in Fig. 151 (p. 169).
A second man now comes with a rod having some melted glass on the
end, and attaches this to the globe at a point opposite to that
where the tube of the first man is joined to it. There now comes a
boy, and, giving this tube a quick blow, severs its connection with
the globe, leaving a hole in the globe where the glass breaks out.
The second man, having the globe attached to his rod, carries it to
a blazing furnace, and resting the rod on a bar at its mouth, puts
the globe directly into the flame. The glass is soon softened,
and he whirls the globe continually around. The hole in the globe
enlarges by the centrifugal force, and at length by this force the
globe is changed into a flat, circular disk. Panes of glass which
are called bull's-eyes are cut from the centres of these disks.

[Illustration: Fig. 152.]

217. =Steam-Governor.=--The operation of the centrifugal force is
beautifully exemplified in this regulator of the steam-engine. It
consists of two heavy balls, Fig. 152, suspended by bars from a
vertical axis, the bars being connected to the axis by hinges. The
bars have also a hinged connection at their lower ends with two
smaller bars, and these latter have a similar connection with a
collar that slides up and down on the axis. Now the faster the axis
turns the farther the balls fly out from it, from the centrifugal
force, and the higher the collar slides up on the axis. From the
collar extends, as you see, a lever. This is connected with a
valve in the steam-pipe, and so regulates the amount of steam that
enters the working part of the engine. The object of this ingenious
contrivance is to make the engine regulate its own velocity. When
it is not working too fast the valve in the steam-pipe is wide
open. But the moment that it works too rapidly the balls extend
out far from the axis, so that the collar rises, and by the lever
partly closes the valve. Less steam, therefore, can come to the
engine, and the engine working in consequence less rapidly, the
balls fall again, opening the valve. You see, then, that the
regulation of this valve by the governor effectually prevents the
action of the engine from becoming too rapid.

[Illustration: Fig. 153.]

[Illustration: Fig. 154.]

218. =Shape of the Earth Influenced by the Centrifugal Force.=--If
the potter should make a ball of soft clay revolve rapidly around
on a stick run through it, the ball would bulge out at the middle,
where the centrifugal force is greatest, and would be flattened at
the ends where the stick runs through it. This is precisely what
has happened to the earth. At the equator, where the centrifugal
force is greatest, it has bulged out about thirteen miles, while it
is flattened at the poles. This shape was of course assumed before
the earth became solid. In Fig. 153 we have the shape of the earth
represented, N S being the polar diameter, and E E' the equatorial
diameter. The tendency to take this shape from the centrifugal
force may be illustrated by the instrument represented in Fig.
154. It consists of a set of circular hoops of brass, with an axis,
_b a_. The hoops are fastened to the axis at _a_, but are left
free at _b_. By a little machinery at the top they can be made to
revolve rapidly, and bulging out at the sides by the centrifugal
force, they slide down on the axis at _b_.

[Illustration: Fig. 155.]

219. =Projectiles.=--I have already spoken of projectiles in § 211.
You saw there that any body, as a cannon-ball, which is projected
horizontally, falls to the earth in a curved line. Two forces act
on the ball; viz., the projectile force given by the powder and
the force of gravitation. The force of gravity being always the
same, the shape of the curve which the projected body describes
must depend on the force with which it is projected. This is very
strikingly exemplified in the curves described by the different
streams of water in Fig. 141. But whether the projectile force be
great or small, the moving body thrown horizontally will in every
case reach the ground in the same time. Thus if two cannons stand
side by side on a height, one of which will send a ball a mile and
the other half a mile, the two balls, if fired together, will reach
the ground at the same instant, though at first thought it would
seem that the ball which travels twice as far as the other would
take a longer time to do it in. This is because the _horizontal_
force of the ball does not oppose in the least the _downward_ force
of gravity. If it were thrown upward instead of horizontally, the
projectile force would be opposed to gravity, and in proportion
as the direction came near to being vertical. As horizontal force
does not interfere with the action of the force of gravity, it
follows that a ball dropped at the instant at which another is
fired will reach the ground at the same instant that the fired ball
does. This can be made clear by Fig. 155. Suppose it takes three
seconds for a ball to fall from the top of a tower to its foot. In
the first second it falls to _a_. The ball projected horizontally
from the cannon, being operated upon by the same force of gravity,
will fall just as far, and will be on a level with it at _b_. Both
balls fall farther and farther each second, both being accelerated
in the same degree because it is done by the same force. The
projected ball will reach _d_ when the falling ball is at _c_, and
the plain at _f_ when the falling ball is at _e_, the foot of the
tower. The same holds true in all cases. A bullet dropped from a
level with the barrel of a gun, paradoxical as it may seem, will
fall to the ground no sooner than one which is shot from the gun.

[Illustration: Fig. 156.]

220. =All Falling Bodies really Projected.=--When a body falls
from any height, it does not, as you have already seen in § 187,
fall in a straight line, as it appears to do. It falls in a curved
line, for, like all projectiles, it is acted upon by a horizontal
force as well as the force of gravity. But what is this horizontal
force? It is the motion which the body has in common with the
earth in its rotation on its axis. In this rotation the height
from which the body falls goes to the eastward 1500 feet in a
second. If, therefore, the body did not partake of the motion of
the earth, and went to the ground in a _straight_ line in a second,
it would be when it reached the ground 1500 feet westward from the
foot of the height from which it fell. But it does partake of the
earth's motion, and goes eastward as fast as the height does, and
so describes the curved line of a projectile. Suppose a ball falls
from a height A, Fig. 156, and in a second of time that height
passes to C. The forward or projectile force would tend to carry
the ball to C, and the force of gravity would tend to carry it
to B. But both forces acting together, it pursues a middle path,
and this path is a curved line, because one of the forces is a
continued force, § 213. For the same reason, if a ball be dropped
from a railway car in motion, and it takes a second for it to
fall, it will be at the end of that second just under that part
of the car from which it fell. Although the car may have moved a
considerable distance, the dropped ball, partaking of its motion,
goes along with it in its fall. So a ball dropped from a mast-head
when a ship is in motion goes along with the ship in its fall. The
ball in each of these cases describes in its fall a curved line.

221. =Motion in Orbits.=--Why is it, let us ask, that a cannon-ball
shot horizontally from some great height will not revolve around
the earth like the moon. It has the same two forces acting upon it
as the moon has--viz., a projectile force, and the attraction of
the earth--and both ball and moon describe a curve in their motion.
But the curve of the ball bends to the earth, while that of the
moon ever sweeps around the earth. Why is this? First, there is
the resistance of the air continually retarding the velocity of
the ball. But, secondly, even if the ball could be projected from
an elevation sufficiently high to be outside of the atmosphere,
the force of the projection would not be great enough. We know,
from the rate of progress of the heavenly bodies in their orbits,
that it would require an immense velocity to keep the ball from
being brought to the earth by its attraction. The Creator of
these worlds, when he launched them into their orbits, gave them
precisely that impulse which is needed to balance the centripetal
force of attraction, and so they pursue a middle course between the
two directions in which these two forces tend to carry them. And
as their velocities have never been retarded by the resistance of
air or any other substance, they have been ever the same from the
beginning.




CHAPTER XI.

THE MECHANICAL POWERS.


222. =Machines not Sources of Power.=--The Mechanical Powers, as
they are termed, are six in number--viz., the Lever, the Wheel
and Axle, the Inclined Plane, the Screw, and the Wedge. They
are not, strictly speaking, powers; for, as you will see in the
course of our investigation, they are merely means of applying
power to advantage, and are not in reality sources of power. The
true sources of power are the causes of motion treated of in §
181. The instrument or machine can not create power, and the only
use of all the variety of tools and machinery is to enable us to
_apply_ power in such a manner, with such a velocity, and in such a
direction, as to effect the objects which we have in view. The term
Mechanical Power, then, is not strictly proper as applied to those
contrivances which commonly have this name; but the term is in so
general use that it would not be well to alter it.

Every instrument, however simple and insignificant, and every
machine, however large or complicated, is an example of some one
of the six Mechanical Powers, or of a combination of them. I will
proceed to consider each of these separately. In doing this certain
terms will be used which I will first explain. _Power_ is the
force by which a machine or instrument is moved. _Weight_ is the
resistance to be overcome. If the resistance be in some other form
than that of weight it is called technically by this name. So what
is called Power may be in the form of weight. The _fulcrum_ is the
point on which the instrument or machine is supported while it is
in motion.

223. =The Lever.=--The Lever is the most simple of all the
Mechanical Powers, and is therefore in universal use. Though the
savage makes use of but few tools in comparison with the civilized
man, he uses the lever almost constantly in some form or other. The
wedge is the only one of the other Mechanical Powers that he uses
to any great extent. Levers are of three kinds, which I will notice
separately.

[Illustration: Fig. 157.]

[Illustration: Fig. 158.]

224. =Lever of the First Kind.=--In the lever of the first kind
the fulcrum or prop is between the weight and the power. The
common crow-bar or hand-spike is a familiar example, as seen in
Fig. 157--the stone, S, or other heavy body to be moved being the
weight, the stone or block of wood, F, on which the bar rests
being the fulcrum, and the pressure of the hand, H, the power. The
nearer the fulcrum is to the weight, or the farther is the power
from the fulcrum, the greater is the force of the lever. This may
be illustrated on Fig. 158. Here the short arm of the lever, as
it is called, C W, is one eighth of the length of the long arm, A
C. If the weight hanging at the end of the short arm be 72 pounds,
a weight of 9 pounds, or the force of a hand amounting to this,
will balance it at the end of the long arm. But if the power should
be applied at only four times the distance from the fulcrum at
which the weight is, then it would require a force of 18 pounds to
balance the 72 pounds on the short arm. Similar variations can be
made by altering the length of the short arm. The power and the
weight will balance each other if the weight multiplied by the
length of the short arm, and the power multiplied by the length of
the long arm, give equal products.

225. =Scales and Steelyards.=--In the common scale-beam we have a
lever, the two arms of which are equal, and therefore equal weights
suspended at the ends balance. If they be not exactly equal, a
heavier weight will be necessary on the shorter arm than on the
longer. The inequality will injure the buyer if the prop be too
near the scale in which the weights are placed, and the seller
if it be too near that which holds the article to be sold. Any
difference can be easily detected by changing the places of the
article and the weights. Whenever cheating is practiced by the
"false balance," it is of course done in a small way, to avoid any
observation by the eye of the inequality of the two arms of the
scale-beam, and the weight of the scale hanging from the shorter
arm is made a little greater than that of the other, so that they
may balance. Scales may be rendered very accurate by making the
fulcrum or pivot of hardened steel, and of a wedge shape, with
a sharp edge, in order to avoid friction as much as possible.
The steelyard differs from the scale-beam in having the arms of
different lengths. The principles on which this instrument is
constructed were developed in what I said of Fig. 158. When either
with the balance or the steelyard two weights balance each other
the centre of the weights and the apparatus taken together is
just over the fulcrum, § 195. We see in this the reason that it is
necessary to have the prop near the large weight when we wish to
balance it by a small one.

226. =Other Examples.=--Scissors are double levers of the first
kind. The fulcrum is the rivet, the weight or the resistance to
be overcome is the article to be cut, and the power is applied to
the long arms of the levers by the fingers. With large shears hard
substances can be cut. Even plates of iron are cut like paper by
shears which are worked by a steam-engine.--Pincers are double
levers. The hinge, or rivet, is the fulcrum.--The common hammer, as
used in drawing nails, is a good example of the power of this kind
of lever. Though crooked, it acts in the same way with a straight
lever. The fulcrum is the point on the board where the hammer
rests, and this is between the resistance to be moved, the nail,
and the power, that is, the hand which grasps the handle.

[Illustration: Fig. 159.]

227. =No Gain of Power in this Lever.=--I will now illustrate the
truth that there is no gain or saving of power in this lever,
though at first thought it would seem that there is. Let _a b_,
Fig. 159, represent a lever, and _e_ its fulcrum. Let the arm _a
e_ be twice as long as _e b_. A pound, therefore, suspended from
_a_ will balance two pounds at _b_. If, now, when the weights are
suspended, the long arm be raised so that the lever shall be in the
position represented by the line _c d_, and then let go, the one
pound at _c_, balancing the two pounds at _d_, will bring the lever
again to the position _a b_. It will be perceived that the end of
the long arm of the lever moves through the space _a c_, which is
larger than _b d_, through which the end of the short arm moves,
in the same time. The one-pound weight, in fact, falls two feet
in raising the two-pound weight one foot, and it moves twice as
far as a one-pound weight suspended at _i_ would. If a one-pound
weight could raise a two-pound weight without thus moving through
twice as much space we might then say that there is an actual gain
of power in the lever. But it evidently makes no difference whether
one pound moves through two feet or two pounds through one foot;
the force is the same in both cases. For the momentum or force of
a moving body is in proportion to its weight and velocity, § 201;
and therefore the pound weight, moving through two feet, has as
much momentum as the two-pound weight moving through one foot in
the same time. The small weight does the same amount of work that
the larger one would by moving twice as far in the same time as the
larger, just as a boy, who carries a load half as large as a man,
will do as much work as the man if he carry it twice as fast.

[Illustration: Fig. 160.]

228. =The See-Saw.=--We see the same thing illustrated in the
see-saw, Fig. 160. The man, being much heavier than the boy, is
nearer the prop, and as they move up and down the boy passes over a
much larger space than the man, describing an arc in a much larger
circle.

229. =Archimedes's Lever.=--Archimedes said that if he could have a
lever long enough and a prop strong enough he could move the world
by his own weight. But he did not think how far he would have to
move to do this, from the vast difference between his weight and
the weight of the earth. "He would have required," says Dr. Arnot,
"to move with the velocity of a cannon-ball for millions of years
to alter the position of the earth by a small part of an inch."

230. =An Analogy.=--You will remember that in the case of the
Hydrostatic Paradox, the Hydrostatic Bellows, and Bramah's Press
(§ 131, § 132, and § 133), great effects are produced by a small
power. But this small power has to execute an extensive motion in
order to produce these effects. Thus, as stated in § 132, if the
area of the top of the Hydrostatic Bellows be one thousand times
the area of the tube, though the water poured into the tube will
raise a very great weight on the bellows, the water in the tube
must fall ten inches in raising the weight the hundredth part of an
inch. So when the pressure of the hand on the long arm of a lever
moves a great weight, as a heavy stone, the weight is moved but a
little, while the extent of the hand's motion is comparatively very
great.

[Illustration: Fig. 161.]

[Illustration: Fig. 162.]

231. =Lever of the Second Kind.=--In the second kind of lever
the weight is between the fulcrum and the power, as you see in
Fig. 161. The same rule of equilibrium applies here as in the
case of the lever of the first kind. The 72 pounds of weight can
be sustained by 8 pounds of power, because the power acts on the
lever at 9 times the distance from the fulcrum that the weight
does, for 1×72 = 9×8. The common wheel-barrow, Fig. 162 (p. 180),
is an example of this kind of lever. The point at which the wheel
presses on the ground is the fulcrum, and the weight is the load,
its downward pressure from its centre of gravity being indicated at
M. Of course the nearer the load is to the fulcrum the easier it
is, on starting, to raise the handles. The crow-bar can be used as
a lever of this kind when its point is placed beyond the weight to
be raised. The chipping-knife, Fig. 163, is another example. The
end, F, attached to the board, is the fulcrum, the hand pressing
at P the power, and the resistance of the substance R, which is to
be cut, is the weight. Nut-crackers have a similar arrangement. In
shutting a door by pushing it near its edge we move a lever of this
kind. The hinge is the fulcrum, and the weight is between this and
the hand.

[Illustration: Fig. 163.]

We see, then, the reason that the slight push of a hand shutting
the door may even crush a finger when caught in it at the side
where the hinges are. The finger is a resistance so near the
fulcrum that the power moving through a great space acts upon it
with immense force. The same explanation applies to the severe bite
of the finger when it is caught in the hinge of a pair of tongs.
The oar of a boat is a lever of this kind, the weight to be moved
being the boat, which is between the power, the hand of the rower,
and the fulcrum, the resisting water.

[Illustration: Fig. 164.]

232. =Lever of the Third Kind.=--In the third kind of lever the
power is between the fulcrum and the weight, as seen in Fig. 164.
In the first two kinds of lever the power may be less than the
weight, but in this the power must always be greater than the
weight. This lever has, then, no mechanical advantage, as that
expression is commonly used. Applying the same rule here as in the
other levers, see what is the result. If the weight, as in Fig.
164, be 9 times as far from the fulcrum as the power is, it will
require a power equal to a weight of 648 pounds to sustain a weight
of 72 pounds, for 9×72=1×648.

[Illustration: Fig. 165.]

233. =Examples.=--When a man puts his foot against the end of a
ladder, and raises it by taking hold of one of the rounds, the
ladder is a lever of this kind. It is evident that he spends his
force upon it at a great mechanical disadvantage, for the power is
applied much nearer to the fulcrum than the weight of the ladder,
taken as a whole, is. If you push a door to by placing your hand
very near the hinges, you do not shut it as easily as when you take
hold of it at its edge. In the first case it is a lever of the
third kind, and the hand moves through a small space, and therefore
must exert a considerable force; while in the latter case the door
is a lever of the second kind, and the hand, moving through a
greater space, puts forth less force. When we use a pair of tongs
we use a pair of levers of the third kind. They are an instrument
in which convenience rather than power is needed. We can not grasp
any thing very firmly with them because the power is so much nearer
to the fulcrum than the weight to be lifted. For this reason a
pinch with the ends of the tongs is nothing compared with one in
the hinge. The most beautiful example of this lever we have in
the moving apparatus of animals. Take, for example, the principal
muscle which bends the elbow, as represented in Fig. 165 (p. 182).
This comes down from the shoulder in front of the bone of the arm,
and is inserted just below the elbow-joint into one of the bones
of the forearm. It pulls upon the forearm very near the fulcrum,
which is the elbow-joint, and so acts at a great mechanical
disadvantage. The object of this arrangement is to secure quickness
of movement, which is here, as in almost all muscular motions, of
more importance than great strength. When great weights are lifted
the fact that the muscles act at such mechanical disadvantage makes
the exhibition of power wonderful.

[Illustration: Fig. 166.]

[Illustration: Fig. 167.]

234. =Compound Levers.=--When several levers are connected together
we call the whole apparatus a compound lever. Let each of the
levers in Fig. 166 be 3 inches long, the long arms being 2 inches,
and the short ones 1 inch. One pound at A will, according to
the rule, balance 2 at B, and 2 at B will balance 4 at C, and 4
at C will balance 8 at D. Therefore 1 pound at A will balance 8
pounds at D. And you see that an equilibrium is effected when the
power is to the weight as the product of all the short arms is to
the product of all the long arms. The compound lever is used in
weighing heavy loads--as hay, coal, etc. You have a representation
of the arrangement in Fig. 167. The load, W, stands on a platform,
A B, which rests upon two levers, E D and E C. The long arms of
these levers are E G and E F, and the short arms are G D and F C.
The ends of the long arms press upon the fulcrum of the lever, H I.
The pressure is now transmitted from the end of the long arm by the
rod, I K, to a small lever, K L, where a small weight or power, P,
balances the weight of the heavy load, W. The two objects secured
by this arrangement are accuracy and the occupation of a small
space.

[Illustration: Fig. 168]

235. =Wheel and Axle.=--The mechanical power next in simplicity to
the lever is the Wheel and Axle. The most familiar applications of
this power we see in drawing water and in raising heavy articles
in stores. The principle of this power is the same as that of the
lever, as may be shown in Fig. 168, which represents a section of
the wheel and axle. The power, P, hangs by a cord which goes round
the wheel, and the weight, W, by a cord around the axle. We may
consider the power as pulling on a lever represented by A B, the
long arm of which is A C, and the short arm B C. You see that the
wheel and axle, then, may be viewed as a constant succession of
levers, and it is therefore sometimes called the perpetual lever.
And the same rule of equilibrium applies here as in the simple
lever.

[Illustration: Fig. 169.]

236. =Windlass.=--In the common windlass the power is applied to a
winch or crank, D C B, Fig. 169, instead of a wheel. In estimating
the power of this arrangement B C must be considered the long arm
of the lever, and half of the diameter of the axle, B A, as its
short arm.

[Illustration: Fig. 170.]

[Illustration: Fig. 171.]

237. =Capstan.=--In the capstan, represented in Figs. 170 and 171,
the axle is in a vertical position. The top of it is pierced with
holes, into which levers are introduced. In Fig. 170 you see the
instrument as it is commonly used in moving buildings. Sometimes
horse-power is applied at the ends of the levers. Great power is
exerted by this instrument; but we have the same fact here as in
all cases where a small force produces a great effect--the effect
is slow, and the force passes over a great space in producing it.
The moving of a building a foot requires many circuits of the horse
around the axle. Fig. 171 gives us the capstan as it is commonly on
board ship. The head of it is circular, with many holes for levers,
so that many men can work together in raising a heavy anchor.

[Illustration: Fig. 172.]

238. =Fusee of a Watch.=--In the fusee of a watch we have a wheel
and axle of a peculiar construction. When we wind up a watch the
chain is wound around the spiral path-way on the fusee, B, Fig.
172, and at the same time the spring is coiled up tightly in the
round box, A. The spring, in gradually uncoiling itself, turns
this round box around, and thus pulls upon the chain, _c_, making
the fusee to revolve, and so give motion to other parts of the
machinery. Now the spring, in its effort to uncoil, acts strongest
at first; and therefore if the fusee were of uniform size the watch
would go fastest when first wound up, and go gradually slower as it
run down. This difficulty is obviated by giving the power a small
wheel to pull on at first, and gradually enlarging the wheel as
the spring uncoils. This is because, in order to produce a certain
effect on a given weight by a power, the less the power is the
longer must be the arm of the lever on which the power acts.

[Illustration: Fig. 173.]

[Illustration: Fig. 174. Fig. 175.]

239. =The Pulley.=--The third mechanical power is the Pulley.
Pulleys are _fixed_ or _movable_. In Fig. 173 you have a fixed
pulley. There is no mechanical advantage in this pulley, for its
action may be conceived of as the action of successive levers of
equal arms, B F and A F, and therefore equilibrium requires an
equality of the power and weight. But this pulley is often a great
convenience. For example, a man can raise himself or some weight
to any desired elevation, as seen in Fig. 174. It is used also
in effecting descents. With two fixed pulleys a horizontal force
may be used in raising a weight vertically, as seen in Fig. 175.
In using a fixed pulley either one or the other of two objects is
attained--applying force where we could not otherwise apply it, and
changing the direction of its application.

[Illustration: Fig. 176.]

240. =Movable Pulley.=--You have a representation of a movable
pulley in Fig. 176 (p. 186). It is evident here that the force
of the weight is equally divided between the cords, A B, so that
the cord B, extending over the fixed pulley, needs to have a
weight, P, but half of the weight W to balance it. A movable pulley
is sometimes called a "runner," and a fixed pulley is commonly
connected with it, in order to give the desired direction to the
force. Many pulleys are often connected together in various ways,
as seen in Fig. 177. It is easy to estimate in such cases the
relation of the power to the weight on the principles developed in
relation to the lever. If, for example, in the system of pulleys
on the left, the weight be 36 pounds, the two cords of the first
pulley will each sustain a weight of 18 pounds, those of the next
pulley each 9 pounds, and those of the next each 4½ pounds. The
weight W then will be balanced by the weight P if it weigh 4½
pounds.

[Illustration: Fig. 177.]

[Illustration: Fig. 178.]

241. =Inclined Plane.=--The fourth mechanical power is the
Inclined Plane. This being a very simple contrivance is much
used, especially when heavy bodies are to be raised only a small
height, as in getting large boxes and hogsheads into stores. The
mechanical advantage of the inclined plane may be illustrated
on Fig. 178. The line A _c_ represents an inclined plane. If a
weight be drawn up this plane it is raised only the height B _c_.
A smaller power is requisite to draw the weight up the plane than
to raise it perpendicularly; and the power necessary will be the
less the longer the plane. A power which would balance a weight
on an inclined plane would be to the weight as the height of the
plane to its length. Thus if A _c_ be twice as long as B _c_, a
weight of four pounds on the plane may be balanced by a two-pound
weight suspended by a cord passing from the weight over the summit
of the plane. A flight of stairs is an inclined plane in regard
to the principle on which the ascent is effected, the projections
in it being for the purpose of affording a sure footing in making
the ascent or the descent. So likewise hogsheads are let down the
steps of a cellar-way by ropes, and it makes no difference in the
principle of the operation whether the steps have or have not
planks laid along them. It is supposed that the immense stones in
the pyramids and other massive Egyptian structures were put into
their position by means of the inclined plane. Roads, when they are
not level, are inclined planes, and the steeper the inclination
the more power is required to draw a load up the road. Great
mistakes were formerly made in carrying roads too frequently over
high hills. Besides failing to take advantage of the principles of
the inclined plane, in many cases the horse in going over a hill
passes over quite as much space as he would if the road were made
to go round the base of the hill, and sometimes even more. If the
hill were a perfect hemisphere, a road over it would be just equal
in length to a road around its base to the opposite point.

[Illustration: Fig. 179.]

242. =The Wedge.=--This is the fifth of the mechanical powers. It
may be considered as two inclined planes placed with their bases
together, as seen in Fig. 179. Indeed, sometimes the wedge has one
side only inclined, it being only half of the ordinary wedge. The
difference between the inclined plane and the wedge in operation
is, that in the first the inclined plane is fixed, and the weight
is made to move up along its surface, while in the latter the
weight, that is, the resistance, is stationary, and the surface of
the plane is made to move along upon it. The power of the wedge
is estimated just as the power of the inclined plane is, that
is, by comparing the thickness of the wedge with the length of
its side. The less the thickness of the wedge compared with its
length, obviously the more powerful is the wedge as a penetrating
instrument. The wedge is used for splitting blocks of wood and
stone, for producing great pressures, for raising heavy bodies,
etc. All cutting and piercing instruments, knives, razors, axes,
needles, pins, nails, etc., act on the principle of the wedge.

243. =The Screw.=--This is the sixth mechanical power. The
principle of it is essentially that of the inclined plane. The
"thread" running around the screw is an inclined plane which is
spiral instead of straight, and so is also the corresponding part
in the nut an inclined plane running in the opposite direction. In
the common screw the nut is fixed, and the screw is made to play up
and down in it; but sometimes the screw is fixed, and the nut is
made to play around it. The screw acts like a wedge, and has the
same relation to a straight wedge that a road winding up a hill has
to a straight road of the same length and rise. Especially does the
comparison hold when the screw is forced into wood; the wedge goes
straight into the wood, but the edge of the screw's thread enters
the wood spirally.

[Illustration: Fig. 180.]

To estimate the force of the screw we compare the length of one
turn of the thread around it with the height to which the thread
rises in going round. Let _a b_, Fig. 180, represent one turn of
the thread, and _b c_ the height to which it goes. It is clear from
the figure that the principle which applies to the inclined plane
and to the wedge applies here also. As the less is the height of
the plane the easier it is for a weight to be drawn up it; and
as the less is the depth of the wedge the less is it resisted;
so, also, the less the height of the turn of the screw's thread
the easier is it to move the screw, and the greater is the force
which it exerts. Hence the prodigious power of a screw with a
thread which rises very slowly in its spiral turns. Screws are
much used when great pressure is required, as in pressing oils
and juices from vegetable substances, in compressing cotton into
bales, in bringing together with firm grasp the jaws of the vice,
etc. In turning the screw a bar is used, so that we have in this
instrument the combined advantages of the screw and the lever.
That you may have some idea of the power of these two instruments
acting together I will suppose a case. Let the weight to be raised
by a screw be 10,000 pounds. Let a turn of the screw be 10 inches
long, and the rise be but one inch. Then, so far as the screw is
concerned, the power requisite to raise the 10,000 pounds will be
1000--the ratio of the height of the thread's turn to its length.
But the power of the lever is yet to be estimated. Let the length
of the lever, passed through the head of the screw so that it is
equal on each side, be 30 inches. The diameter of the screw is
about three inches, or one-tenth of the diameter of the circle
described by the end of the lever. It will now take but a power
of 100 pounds to raise the weight, the ratio of the radius of the
screw to half the length of the lever.

244. =Truly but Three Mechanical Powers.=--The Wheel and Axle, you
have seen, is merely a modification of the Lever, and the Wedge and
the Screw are modifications of the Inclined Plane. The Mechanical
Powers are, then, in reality but three--the Lever, Pulley, and
Inclined Plane. And these are the elements of all machinery, from
the simplest tool that is used for the most common purposes to
the most complicated and powerful engine which the ingenuity of
man ever designed. The principle upon which a pin is shaped is
identical with that of the wedge, by which large masses are cleft
in two; and the instrument by which the finest textures are cut
by delicate fingers is arranged on the same principle with those
varied contrivances by which immense weights are raised by a
comparatively small power, viz., the principle of the lever.

245. =Friction in Machinery.=--You have seen, as we have proceeded,
that the Mechanical Powers, though thus named, do not generate
power. So far from this, there is really a loss of power in their
use, chiefly from friction. In raising a weight, for example,
directly by the hand, there is no loss from this cause; but if
you use a pulley you have the friction of the cord upon it, and a
loss of power in proportion to the amount of friction. In some
cases the loss of power from this cause is so great as to call for
a considerable variation from such calculations as we have made
in this chapter in regard to the relations of power and weight in
machinery. In the operations of the screw friction has a great
influence in diminishing the power of the instrument.

246. =The Real Advantages of the Mechanical Powers.=--If there is
then no saving, but a loss of power in tools and machinery, what,
let us inquire, are their advantages?

If one man can do alone by the aid of some instrument what would
otherwise require the exertion of many men, though he be slow in
doing it, yet it is a great advantage. Thus one man can with a
lever move a stone which perhaps it would require thirty men to
move without it, and though it take him thirty times as long, it
saves him the trouble of getting a company of men to help him. So
if a man can raise his goods by a wheel and axle to the upper loft
of his store, though he raise them slower than several men would
lift them directly by ropes, it is an advantage to him, as it saves
the hiring of a company of laborers. A few men by a capstan can
raise an anchor which could be raised without it only by a large
company of men.

Another advantage often is that there may be intervals of rest in
applying the force without any loss. This is obvious in the case
of the pulley, but still more so in the case of the screw. It is
friction in both these cases which enables the workman to rest. It
saves to him all that he has gained by opposing any tendency to
slip back. We see the same thing in the wedge. When this is driven
into wood, it remains because it is prevented from returning by
the friction of the wood against its sides. It is the same cause
which holds a nail in its place, and opposes any effort to draw it
out. In driving the wedge the workman can have as long intervals as
he pleases between his blows, because friction saves all that is
gained. This effect is very well exemplified in the capstan, Fig.
170. It requires but little exertion of the man who sits there to
hold the rope, because the few turns of it around the axle prevent
its slipping easily.

A third advantage which often attends the use of tools and machines
is that force may be made to produce motion at various distances,
in various directions, and in various degrees of velocity. Thus as
to distance, a man standing on the ground can raise a weight to
the top of a house by a pulley. So, also, a water-wheel may by the
connections of machinery produce motion at considerable distances
from it. Then as to direction, horizontal motion may be converted
into vertical, rotary into straight, etc. The velocity of motion is
generally varied by cog-wheels. Thus a wheel of 60 cogs revolving
once in a minute, playing on a wheel of 10 cogs, will make it
revolve once in 6 seconds.

[Illustration: Fig. 181.]

Another advantage of tools and machines is that they secure a
better mode of applying power than we otherwise could have. Thus
when several men are pulling on a rope much power is lost by
their pulling irregularly, a difficulty which is removed by the
pulley. The same can be said of applying pressure by the screw. One
man presses more steadily, and therefore more effectually, than
fifty men would without the screw. The arrangements of tools and
machines are so made as to provide convenient ways of applying our
strength. An instrument, for example, for moving a weight by hand
is so shaped as to hold the weight well, and also to afford a good
handle for the hand to grasp. The common claw hammer is a very good
illustration. We grasp the nail by an iron claw, with the handle we
can apply not merely the force of the hand, but that of the whole
arm, and then we have the immense lever power of the instrument.
We have a good illustration of convenience in an instrument, in
what is called a Lewis, represented in Fig. 181. It is used for
raising blocks of stone in building. It has three parts, A B C. It
is used in this way: A hole is made in the upper part of the block
of stone to be raised in shape like the instrument; then A and C
are inserted, and B is pushed in between them. With the ring, D,
bolted through the instrument the stone is raised to its place by
the ordinary machinery. The principle of the instrument, you see,
is that of the wedge.

247. =Man a Tool-Making Animal.=--Though there is no actual
saving of power in the tools and machines which man uses, yet
so great are the advantages which he reaps from them, that more
than two thousand years ago a philosopher thought that man could
not be better distinguished from brutes than by calling him a
tool-making animal. If the distinction was so striking in the
time of Aristotle, when tools and machines were so few in number
and so rudely contrived, and so few of the sources of power were
appropriated by man to his use, how much more striking is it now,
with all the variety and perfection of instruments and machinery,
and with the ever-extending appropriation of the sources of power
furnished by the elements. The power which air and water and
gravitation give is applied constantly with more and more variety
and effect; and the appropriation of that mighty source of power,
steam, is wholly a modern invention.




CHAPTER XII.

SOUND.


248. =What Sound is.=--Sound is such a vibration of substances
as can, on being transmitted to the ear, act upon the sense of
hearing. I say _such_ a vibration, because there may be vibrations
which will not produce the sensation of sound, Vibrations which are
either very slow or very quick will not do it. Thus if a plate of
metal or a string make less than 15 or more than 48,000 vibrations
in a second, no effect is produced upon the ear. The capacity of
hearing differs, however, in different persons, so that although
few can hear vibrations which are beyond the range which I have
mentioned, there are many whose capacity falls much within it
either at one end or both ends of the scale. The range for animals
is not the same as that for man. Thus the lion and the elephant can
hear a sound when the vibrations are too infrequent to make any
impression upon our ears; while small animals have a susceptibility
in the organ of hearing for vibrations so quick that we can not
hear them, and at the same time are not susceptible to the slower
vibrations. How far the range varies in different animals has not
been ascertained to any extent.

[Illustration: Fig. 182.]

249. =The Vibration of Sounding Bodies Manifest to the Senses.=--If
we place the hand upon a large bell that has been struck we can
feel the vibration. If we strike one of the ends of a tuning-fork
upon some hard body we can see the vibration, as represented in
Fig. 182 by the dotted lines. If we look in upon the strings of a
piano as it is played, the vibration of the larger strings is very
observable to the eye. If we rub the edge of a drinking-glass so as
to produce a musical sound, the water which is in it will be thrown
into waves from the vibration of the glass.

[Illustration: Fig. 183.]

250. =Wind Instruments.=--In wind instruments, as the flute, horn,
etc., it is the vibration of the body of air in the instrument
which causes the sound. In the common tin whistle or bird-call,
Fig. 183, the sound is produced by the vibration imparted to the
contained air by the impulse of the breath through the orifice, B.

[Illustration: Fig. 184.]

251. =An Analogy.=--The vibration of a sounding body is much like
that of a pendulum. The end of the tuning-fork, Fig. 182, on being
struck passes to _b_, and in returning passes by the point of rest,
A, as the pendulum does, and reaches _a_. So, also, if a string,
A B, Fig. 184, be drawn aside to D, as it flies back to C it will
by its inertia pass on to E, and so will continue to vibrate back
and forth for some time. The same rule also applies to the extent
of the vibrations here as in the case of the pendulum, § 209. The
quickness of the vibration is not at all affected by its width.
The farther the string, A B, is drawn to one side the greater will
be the force with which it will return, and hence it will arrive
at its position on the other side of the middle line as soon when
drawn far away from this line as it would if drawn but little away.
The same thing is true of the vibrations or waves of air, though it
can not so easily be made plain to you.

252. =How the Sensation of Sound is Produced.=--The vibration of a
sounding body is transmitted to the ear ordinarily through the air,
and there strikes upon a little drum, a membrane at the bottom of
the external cavity of the ear just like a common drum-head. Here
the vibration of the air is communicated to this drum, and from
this to a chain of very small bones. From the last of these bones
it is transmitted to another very small drum, and from this to a
fluid in some very complicated passages in the most solid bone
in the body. These may be called the _halls of audience_. In the
fluid contained in them are spread out the branches of the nerve
of hearing, which receive the impression of the vibration, and
transmit it to the brain, where the mind takes knowledge of it.
Observe that the vibration, transmitted first through the air, then
through the drum, then the chain of bones, then another drum to a
fluid, stops at the fluid. What is transmitted from this to the
brain by the nerve we know not, and so we call it an impression.

253. =Sound Transmitted through Various Substances.=--In ordinary
hearing sound, as you have seen, is transmitted through various
substances before the vibration arrives at the liquid in the halls
of audience. But sound need not take this course in all cases to
arrive at the nerve of hearing. If, for example, you place a watch
between your teeth, the sound will go through the solid teeth and
the bones of the jaw directly to the halls of audience by a short
cut, instead of going round through the outer ear-passage to the
drum, and so through the chain of bones. Fishes in hearing receive
the vibration through water. If you place your ear at the end of a
timber, while some one scratches with a pin at the other end, you
hear the sound distinctly, for the vibration is transmitted through
the timber; as in the case of the watch between the teeth, it goes
through the solid bone.

[Illustration: Fig. 185.]

254. =Sound not Transmitted through a Vacuum.=--As sound is a
vibration of some substance it can not be transmitted through
absolute space. This can be proved by an experiment with the
air-pump, as represented in Fig. 185. The apparatus in the receiver
is so arranged that the bell, _a_, can be struck by pressing down
a sliding rod, _h_. If it be struck before the air is exhausted
the sound is heard through the glass. But the more you exhaust the
air the fainter will be the sound; and at length, if you keep on
pumping, it can not be heard at all. The same experiment can be
tried with a music-box. It is from the thinness of the air on high
mountains, and at the great heights reached by balloons, that all
sounds are so faint. The report of a pistol fired off on top of
Mont Blanc is a mere crack compared with its report when fired in
the valley below.

255. =Motion of the Heavenly Bodies without Noise.=--Sound is
often heard at a very great distance on the earth. The sound of an
eruption of a volcano has been heard in one case at the distance
of 970 miles. But suppose that the same sound should occur at the
same distance from the earth, that is, over 900 miles beyond the
atmosphere that enrobes the earth, no inhabitant of our world could
hear it, for the same reason that you do not hear the bell ringing
in an exhausted receiver. If, therefore, any sound, however loud,
should be given forth by any of the heavenly bodies we could not
hear it. The course of these bodies in their orbits is noiseless,
because they meet with no resistance from any substance. Bodies
passing rapidly through our atmosphere cause sound, from the
resistance which the air gives to their passage. The whizzing of
a ball is an example of this. It is the passage of the electric
fluid through the air which produces the thunder. But the heavenly
bodies, having no such resistance, make no sound in their course,
though their velocity be so immense. In the expressive language of
the Bible, "their voice is not heard."

256. =Velocity of Sound.=--The velocity of sound varies in
different media. Thus it passes through water four times as rapidly
as it does through air. Dr. Franklin, with his head under water,
heard distinctly the sound of two stones struck together in the
water at the distance of more than half a mile. Sound passes
through solids much more easily, and therefore more rapidly, than
through liquids. Thus its velocity through copper is twelve times
and through glass seventeen times greater than through air. If you
place your ear against a long brick wall at one end, and let some
one strike upon the other end, you will hear two reports, the first
through the wall and the second through the air. Indians are in the
habit of ascertaining the approach of their enemies by putting the
ear to the ground. When the eruption of a volcano is heard at a
great distance the sound comes through the solid earth rather than
through the air. The ready transmission of sound through solids
furnishes us with a very valuable means of examining diseases of
the lungs and heart. The sounds occasioned by the movement of the
air in the lungs and by the action of the heart are very distinctly
heard through the solid walls of the chest.

257. =Measurement of Distances by Sound.=--It makes no difference
with the velocity of sound whether it be loud or not. Thus the
sounds of a band of music at a distance all reach your ear at the
same time, the sounds of the instruments that can scarcely be heard
keeping exact pace in the air with the sounds of the loudest. So,
also, the velocity of sound is uniform throughout its whole course,
being just as rapid when it is about to die away as it was when it
began. It is from this uniformity in the velocity of sound that we
can estimate the distance of the object by which any sound is made.
We do it by a comparison between light and sound. Sound moves at
the rate of 1120 feet in a second. Now light moves 192,000 miles a
second, and therefore, for all ordinary distances on the earth, we
need make no allowance of time for light in comparison with sound.
If we see, then, the operation by which a sound is produced we can
estimate its distance from us by the length of time which elapses
between what we see and what we hear. In this way we can estimate
very accurately the distance of a cannon that we see fired, or the
distance of a flash of lightning.

258. =Loudness of Sound.=--The loudness of sound depends upon the
width of the vibrations producing it. The harder you strike the
end of the tuning-fork, Fig. 182, the farther will it vibrate the
one way and the other, and the louder will be the sound. The same
thing is true of the strings of a piano. A round bell, when it
is struck, tends in its vibration to take an oval form, and the
extent of its vibration back and forth as it does this governs the
loudness of the sound. As sound passes from the sounding body the
vibration gradually lessens, and at length dies away. It is like
the successive vibrations or waves of water produced by dropping
a stone in it. The louder the sound is the larger are the first
vibrations, and the farther will the vibrations extend, as in water
a large stone dropped into it will produce larger waves than a
small one, and the waves will extend over a greater space.

259. =Diffusion of Sound.=--When there is no hindrance sound
spreads equally in all directions. It is in this respect with the
vibrations or waves of air as it is with the waves of water when
a stone is dropped into it. Light is also diffused in the same
manner, as you will see in another chapter.

[Illustration: Fig. 186.]

260. =Reflection of Sound.=--As waves of water striking against
any object bound off, so it is with the vibrations or waves of
sound. And the same is true of this as of all motion, as stated
in § 206, that the angle of incidence is equal to the angle of
reflection. The reflection of sound is the cause of _echoes_. In
order that an echo be perfect the sound must be reflected back
to the ear from some plane surface of some size. Sometimes when
there are successive plane surfaces of rocks along a river there
are successive echoes. Thus in Fig. 186 (p. 200) is represented
a locality on the Rhine where a sound is reflected at successive
places, 1, 2, 3, 4. The rolling of thunder, though sometimes
caused by the different distances of parts of the same flash of
lightning, is commonly owing to reflections of the sound among
the clouds. From this cause the report of a cannon is more apt
to be a rolling sound when there are clouds above than when the
sky is clear. Sound is continually reflected in every variety of
direction from obstacles with which it meets. Thus in a room it is
reflected from the walls and from all the objects in the room;
and the more varied are the surfaces the more varied and confused
are the reflections. You know that a voice has a very different
sound in a room when it is empty from what it has when the room
is filled with an audience. Indeed, a blind speaker can estimate
very nearly the size of his audience by the sound of his own voice.
The explanation is, that with a full audience the surfaces for
reflection are vastly multiplied, and so deprive the sound of the
sharp and ringing character which is given to it by reflection from
comparatively few surfaces which are plane and firm. The effect
produced by an audience upon the voice of the speaker is quite
analogous to that of muffling upon the sound of a drum.

[Illustration: Fig. 187.]

261. =Whispering Galleries.=--The reflection of sound from curved
surfaces gives us some interesting phenomena. The waves of sound
in being reflected from a concave surface are gathered together
to some point. If the surface be a perfectly spherical one, and
the sound issue from the centre, the reflection will be from all
points to the centre. But suppose the concave surface have the
curve of an ellipse, as represented in Fig. 187. This, instead
of having a centre, has two foci, _c_ and _g_. Now if a sound
proceed from one focus, _c_, the waves of sound, as represented by
the lines _c d_, _c e_, _c f_, _c h_, will all be reflected to the
other focus, _g_; so that if a person speak in a very low tone or
even whisper at _c_, he may be heard distinctly by another at _g_,
though persons at other points may hear nothing. We may have this
result with a curved wall extending even several hundred feet; and
such structures are called whispering galleries. If in one of these
galleries a person standing in one focus speak loudly he will be
heard by others at any point by the _direct_ waves of sound; but
the reflected sound will be added to the direct in the case of one
standing at the other focus.

[Illustration: Fig. 188.]

[Illustration: Fig. 189.]

262. =Concentration of Sound.=--It is by the reflection of sound
that it can be concentrated in various ways. Thus in using a
speaking-trumpet the waves of sound, instead of moving in all
directions as soon as they escape from the mouth, are reflected by
the sides of the instrument toward a central line as represented
in Fig. 188. The waves or vibrations, being thus concentrated have
more intensity and are thrown to a greater distance than if they
issued directly from the mouth. So a speaking-tube, confining the
vibrations, carries the voice to distant parts of a building. For
the same reason the voice can be heard much farther through a
narrow street than in an open space. So, also, a speaker can be
heard more distinctly in a hall than when addressing an audience
of the same size in the open air. The "sounding-board," once
so fashionable in churches, was really of considerable service
in preventing the escape of the vibrations of the voice of the
preacher upward, and directing them downward upon the audience. In
the hearing-trumpet, Fig. 189, the vibrations are collected in
the broad open end of the instrument, and by reflection are thrown
together into a narrow compass before they enter the ear to strike
upon the drum. We often instinctively make the palm of the hand act
as an ear-trumpet when we do not hear distinctly. Many animals have
the external ears movable, so that they can direct their concave
surface toward the point from which they wish to hear. Such ears
are movable ear-trumpets.

263. =Difference Between a Musical Sound and a Noise.=--The
difference between a musical sound and a noise is very analogous to
the difference between a crystal and the same substance destitute
of the crystalline arrangement. In both there are vibrations, but
in the musical sound they have perfect regularity, while in a noise
the vibrations are irregular, and there is confusion. Indeed so
regular are the vibrations of musical sounds that the rules and
principles of music have all the rigid exactness of mathematics.

264. =How Different Notes are Produced.=--The quicker is the
vibration the higher is the note. Thus a short and small string on
a violin or in a piano gives a higher note than a long and large
string, because its vibrations are quicker. The tension of the
string also has an influence, the note being raised by increasing
the tension. In tuning a violin the right pitch is given to each
string by lessening or increasing the tension by means of the
screws to which the strings are attached. In playing upon it
various notes are made upon each string by shortening the vibrating
portion more or less by pressure of the finger.

[Illustration: Fig. 190.]

In wind instruments the note depends on the length and size of the
column of air contained in them. This may be illustrated by an
organ-pipe, Fig. 190 (p. 203). It is one of the pipes of what is
called the flute-stop. It is constructed very much like a boy's
willow whistle. The air from the bellows of the organ enters at P,
and causes a vibration of the whole column of air in the pipe, the
sound issuing at _t_. In the upper end is a movable plug, _s_, by
which, in tuning, the note of the pipe is regulated. If the note be
too grave this plug is pushed downward, so as to shorten the column
of air.

It is from difference in rapidity of vibration that a large bell
gives a graver note than a small one. So, too, when musical sounds
are produced by passing the moistened fingers over the edges of
glass vessels, the larger the vessel the graver is its note. A
tumbler will give a graver note than a wine-glass.

265. =Human Voice.=--The principles which I have developed in
relation to musical instruments apply to the voice. The musical
instrument of man, by which the voice is produced, is contained
in a very small compass. It is that box at the top of the throat
commonly called Adam's apple. Across this, from front to rear,
stretch two sheets of membrane, leaving a space between their
edges. In our ordinary breathing these membranes are relaxed, and
the space between their edges is considerable, to allow the air to
pass in and out freely. But when we speak or sing these membranes,
or vocal chords, as they are termed, are put into a tense state
by muscles pulling upon them, and the opening between them is
lessened. The voice is produced by the air that is forced out from
the lungs, which, striking on the chords, causes them to vibrate.
The nearer their edges are together, and the more tense they are,
the higher is the note. The sounds are produced precisely as those
of the Æolian harp are, the air causing in the one case a vibration
of strings, and in the other of edges of membranes.

266. =Harmony.=--When notes, on being sounded at the same time,
are agreeable to the ear, they are said to harmonize. Now this
harmony depends on a certain relation between the vibrations.
The more simple is the relation the greater is the harmony. For
example, if we take the first note, termed the fundamental note, of
what is called the scale in music, it harmonizes better with the
octave than with any other of the eight notes, because for every
vibration in it there are just two in the octave. Take in contrast
with the octave the second note. Here to every eight vibrations of
the first note we have nine of the second, and the consequence is
a discord when they are sounded together. The difference between
the two cases is this: In the first case the commencement of every
vibration in the fundamental note coincides with the commencement
of every second vibration in the octave. But in the other case
there is a coincidence at only every eighth vibration of the first
note with every ninth of the second. Next to the octave, the most
agreeable harmony with the fundamental note is that of the fifth
note of the scale. Here we have three vibrations to every two of
the first note, and so every second vibration in the first note
coincides with every third vibration of the fifth. Next comes the
harmony of the fourth, there being here a coincidence at every
third vibration of the fundamental note. The more frequent, you
see, are the coincidences between the vibrations the greater is the
harmony. In the three cases just stated the coincidence is in the
first at the commencement of _every_ vibration of the fundamental
note, in the second case at the commencement of every _second_
vibration, and in the third at the commencement of every _third_
vibration.

267. =The Diatonic Scale.=--In order that you may see the relative
numbers of the vibrations for each of the notes I will give them
for the whole scale. They are as follows:

  1  9/8  5/4  4/3  3/2  5/3  15/8  2
  C   D    E    F    G    A     B   C.

According to this the note D has nine vibrations to every eight
vibrations of C, E has five to every four of C, etc., the octave
C having just twice the number of vibrations that the fundamental
note C has. You have here expressed the _proportion_ between the
numbers of vibrations in the different notes. Suppose, then,
that you know the number of vibrations in a second that C, the
fundamental note, has, you can readily calculate the number of
vibrations of each of the other notes. It is done by multiplying
the number which C has by the fractions over the other notes. Thus
if the number of vibrations in a second in the fundamental note be
128, by this process we make the vibrations of all the notes to be
thus:

   C    D    E    F    G    A    B    C
  128  144  160  170  192  213  240  256.

There are really but seven notes in what is called the diatonic
scale, the eighth note, C, being truly the first of seven other
notes above, having relations to each other similar to those of
the notes below, and constituting another octave. So we may have
several octaves, one above another.

It is interesting to observe that the proportionate lengths of
strings required to produce the eight notes of the scale have an
exact numerical relation, but the _reverse_ of that of the numbers
of the vibrations. Thus if you have eight strings of the same size,
their vibrating lengths required for the notes are as follows:

  C   D    E    F    G    A    B     C
  1  8/9  4/5  3/4  2/3  3/5  8/15  1/2.

For the notes of the octave above the lengths are thus:

   C    D    E    F    G    A     B     C
  1/2  4/9  2/5  3/8  1/3  3/10  4/15  1/4.

268. =Unison.=--In tuning instruments so as to make them harmonize
the result is obtained when the corresponding parts of the
instruments have the same number of vibrations. Thus the string in
one violin that gives any particular note must vibrate just the
same number of times in a second that the strings giving the same
note in other violins do, or it will not be in perfect unison with
them. The same is true of other strings for other notes, and also
of the corresponding parts of all kinds of instruments which are
to be played together. When, in tuning instruments together, it is
said that a string of a violin, for example, is too _flat_, the
difficulty is that it does not vibrate with sufficient rapidity,
and it is therefore screwed up to make its note _sharp_ enough, as
it is expressed, to be in unison with the note of the corresponding
strings or parts of other instruments.

269. =Mysteries of Sound and Hearing.=--There are many things of a
mysterious character in relation both to sound and the manner in
which it causes the sensation of hearing. I will barely notice but
two of these. The effect, or rather the chain of effects, resulting
in hearing is wholly mechanical, until we come to the nerve of
hearing, which branches out with minute fibrils in the halls of
audience of the internal ear. It is merely a series of vibrations.
Now how it is that the mere agitation of a fluid inclosed in hard
bone can communicate through fine white fibres to the brain,
and through that to the mind, the idea that we have of all the
various sounds that are produced, is a great mystery. All that we
know is that the nerve is the medium of the communication, but
of the manner in which it performs its office we know absolutely
nothing. Again, while it is sufficiently mysterious that this
information can thus be given to the mind when one sound after
another communicates its vibration to the liquid in the ear, the
mystery is greatly enhanced when various sounds come to the ear
at one and the same time. To get a distinct idea of the very
compound and wonderful character of the process of hearing in such
a case we will suppose that a full band of music is playing, and
at the same time mingled with its sounds there are various other
sounds heard, some of them perhaps discordant. What a diversity of
vibrations we have here! We have the slow vibrations produced in
the grave notes, and the quick vibrations of the higher ones, all
traveling together through the air to the ear, and each preserving
its distinctive character. And more than this, after they arrive
at the ear they are communicated unaltered through the drum, the
chain of bones, the second drum, and the liquid where the nerve is,
so that a correct report of each of all the notes is given through
the nerve to the mind. Then, too, if there be any discord its
vibration travels along with the rest, and so do the vibrations of
other sounds, as the roaring of the wind, the report of cannon, and
the noise of the people. And besides all this, in the multiplicity
of the vibrations thus transmitted through so many different
substances the mind gets a true report of the comparative loudness
of the sounds, and even of their character, so that the sounds of
drum, fife, trumpet, etc., are all accurately distinguished. In
view of such wonders how significant is the question, "He that
planted the ear, shall he not hear?"




CHAPTER XIII.

HEAT.


270. =Heat and Cold.=--In common language we speak of heat and
cold as two distinct and opposite things. That this is not
strictly correct may be shown by the following experiment: Take
three vessels, and fill the first with ice-cold water, the second
with hot water, and the third with tepid water. If you place
your right hand in the first and the left in the second, and let
them remain a little time, on taking them out and plunging them
together into the third vessel, the water in it will feel warm to
the right hand and cold to the left. So the air of a cellar seems
warm to you in winter and cold in summer in contrast with the air
outside. For the same reason water of a temperature that would
ordinarily be refreshingly cool to us seems warm when drank after
eating ice-cream. It is manifest, then, that there is no fixed
dividing-line between heat and cold. There is, in fact, no such
thing as cold. Substances are cold from being deprived of heat; and
no substance ever has all its heat taken from it. Sir Humphrey Davy
proved that there is heat in ice by rubbing two pieces together in
a very cold room. They were gradually melted. Now this was not done
by the air, for that was at a temperature below the freezing point.
The heat which melted the ice came from the ice itself by means of
the rubbing.

271. =Nature of Heat.=--There are two theories in regard to the
nature of heat. One is that heat is an imponderable (§ 16), and of
course a very subtile substance, which pervades all matter. Its
particles are supposed to repel each other strongly, and hence
they have a tendency to diffuse themselves, and to separate the
particles of matter from each other. It is in this way that they
are supposed to occasion the expansion of substances. The other
supposition, which is most commonly received, is that heat is a
vibration of the particles of bodies, and that it passes from these
to bodies less warm through a subtile fluid called ether, supposed
to fill all space. You see that if this be the true theory, there
is some analogy between heat and sound.

[Illustration: Fig. 191.]

272. =Sources of Heat.=--The principal of the sources of heat on
our earth is the _sun_, though that body is ninety-five millions
of miles distant from us. As the heat, in traveling all this long
journey, is becoming more and more diffused; or, in other words, as
its rays are all the way separating from each other more and more,
we can have no conception of the concentrated heat that exists
in the sun itself. We can, however, approximate to the idea by
observing the effects of heat when some of its separated rays are
gathered to a point by a powerful lens, as represented in Fig.
191. A lens which concentrated the heat ten thousand times melted
platinum, gold, quartz, etc., in a few seconds. And as the heat
at the sun is supposed to be thirty times more concentrated than
this, none of the most solid substances of our earth would remain
solid if they were there, but would be some of them liquid, and
others even in a state of vapor. The heat which the sun constantly
radiates to the earth pervades all substances, producing motion,
and awakening life every where, so that, in the expressive language
of the Bible, "There is nothing hid from the heat thereof."

Another source of heat is _within the earth itself_. It has been
found as we go down into the earth there is a constant increase of
temperature the farther we go. This internal heat is attributed in
part to subterranean fires and various chemical actions. We see
here and there external evidences of the operation of these causes
in the eruptions of volcanoes, the boiling springs, the jets of
steam and sulphureous vapors, etc. But that the heat in our earth
which comes from these subterranean sources is small compared with
that which comes from the sun, is seen in the fact that the rate
of increase of heat at great depths is much less than it is nearer
the surface. This would seem to show that although fires within
the earth may have considerable influence in heating its crust, on
which we live, it derives the most of its heat from the sun, at
least to a very great depth.

How great a source of heat _electricity_ is we know not, but that
considerable heat comes from this source is evident from the
melting and burning effect which we often see resulting from the
passage of the electric fluid.

Another very common source of heat is _chemical action_. We see it
continually produced in chemical experiments. Combustion, which, as
will be shown to you in the Second Part of this Series, is nothing
but an example of chemical action, is the most common of all the
chemical sources of heat. Animal heat is also, for the most part, a
result of chemical action.

_Mechanical action_ is a common source of heat. The rubbing of
a match producing heat enough to occasion flame is a familiar
example. The spark produced in what is called striking fire is the
burning of a particle of steel set on fire by the blow. The Indian
was accustomed to light his fire by the rubbing together of two dry
sticks till he learned an easier way from civilized neighbors; and
the blacksmith, previous to the invention of phosphorus matches,
often lighted his fire by touching a sulphur match to a nail made
red-hot by rapid and continued hammering. Machinery has sometimes
been set on fire by friction, and the water around a mass of metal
has been so heated by boring as even to boil. If you stretch a
piece of India rubber several times in quick succession, and then
apply it to your lips, you will perceive that the motion has warmed
it.

273. =Relations of Heat and Light.=--Heat is sometimes alone, and
is sometimes in intimate union with light. All substances have
some amount of heat, and it passes from them to other bodies in
their neighborhood that happen to have less heat in them. In doing
this it may or may not have the company of light. In the radiation
of heat from a stove, unless it be heated to redness, there is
no light with the heat; but from an open, burning fire the light
and heat come together. But the rays of the sun give us the best
example of the union of light and heat. Traveling together at an
equal pace they are most curiously mingled, as you will see when I
come to speak particularly of light.

I will now proceed to notice the principal effects of heat; viz.,
expansion, liquefaction, and vaporization.

[Illustration: Fig. 192.]

274. =Expansion in Solids.=--Heat, you have seen in § 23, acts in
opposition to the attraction of cohesion, tending to separate the
particles, and so produces an expansion of any substance. This may
be exemplified in the experiment represented in Fig. 192, in which
A B is an iron rod, which is of such a size that at the ordinary
temperature it will fit into the space, C D, in a bar of iron, and
easily pass through the hole, E. If the rod be heated it will be
enlarged or expanded in all directions, so that it will neither
fit into C D nor pass into the hole, E. When the wheel-wright
puts a tire upon a wheel he uses the expansion of heat to make it
fit tightly and firmly. The tire is made a little too small to
have it fit upon the wheel as it is. But by being heated it is
so expanded that it will readily go on to the wheel, and then in
contracting as it cools it so compresses the fellies as to hold on
very tightly. Water is poured on to cool the iron quickly, and thus
prevent it from burning the wood. Iron hoops are put on barrels
in a similar manner, the compression caused by their contraction
binding the staves together very strongly. So in fastening the
plates of boilers together, the rivets are put in red-hot, so that
in their contraction they may press the plates closely together.
If an iron gate just shuts into its place in cold weather, its
expansion will prevent its shutting when warm weather comes. In
order to avoid this difficulty, calculation must be made in fitting
it for its place for the expansion to which it will be subjected
by heat. So in laying the rails of a railroad in cold weather
care must be taken not to put the ends too near together. In
constructing iron bridges the expansion by heat must be calculated
for in the arrangement. Nails often become loose after the lapse of
years from the wear of the wood around them, occasioned by their
alternate expansion and contraction. The leaking of gas-pipes in
the earth is often undoubtedly caused by the loosening of the
joints from contraction and expansion of the pipes by varying
temperatures of the soil, especially where they are not laid very
deep. If a stopper stick fast in a bottle it can be loosened by
the application around the neck of a cloth dipped in hot water,
because the neck becomes expanded at once by the heat. A similar
expedient was once very ingeniously made use of in repairing the
machinery of the steamer _Persia_ at sea, and was perhaps the
means of saving the vessel and the lives of all on board. The
accident which occurred was the breaking of the port crank-pin of
the engine. The problem to be solved was the removal of this pin,
which weighed nearly a ton, and the substitution of a sound one
which they had on hand in its place. But it was found impossible to
start the broken pin from its socket with all the force which could
be brought to bear upon it by a sort of battering-ram constructed
extemporaneously for the purpose. It was determined now to try
the expansive force of heat. An iron platform was built under the
socket, and a brisk fire made upon it. The socket soon expanded,
and the pin was now readily knocked out by the battering-ram, just
as the stopper of the bottle is easily removed when the neck is
heated. The walls of a very large building in Paris, which had
bulged out and were in danger of falling, were restored to their
upright position by the expansion of heat.

[Illustration: Fig. 193.]

It was done in this way: Long rods of iron were run through the
walls after the plan represented in Fig. 193 (p. 213), their ends
being made with a screw-thread, with nuts fitted to them. The rods
marked _a_ were first heated, and as they lengthened the nuts were
screwed up tight to the walls. On cooling, their contraction would
of course draw the walls together. The other bars, _b_, were now
heated and managed in the same way. The one set, you see, were made
to hold on by their nuts to what had already been gained, while the
other were expanding. By many repetitions of this process the walls
were righted and the building saved. The same mode has been adopted
successfully in other cases of a similar character.

275. =Expansion in Liquids.=--Liquids are expanded by heat more
than solids are. But they are very unequally expanded by it. Thus
water is expanded more than twice as much as mercury, and alcohol
six times as much. We have a frequent example of the expansion of
water by heat in our kitchens. If the tea-kettle be put over the
fire filled to the brim, it will run over long before the water
begins to boil. All liquids occupy more space in summer than in
winter, and in the former case weigh less--that is, have less of
real substance in them than in the latter. If, therefore, alcohol,
or oil, or molasses be bought by the gallon in winter and sold in
summer, there will be a profit afforded by the expansion. Twenty
gallons of alcohol in winter becomes twenty-one in mid-summer.

[Illustration: Fig. 194.]

The influence of the expansion of heat upon the specific gravity
of liquids may be very prettily shown by the following experiment:
Let some little bits of amber--a substance which is nearly of the
same specific gravity with water--be thrown into water in a glass
vessel, and let the water be heated, as represented in Fig. 194, by
a spirit-lamp. That portion of the water which is heated passes
upward because it is made specifically lighter, and colder water
continually comes down to take its place. The upward and downward
currents are as indicated by the arrows, the upward passing up in
the middle, the downward coming down at the sides. This will be
made manifest by the little bits of amber.

276. =Thermometers.=--It is the expansion of liquids by heat that
in the thermometer gives us the measure of temperature. The liquid
metal mercury is commonly used for this purpose, and answers well
except in the extreme cold of the arctic regions. There, as mercury
becomes solid at 39 degrees below zero, it is necessary to use a
thermometer with alcohol in it, as this fluid can not be frozen
by any degree of cold. The operation of the thermometer is simply
this: Heat expands the fluid in the bulb, and the only way in which
it can occupy more space as it expands is by rising in the tube.
The abstraction of heat, on the other hand, causes contraction, and
of course a proportionate falling of the fluid.

277. =Fahrenheit's Thermometer.=--The thermometer was invented in
the beginning of the seventeenth century, but it is not decided
who was the inventor. There may have been in this case, as in
others, more inventors than one, the same ideas having, perhaps,
entered several inquiring minds at the same time. Various fluids
were used by different persons. Sir Isaac Newton used linseed
oil. Fahrenheit, a native of Hamburg, who flourished in the first
part of the last century, was the first to use mercury. Though
various propositions were made by Newton and others in regard to
the measurement of heat by thermometers, no thermometric scale
seems to have met with general reception till that of Fahrenheit's,
which was put forth about 1720. The plan of it is this: His zero
is the point at which the mercury stood in the coldest freezing
mixture that he could make; and he supposed that this was the
greatest possible degree of cold, as it was the greatest that he
knew. He next found the point at which the mercury stood in melting
ice. This he called the freezing point, because the temperature
is the same in water passing into the solid from the fluid state
as in water passing into the fluid state from the solid. In other
words, this point in the scale marks the transition line between
the two states. From this point Fahrenheit marked off 32 equal
spaces or degrees down to zero. He now found the point at which
the mercury stands in boiling water, and called this the boiling
point. Marking off the space on the scale between this, and the
freezing point in the same manner, there are 180 degrees--that is,
the boiling point is 212 degrees above zero. The degrees above zero
are commonly designated by the mark +, plus; and those below by the
mark -, minus. Thus, +32° signifies 32 degrees above zero, and -32°
signifies 32 degrees below.

[Illustration: Fig. 195.]

278. =Other Thermometers.=--Fahrenheit's thermometer is the
one commonly used in this country. But there are several other
thermometers on different scales, as the Centigrade, Reaumur's, and
De Lisle's. In Fig. 195 you see the plans of the scales of these
thermometers placed side by side. In the Centigrade thermometer,
which is in use in France, and indeed in a large part of Europe,
the zero, you see, is placed at the freezing point; and the space
between this and the boiling point is divided into 100 degrees,
which gives it the name Centigrade. Reaumur's, which is in use
in Russia, has the same zero, but he has only 80 degrees from
this to the boiling point. De Lisle's, which has gone entirely
out of use, has its zero at the boiling point. The arrangement of
Fahrenheit, although its zero is a mere arbitrary point, is, on the
whole, the best, because its degrees are of such a size that they
mark differences of temperature with sufficient minuteness for all
practical purposes of an ordinary character without resorting to
fractional parts.

[Illustration: Fig. 196.]

279. =Expansion in Aeriform Substances.=--Heat produces a vastly
greater expansive effect in air, the gases, and vapors, than it
does in liquids. The expansion of air by heat may be shown very
prettily in this way: Take a glass tube that has a bulb on one
end, and, placing the other open end in water (as represented in
Fig. 196), apply the palm of your hand to the bulb. The heat of
the hand being communicated to the bulb will expand the air, and
so, as you see, bubbles of air will escape through the water. On
removing the hand, and allowing the bulb to cool, the air in it
will be condensed, and water will pass up in the tube in proportion
to the amount of air which has escaped. A bladder partly filled
with air will be made to swell out to plumpness if it be heated
sufficiently, and a full one may be so heated as to burst from the
expansion of the air. Porous wood, as chestnut, snaps very much
when burned, because the heat expands the air contained in the
pores.

280. =Balloons.=--The first balloons that were used were filled
with heated air. You have already seen, in § 149, why it is that
balloons rise. Now in the hot-air balloon it is the expansion of
the air by heat that makes it lighter than the surrounding air. Of
course such a balloon is not as effective as the gas balloon, for
the air within it loses its comparative lightness as it becomes
cooled; while the gas which is used, being very much lighter than
air at the same temperature, does not lose its lightness as the
balloon goes up. You learned in § 152 that the atmosphere becomes
thinner as we go upward. The gas balloon, therefore, rises until it
arrives at that point where the air is of about the same specific
gravity with the gas, and there it stops. It is made to descend
by letting out some of the gas from a valve. Gas was not used for
balloons till 1782. Hydrogen gas was employed at first, being over
fourteen times lighter than air. Of late the common burning gas,
carbureted hydrogen, has been generally used, because it can be so
readily obtained where there are gas-works.

[Illustration: Fig. 197.]

281. =Currents in the Air from Heat.=--Heat is the grand mover
of the atmosphere. Any portion of it that becomes warmer than
surrounding portions rises, or rather is pushed up, for the same
reason that a hot-air balloon rises, the only difference between
the two cases being that in the one the air is confined, and in the
other is left free, and so becomes diffused. And it is this rising
of the air from expansion that causes nearly all the movements
that we witness in the air. We see this exemplified in various
ways wherever there is a fire. The air that is heated by the fire
is forced upward by the colder air, which, on the principles of
specific gravity, seeks to get below the warmer and lighter air.
The hot air that comes through the registers of a furnace is pushed
up by colder air below. For the same reason the heated air around a
stove-pipe is constantly going upward. This is very prettily shown
by the toy represented in Fig. 197 (p. 218), which is a paper cut
spirally, and suspended, as you see, upon the point of a wire. The
upward current makes the paper revolve rapidly around the wire. It
is from the rising of warm air that the galleries of a church are
warmer than the space below. In a common room the disposition of
the air is continually to have its warmest portions above and the
colder below. It is for this reason that we have our arrangements
for producing or introducing heat at as low a point as possible.

282. =Chimneys.=--We speak of the _draught_ of a chimney, and we
say of one that does not smoke that it _draws_ well, as if the
smoke were in some way actually drawn up. But the same principles
apply here as are developed in § 281. The smoke, which is a
combination of heated air and gases with some solid matters in a
fine state, is _forced_ up the chimney. When a chimney does not
draw well we open a door or a window for a little while until
the fire gets thoroughly agoing. Why is this? It is that we may
have denser air than there is in the room, so that the smoke may
be pushed up more forcibly. When the chimney becomes well heated
there is ordinarily no difficulty, because then the smoke in it
is not obliged to part with much of its heat to the walls of the
chimney, and therefore is so much lighter than the air in the room
that it is very easily forced upward. The principal reason that a
stove-pipe generally draws better than a chimney is that there is
much less heat expended in establishing and maintaining the upward
current. Especially is this true if the chimney be a large one. In
such a case there are both a great extent of brick and a large body
of air to be heated to establish the upward current, and these must
be kept warm in order to maintain it.[3]

[Illustration: Fig. 198.]

[Illustration: Fig. 199.]

283. =Winds.=--If you open a door of a heated room a candle held
near the floor will have its flame blown inward, while one held
near the top of the door will have its flame blown toward the cold
entry. Here you have a good illustration of the manner in which
winds are produced. Wherever the wind blows it is air pushing
out of the way other air that is warmer, in order that it may,
in obedience to gravitation, get as near the earth as possible.
Take, for example, the land and sea breezes, as they are called.
During a hot summer's day the sun heats the earth powerfully,
while the ocean receives but little of its heat. The heated land
heats the air above it; and as the air over the ocean is cooler,
and therefore heavier, it pushes upward the air of the land, for
the same reason that water pushes up oil; and as this goes on
continuously a regular current is established. The wind blows in
upon the land, as represented in Fig. 198, while the warmer air
passes upward into the higher regions of the atmosphere, and turns
toward the sea. The arrows show the course of the currents. The
resemblance of all this to the effect upon the candle held near
the open door is very obvious, the cold air from the entry blowing
in below representing the breeze from the ocean, and the warm air
of the room blowing out above representing the passage of the warm
air of the land out toward the ocean. At night this is apt to be
reversed. The earth becomes cooled, and with it the air that is
over it. The result is that the cooled air of the land now pushes
upward the warmer air of the sea, as seen in Fig. 199.

[Illustration: Fig. 200.]

[Illustration: Fig. 201.]

284. =Winds as Affected by the Rotation of the Earth.=--The heat
of the vertical sun upon the tropics causes a rise of heated air
into the upper regions, while there is a rush of colder air toward
the equator from both north and south. This effect is represented
in Fig. 200 (p. 221), E being the sun, N the north pole, and S
the south pole. An effect similar to that represented in Figs.
198 and 199 is produced here, but it is on a much larger scale.
But the diagram does not present the matter in its true light in
all respects. The prevailing winds in the equatorial regions are
not north and south winds, as would appear from this diagram; but
they are from the northeast and southeast. I will explain this by
Fig. 201. As the earth turns on its axis it is plain that there
is no part of the surface of the earth that moves so rapidly as
the equator, E W, for that moves in a larger circle than any other
part. And the nearer you go to either pole, N or S, the less is
the rapidity of the revolution. Now the atmosphere, as stated in §
188, partakes of the motion of the earth. The air, therefore, at
the equator is moving from west to east with the rotation of the
earth faster than it is any where else, and the nearer you go to
either pole the slower is its motion. It follows from this that
any portion of air blowing from the north or the south toward the
equator, as it comes from where it was moving east slower than air
at the equator is, would from its lesser momentum lag behind the
air of the equator, the wind would be curved toward the westward,
as indicated by the arrows. The result would be that the northern
wind would be converted into a northeaster, and the southern into
a southeaster. All this can be made more clear with a globe, or,
indeed, with any round object.

285. =Liquefaction.=--The change of solids into liquids is one of
the most observable effects of heat. This change requires different
degrees of heat in different substances. Thus while iron melts at
the high heat of 2786°, lead melts at 633°, sulphur at 239°, ice
at 32°, and mercury at 39° below zero. Mercury is never found in a
solid state, but it sometimes becomes solid in the arctic regions
when carried there and exposed in the open air. We are apt to think
of water as being in a more natural state when liquid than when it
is solid, just as we think of iron as being naturally solid and
mercury as naturally liquid. But in all these cases the state of
the substance depends on its temperature, and this is varied by
circumstances. Water at the equator is always liquid, and the idea
of ice there is exceedingly unnatural; while near the poles it is
the reverse, ice and snow reigning every where throughout the whole
year.

286. =Evaporation.=--There are two ways in which the change of
a liquid into a vapor occurs. One is a rapid change when heat
is so applied as to raise the liquid to its boiling point. This
is commonly termed vaporization. The other mode is the ordinary
gradual evaporation which goes on from the _surface_ of the liquid.
This process is going on continuously, not requiring any particular
degree of heat, but occurring under all degrees of the temperature
of a liquid. Its rapidity, however, is in proportion to the degree
of heat, as may be seen by the rise of vapor from water that is
being heated, long before it begins to boil. The same thing can
also be seen in a bright summer's morning, when the heat of the sun
causes the moisture gathered from rain or dew to rise so abundantly
from fences, and boards, and roofs as to be visible like smoke.

287. =Solution of Water in Air.=--Evaporation is constantly going
on from every wet surface, except when the air is so loaded with
moisture that it can take up no more. The vapor is not ordinarily
visible, the particles of water passing quietly upward among those
of the air, being dissolved in the air just as some solids are
dissolved in water. It becomes visible only when so much of it
rises that the solution of the water in the air is not readily
effected. The readiness with which the solution takes place
depends much upon the temperature of the atmosphere. Some very
common phenomena illustrate this. In a very cold day the breath
of animals, as it comes out of the mouth, seems to be loaded with
moisture. Why? It is not because there is more moisture in it than
in warm weather, but because cold air can not hold in solution so
much water as warm air can. The same explanation applies to the
smoking of wet fences and roofs in the sun of a summer's morning.
The moisture is heated by the sun, but the air, not having become
very warm as yet, can not readily dissolve all the moisture that
rises. The phenomenon is not apt to occur when the hot sun shines
after a shower at mid-day or in the afternoon, because then the air
is warm enough to take up all the moisture that is sent up into it.

How water, being heavier than air, rises in the atmosphere is a
mystery. It has been supposed by some that it was owing to a kind
of affinity existing between water and air. But in opposition to
this is the fact that evaporation takes place more rapidly under
the exhausted receiver of an air-pump, where there is almost no
air, than it does where it is freely exposed to the atmosphere.

288. =Clouds.=--The water which goes up in the air in evaporation
is variously disposed of. Some of it is deposited as dew or frost.
Some of it forms fog. Some of it also mounts far upward and forms
the clouds, which are really collections of fog made high up in
the air. In fog and in clouds the water which in its evaporation
is invisible becomes visible. Let us see how this is. There is
always more or less of water in clear air, but the particles are
so minutely divided and so thoroughly mingled with the particles
of the air that they can not be seen. But in a fog or cloud the
particles of water are gathered together in little companies, as we
may express it. And it is supposed, some think ascertained, that
each of these companies of particles is globular and hollow. If
so, then we may regard every cloud as a vast collection of minute
bubbles or balloons careering through the air.

[Illustration: Fig. 202.]

[Illustration: Fig. 203.]

289. =Shapes of Clouds.=--Clouds have a very great variety of
shape, the causes of which are for the most part not understood.
They are generally divided into four classes: Cirrus, Cumulus,
Stratus, and Nimbus. The _Cirrus_ is represented in Fig. 202 (p.
225). It is a light, fleecy cloud, having graceful turns like
curls, and hence its name, which is the Latin word for curl. Such
clouds are commonly very high up in the air. The _Cumulus_ (Latin
for heap) you see in Fig. 203 (p. 225). Clouds taking this form
appear as heaps rounded upward, and often appear like mountains
of snow when they are illuminated by the sun. We see such clouds
mostly in summer. The _Stratus_ (Latin for covering) is seen in
the same figure under the Cumulus. Clouds of this form lie low in
the horizon, stretched along like a sheet. They often form in the
latter part of the day, and increase in the night, but the rising
sun dissipates them. The _Nimbus_, or rain-cloud, is represented
in Fig. 204 (p. 226). It has a uniform gray or dark color. We
often have two forms of cloud mingled together. Thus in Fig. 205
(p. 226) we have a mixture of the Stratus and the Cirrus, termed
_Cirro-Stratus_. This is commonly called the mackerel-sky, and is
quite a sure prognostic of rain. Then we have the _Cirro-Cumulus_,
Fig. 206 (p. 227), and the _Cumulo-Stratus_, Fig. 207 (p. 227).

[Illustration: Fig. 204.]

[Illustration: Fig. 205.]

Water is gathered into clouds undoubtedly, in part at least, from
the influence of attraction. But what the circumstances are that
give them all these various shapes we know not. Whatever they are,
they sometimes operate very extensively, giving a similar shape
to all the clouds that cover the whole arch of the heavens; and
at other times they operate variously in different localities,
producing different shapes, sometimes even in near neighborhood to
each other. Sometimes the edge of a cloud is irregular, or curved,
or feathery; and at others it is a well-defined line, stretching
along over a large portion of the horizon. In all these cases we
have only divers arrangements of the same thing--a collection of
vesicles of water containing air, which is made lighter than the
air outside of the cloud by means which I shall speak of in another
part of this chapter.

[Illustration: Fig. 206.]

[Illustration: Fig. 207.]

290. =Rain, Snow, and Hail.=--When it rains the vesicles or minute
bubbles of which the clouds are composed are broken up, and each
drop of rain contains the water which came from a multitude of
these vesicles. But let us see exactly how this result is produced.
Rain comes from the contraction of the clouds by cold. A cold
current of air coming in contact with a cloud will condense its
bubbles into drops, and these of course will fall. The same result
occurs if a cloud passes into a cold stratum of air. But let us
look at the process more minutely. Let us see what the effect of
cold is upon the bubbles.

[Illustration: Fig. 208.]

The first effect may be made clear by Fig. 208. If a bubble be
contracted by the influence of cold, the water of its wall being
made thicker, there will be a gathering of it from gravitation at
the lower part, as represented by the dotted line. You often see a
similar effect in the soap-bubble. It rises filled with the warm
air from your lungs, and as it goes up it is contracted by the
colder air which is around it. This contraction makes the water
hang downward from the bottom of it. And as the soap-bubble at
length perhaps bursts in the air from the weight of this water, so
it is with the vesicles in the cloud. And many of these, united
together by attraction, form a drop. When the cold is sufficiently
severe it makes the water of the ruptured vesicles of the cloud
arrange itself in snow-crystals instead of drops. And when the cold
acts with great rapidity upon a cloud it presses the particles
of the water together so suddenly that there is not time for the
crystalline arrangement, and hail is formed.

291. =Vaporization.=--The production of vapor by boiling differs in
some respects from quiet evaporation. Here the liquid is raised in
temperature to its boiling point, and the formation of vapor is not
confined to the surface. In water the boiling point is 212°, but it
varies more or less from this in other liquids. Thus the boiling
point of alcohol is 173°, of ether 95°, oil of turpentine 568°,
and mercury 652°.

[Illustration: Fig. 209.]

292. =Influence of Pressure upon the Formation of Vapor.=--Pressure
restrains the production of vapor, whether it be formed by
evaporation or vaporization. We know by experiments with the
air-pump that the less pressure of air there is upon the surface of
a liquid the more rapidly will evaporation from it go on. I have
already spoken of the influence of pressure upon the boiling of
liquids in § 171. I will give here a few additional illustrations.
Ether boils when it is heated to 95°, three degrees below the heat
of the blood in our bodies. If we place some of it in a vessel
under the receiver of an air-pump, by exhausting the air we can
so take off the pressure that the ether will boil at the ordinary
temperature of the air in a room. The restraint of pressure upon
boiling is very strikingly shown in the _digester_, Fig. 209. This
is a strong boiler, _a_, partly filled with water. A thermometer,
_d_, is fastened into it so as to indicate the heat of the water.
There is also a tube, _c_, extending to near the bottom of the
boiler into a small quantity of mercury which is there. Let, now,
the boiler be heated till the water boils, the air being left to
escape by the stop-cock, _b_. If the stop-cock be shut, and we
continue to apply the heat, we can raise the water to a very high
temperature without having it boil at all, because of the pressure
of the condensed steam upon its surface. An apparatus somewhat
after this plan, called _Papin's digester_, has been used sometimes
in cooking. The great heat to which water can thus be raised causes
it to extract the nutritious matter from bones and cartilages,
affording material for soup from what is commonly thrown away. To
guard against the danger of explosion a safety-valve is provided,
having a weight upon it which will keep it shut until a certain
amount of pressure accumulates, and then it is forced open, letting
out some of the steam.

293. =Steam.=--The cloud of steam, so called, which you so often
see escaping from a locomotive is not really steam. Steam is
transparent and invisible. You can see that it is so if you observe
it issuing from the spout of a tea-kettle. It is only after it gets
an inch or more from the spout that it becomes visible, and then it
is really changed from steam into water by the condensing influence
of the cold air. And the water in the cloud thus formed is probably
in the same condition with the water in the clouds above, as
described in § 288.

[Illustration: Fig. 210.]

294. =The Steam-Engine.=--As compressed or condensed air has
great power by its elasticity, as seen in the air-gun, § 164, so
also has condensed steam. It is steam condensed, and endeavoring,
therefore, in proportion to its condensation, to expand itself,
which constitutes the moving force of the steam-engine. The steam
is generated in a boiler, having, like the boiler of Papin's
digester, a valve with a weight attached to it. This valve is
called a safety-valve, because when the steam has reached a certain
degree of condensation it lifts the valve, and, as some of the
steam escapes, such an increase of pressure as would occasion an
explosion is prevented. The expansive force of steam in a boiler
is estimated in pounds by the weight on the valve, and hence the
common expression that there are so many pounds of steam on. But
the boiler is only the generator of steam, and it remains to show
how the steam is used in moving machinery. This is done by allowing
the steam to pass from the boiler into a cylinder, and then move a
piston back and forth by its expansive force. The manner in which
it does this may be made clear by the diagram, Fig. 210 (p. 231).
Let _e_ be a piston in a cylinder, _f_, which has four openings,
_a_, _b_, _c_, and _d_. These all have valves. The steam is
supplied from the boiler to the cylinder through _a_ and _c_, and
makes its escape from _b_ and _d_. Suppose, now, the piston is near
the bottom of the cylinder, as represented. The valve at _a_ is now
opened that steam may enter to push up the piston, and the valve
at _b_ shuts that the steam may not escape. At the same time, that
pressure may be taken off from the upper surface of the piston, _d_
opens that the steam may escape, and _c_ shuts that none may enter.
When the piston is to be forced downward all this is reversed--_c_
opens to admit the steam, _d_ shuts to prevent its escaping; and
below, _b_ is opened to let the steam escape, and _a_ is shut to
prevent any from entering. This is the plan of what is called the
high-pressure engine. The low-pressure engine differs from it in
having the steam, as it escapes from the cylinder, pass into water
to be condensed. The latter requires less pressure of steam to work
it, and therefore is the safest. The manner in which the motion of
the piston is made to work various kinds of machinery I need not
stop to explain, especially as exemplifications of it may be seen
in every quarter.

295. =Communication of Heat.=--Heat has a constant tendency to an
equilibrium. If therefore any warm substance be in the neighborhood
of one which has less heat, a flow of heat from the former toward
the latter takes place. Now this communication of heat occurs in
three different ways, called Convection, Conduction, and Radiation.
I will speak of each of these separately.

296. =Convection.=--This mode of diffusion of heat is in operation
in those substances whose particles are movable among each
other--viz., liquids and aeriform substances. I have already
alluded to examples of this mode in speaking of the movements which
heat causes in these substances. The heat goes along with the
particles which are moved, or is _conveyed_ along with them, and
hence the term convection. In this movement the heated particles
always ascend, for the reason given in § 275. Of the multitude of
examples of convection I will present but a few.

In the upward current about a stove-pipe you have an example
of convection, the heat generated being carried upward by the
particles of this current. This being so, the heat of a stove has
no effect upon the air _below_ it by convection, though it does
have by radiation, as you will soon see. Any hot fluid becomes
cool chiefly by convection. The air coming in contact with it
taking some of its heat rises, and other air comes in its turn
to be also heated, and so on till the fluid becomes of the same
temperature with the air, and then the currents of air cease.
The liquid cools more rapidly by stirring it, because the air is
brought into contact with a greater extent of surface, and so the
heat is conveyed away more rapidly. The result is the same whether
we disturb the surface by stirring it or by blowing upon it. In the
latter case, however, the effect is increased by making the air
to come more rapidly upon the disturbed surface. So in fanning,
it is the bringing of the air faster upon the surface of the body
that causes the more rapid, convection of heat from it. Every one
must have observed the fact that a buckwheat cake cools much more
quickly than a flour or rice cake. It is because it has so many
pores and little projections, and so presents a much larger amount
of surface to the heat-conveying air than the smoother and more
solid cakes. Viscid fluids, as molasses, oil, etc., when heated do
not cool as readily as water, because their particles are not as
movable, and therefore heat is not conveyed as rapidly upward to be
given off to the air.

[Illustration: Fig. 211.]

297. =Conduction.=--In this mode of diffusion the heat goes through
or among the particles of substances. For example, if one end of a
bar of iron be held in the fire, it travels through or among the
particles to the other end. The gradual progress of the heat may
be seen by the following simple experiment: Take a rod of iron and
attach to it, as seen in Fig. 211, some little balls of wood by
means of wax. By heating one end with a lamp the balls will drop
one after another as the heat passing along melts the wax which
holds them.

[Illustration: Fig. 212.]

298. =Conductors and Non-Conductors.=--Heat is conducted more
rapidly through some substances than through others. There is great
variety in this respect. There is considerable among those which
are reckoned as good conductors, as is shown by the experiment
represented in Fig. 212. Here are cones of the same size of seven
different substances--copper, iron, zinc, tin, lead, marble, and
brick--all tipped with a little wax, and placed on a stove. The wax
will melt on the copper cone first, showing that this is the best
conductor of them all; and on the brick one last, showing that this
is the poorest conductor. The conducting powers of the rest are
according to the order in which I have mentioned them.

Those substances which allow heat to pass through them very slowly
are called non-conductors. The term, though convenient, is not a
strictly correct one, for there are no substances which do not
conduct heat in some degree. Wood is one of these poor conductors,
and hence wooden handles are put upon various instruments and
vessels that are used about fires, as the soldering irons of the
tinman, the metallic tea-pot, etc. As cloth is a non-conductor,
the holder is used in taking off the tea-kettle and in using the
flat-iron. Glass is so poor a conductor that if you hold a rod or
tube of it across the flame of a spirit lamp or gas burner, and
heat it even to redness, you can place your fingers very near to
the heated portion with impunity. I had occasion to-day to bend a
small glass tube in this way, and I observed some water in it quite
near to the heated part which remained undisturbed through the
process. It is the non-conducting quality of glass that makes it so
liable to break, when it is thick, if it be exposed to any sudden
change of temperature. For example, if hot water be poured into
a thick glass vessel, the inner surface is quickly expanded; but
the outer surface not expanding with it, because the heat is not
readily conducted through, this irregularity in expansion causes
a fracture. It is for this reason that the flasks, retorts, etc.,
used by the chemist are made very thin, especially where the heat
is to be applied.

[Illustration: Fig. 213.]

299. =Davy's Safety-Lamp.=--One of the most beautiful applications
of the conduction of heat we have in the Safety-Lamp of Sir
Humphrey Davy, an invention which has been the means of saving the
lives of multitudes of miners. It is represented in Fig. 213. With
this lamp one can go into the midst of the most explosive gases
with impunity. Now all that prevents the flame within from setting
on fire the gases without is a covering of wire-gauze. This, being
a good conductor, conducts off the heat of the flame within so
rapidly that it can not go through the openings _as flame_, and
so does not set fire to the gas without. The fact upon which the
construction of this lamp was based was discovered by trying many
experiments. Among them were the following: A piece of wire-gauze
was held over a candle so that its flame struck against it. The
smoke issued above, but no flame. Then a stream of gas was allowed
to pass through the gauze, as seen in Fig. 214, and was set on fire
above. It burned without inflaming the gas below.[4]

[Illustration: Fig. 214.]

300. =Relation of Density to Conduction.=--Generally the more
dense a substance is the better is its conduction of heat. Thus
the metals are better conductors than wood, marble than brick, the
solids than liquids, and liquids than aeriform substances. We have
frequently a good illustration of the difference between stone and
brick as conductors in the melting of snow on sidewalks. If a light
snow fall in the spring, after the earth has become somewhat warm,
you will see it melted from the stone walks much before it is from
the brick ones. This will be especially the case if the snow be
melted mostly by the warmth of the earth without the agency of the
sun. The explanation is obvious. The stone is a better conductor
than the brick, and therefore the heat of the earth comes up
through the former more rapidly than through the latter.

[Illustration: Fig. 215.]

[Illustration: Fig. 216.]

301. =Conduction in Liquids.=--That liquids are poor conductors of
heat may be shown by an experiment or two. If a thin glass tube
closed at one end be filled with water, and the heat of a spirit
lamp be applied to its upper portion, as seen in Fig. 215, though
the water at this portion may be made to boil, there will not be
the least movement in the lower part. This will be very obvious
if you have some amber-dust in the water. Again, let a little
water be frozen in the lower part of the tube by placing it in a
freezing mixture, and introduce a little oil, and then over that
some alcohol. Hold now the tube over the chimney of a lamp, as
represented in Fig. 216, until the alcohol boils. The ice in the
bottom of the tube will not be in the least affected, and the oil
will be but slightly heated. If the heat were to be applied in
either of the above cases at the lower part of the tube the result
would be different, because convection would then operate in the
diffusion of the heat.

302. =Air as a Non-Conductor.=--Heat is rapidly diffused in air by
convection; but it is only when the air is free that this can be
done. When the air is confined in spaces or pores, or among fibres,
heat makes its way through it very slowly, for it can be diffused
through it then only by conduction. The variety of ways in which
air is of service to us as a non-conductor is almost endless. I
will notice some of them.

303. =Double Windows.=--The efficacy of double windows depends upon
the confined air between them. In the case of the single window a
great deal of the heat inside is lost in this way: The warm air
of the room which comes in contact with the window imparts to it
some of its heat, and, being thus cooled and therefore condensed,
passes downward. As this process goes on continually this downward
current by the window is constant. The current outside is in the
opposite direction. The heat imparted to the window is taken up by
the cold air, and as it thus becomes warmer it passes upward. And
this upward current outside is as constant as the downward current
inside. Now nearly all this is prevented by the non-conducting
quality of confined air in the case of double windows. If a pane
were taken out from the upper part of the inner window, and another
from its lower part, the inner window would be of little use,
for then the heat of the air in the room would be continually
diminished by convection, as when the window is single. The warm
air would pass in at the upper opening, and, being cooled, pass
down through the lower one.[5]

304. =Air as a Non-Conductor in the Walls of Buildings.=--The
spaces included between the outer wall of a building and the
plastering inside being filled with confined air, prevent the heat
of the air in the apartments from passing off readily through
the wall. A house built of brick or stone, with the plastering
placed directly upon the inside of the wall, would be kept warm
with difficulty in winter, because the solid wall would so readily
conduct off the heat to the external air. So, also, such a house
would be very warm in summer, because the heat of the sun and of
the external air would be so rapidly communicated to the air of the
house. In this connection I will mention a contrivance to prevent
the spreading of fires in blocks of buildings, which, though very
effectual, is seldom made use of, partly because it occasions some
trouble and expense, and partly because it takes up a little room.
It is this: A small space is left in the division wall between each
two houses from top to bottom, containing, of course, a body of
confined air, that is, if the space be entirely shut in, which is
as essential here as in the case of the double windows. With such
an arrangement the interior of one house may be entirely consumed
without communicating sufficient heat through the confined air to
set on fire the other.

305. =Fur, Hair, and Feathers.=--Animals that live in cold climates
are provided with suitable coverings for their protection.
Quadrupeds, for example, are covered with fur, and birds have an
abundance of downy feathers. These coverings have no warmth in
themselves, though in common language we speak of them as being
warm. They are simply non-conductors, and so prevent the heat which
is made in the body of the animal from escaping as fast as it
otherwise would. But why are they non-conductors? It is not because
the substance of which they are made is a non-conductor, but
because among their numberless fibres is partially confined that
great non-conductor, air. Let the fur or down be condensed into
a thin hard plate upon the animal, and it would prove of little
service as a protection against cold. Down is much more abundant
on the birds of cold climates than on those in warmer regions,
because more air can be confined among the fibres of down than
among those of common feathers. Quadrupeds that are natives of warm
climates generally have hair instead of fur. When therefore the
horse is taken to a cold climate he requires in winter the defense
of a blanket; and the ox needs under the same circumstances to be
better housed than he ordinarily is. As the elephant is a native
of a climate positively hot, his hairs are scanty and coarse.
Formerly there were elephants in the cold regions of Siberia, as
has been ascertained by remains found there. But the elephant of
Siberia had under its hair, close to the skin, a fine wool to
protect it against the cold. Animals that live in cold climates
have their coverings become finer in fibre in the cold season of
the year, to give them the additional protection which they then
need. And animals with a furry covering, if they are carried into
a warm climate, have their fur become coarse, and approximate the
condition of hair.

306. =Clothing.=--Man has no covering to guard him against cold,
because he is capable of contriving clothing suitable to the
various degrees of temperature to which he may be exposed. The
object of clothing is not to make the body warm, but to keep it so.
The heat of the body is generated continually within itself, and
under all circumstances this heat is maintained quite uniformly at
98°. This, you see, is a much higher degree than the atmosphere
ordinarily has. We are all the time, then, giving off heat to
the air around us, except when the air gets up to 98°. We are
comfortable only when we are giving off heat to a considerable
amount, for the point of temperature which is most agreeable to us
when we are at rest is 70° or a little less, that is nearly thirty
degrees below the temperature of our bodies. When the temperature
is below this we need extra clothing. In making choice of clothing
for various degrees of temperature we practically apply the
principles which I have developed. Those articles of clothing which
can confine or entangle, as we may say, the largest quantity of
air among their fibres are the best non-conductors, or, in common
language, are the warmest. So, too, loose clothing is warmer than
tight, on account of the amount of air between the clothing and
the body. Thus a loose glove is much warmer than a tight one. The
same general fact is exemplified in the coatings of straw which we
put upon tender trees and shrubs in winter. It is the air that is
confined in the tubes of the straw which makes these coverings so
effective a defense. It is probably the air in the pores of the
brick which makes it a poorer conductor than stone, as illustrated
by the fact stated in § 300.

307. =Cocoons.=--Many insects pass through their pupa or transition
state in cocoons. When this is done during warm weather, as in
the case of the silk-worm, the cocoon is simple. But when the
pupa state lasts through the winter special provisions are made
in the arrangements of the cocoon to guard the insect against the
cold. I will cite as an example the cocoon of one of our largest
moths, the Cecropia. This cocoon, fastened to some shrub, keeps its
inmate secure from the rigors of the winter by a very beautiful
arrangement. The real cocoon is similar to that of the silk-worm;
but it has a very dense air-tight outer covering, and the space
between these two coverings of the pupa is filled with a loose
substance, which has air, of course, mingled with its fibres, and
acts, therefore, the part of a blanket for the insect.

308. =Buds of Plants in Winter.=--In the latter part of summer buds
are formed on trees and shrubs, and these contain the germs of the
branches, leaves, and flowers which are to come out the next year.
These of course must be guarded against the cold of winter, and
it is done very much as the pupa is guarded in the cocoon. Each
bud you can see has a covering of scales which is air-tight, and
inside of this there is a soft downy substance, the blanketing of
the bud. In these coverings, which have been called by some one the
"winter-cradles" of the buds, the infant vegetation of another year
rocks back and forth in the wintry winds secure from the cold, till
the warm sun of spring wakes its hidden life into activity.

309. =Snow a Protection to Plants.=--Snow is a good blanket to
the earth, keeping its warmth from escaping into the cold air.
This is because it contains mingled with its feathery crystals
such a quantity of air. If snow come early, before the ground
and the plants in it have become frozen, it will keep them from
freezing through the winter, if it remain during all that time. It
is curious to observe the peculiar arrangement of the snow in the
arctic regions for the preservation of vegetation. First in the
autumn come soft light snows covering up the grasses, and heaths,
and willows. Then as winter advances there are laid on top of these
the denser snows, making a compact, stout roof over the lighter
snows in which the scanty but precious vegetation of those regions
is imbedded. On top of this roof are deposited the snows of spring.
As these melt the water runs off from the icy roof down the <DW72>s,
leaving untouched the plants underneath, which lie there alike
secure from the rush of waters and from the nightly frosts until
the season is sufficiently advanced to bring them out with safety
from their concealment. Then the icy roof melts, and with it the
light snows that have so long encircled the plants, and the sun
wakes them from their long sleep to a new life.

310. =Influence of the Conduction of Heat on Sensation.=--If you
place your hand upon fur hanging at the door of a fur-store it
does not feel as cold as the wood from which it hangs, and the
wood does not feel so cold as the iron bar of the shutter close
by. Why is this, when these substances are exposed to the same
atmosphere, and really have the same temperature? It is because the
iron conducts the heat from your hand more readily than the wood,
and the wood more readily than the fur. So the iron handle of a
wooden pump feels colder than the pump, and the pump colder than
the snow around it. For the same reason, in a cold room the rug or
the carpet will not feel as cold as the poker and the hearth. If
water has stood long enough in a room to be of the same temperature
with the air of the room your hand will feel colder in the water
than in the air, because the water is the better conductor. So much
for the sensation of _cold_. On the other hand, when substances
are so heated as to give us the sensation of heat, the conductors
do this more than the non-conductors. As they receive heat readily
they also readily impart it. For this reason, with a brisk fire the
hearth-stone feels very hot, while the rug before the fire does not.

311. =Radiation of Heat.=--Every substance sends heat into space
constantly in straight lines in every direction. These lines are
radii, and hence the term radiation is applied to heat diffused in
this way. It is very obvious in regard to the sun that it radiates
heat in all directions. The same can be seen in the case of a
heated iron ball. In whatever direction you hold your hand, above,
below, or laterally, you feel the heat. And it makes no difference
whether the ball be red-hot or not. That is, heat is radiated
either with or without light. When a room is warmed by a furnace
it is warmed altogether by convection; but when it is warmed by a
fire, either in a fire-place or a stove, we have both convection
and radiation. The heat which we receive from the sun comes
altogether by radiation.

312. =Connection between Heat and Light.=--The heat and light of
the sun pass together through transparent substances, as air,
glass, water, etc., without heating them to any extent. Thus, when
the heat is transmitted through a lens, § 272, the lens is little
heated, that is, it lets almost all the heat pass through it. The
air is heated by the sun, but not directly to any amount. It is
heated indirectly in this way: the rays of the sun passing through
the air heat the earth, and then the air receives a part of this
heat from the earth, which is diffused through it by convection.

It is otherwise with heat that comes from a common fire. It does
not seem to be so thoroughly united with the light, and therefore
readily parts company with it, as we may say. While the heat and
light of the sun go together through all transparent bodies the
heat of a fire will not go with its light through all of them. So
while the heat of the sun does not warm the glass through which it
passes the heat of a fire will warm it, and therefore glass is an
effectual screen against it. In some operations in the arts a mask
of glass is sometimes worn to ward off the heat. The connection
of light and heat will be farther noticed when I come to treat of
light.

313. =Relation between Radiation and Absorption.=--All surfaces
that radiate will absorb also equally well the heat that is
radiated upon them. All rough and dark surfaces both absorb and
radiate freely; but all light- and polished surfaces do both
slowly. For this reason the black, rough tea-kettle is well fitted
to heat water in; but it is not fitted to retain the heat in the
water. On the other hand, the bright, polished tea-pot absorbs heat
poorly, but retains it well.

[Illustration: Fig. 217.]

314. =Reflection of Heat.=--Radiated heat is reflected; and here,
as in the case of motion, § 206, and of sound, § 260, the angles of
incidence and reflection are equal. Some interesting experiments
in relation to the reflection of heat can be tried with concave
metallic mirrors. Thus, if we take two such mirrors, as represented
in Fig. 217, and place in the focus of one a thermometer, and in
the focus of the other a small flask of hot water, or a heated
iron ball, the mercury in the thermometer will rise, although the
mirrors may be many feet apart. Observe how the effect is produced.
Rays of heat go from the flask directly toward the thermometer, as
represented by the lines in the figure; but that the effect does
not come from these can be proved by removing the mirrors, leaving
the flask and thermometer just as they are. When the experiment is
tried in this way no effect is produced on the thermometer, because
it is too far from the source of heat, the flask, to receive any
perceptible influence in this way. The effect comes from the rays
of heat which go to the mirror near the flask, and are reflected
to the other mirror, and then are reflected upon the thermometer,
all of which is represented by the dotted lines. There is another
way, besides that already mentioned, of showing that it is not
the _direct_ rays that produce the effect. After arranging the
apparatus put a screen between the thermometer and the mirror near
it, and the effect will be prevented because the reflection is cut
off. If a piece of ice be substituted for the flask of hot water
the thermometer will fall--an effect opposite to that produced
in the previous experiment. This would seem to show that cold is
radiated, but as there is no such thing really as cold, § 270,
the effect must be attributed to the radiation of heat from the
thermometer to the ice. If a hot ball be placed in the focus of
one mirror and a piece of phosphorus in that of the other, the
phosphorus will be set on fire, though the mirrors may be twenty or
more feet apart.

[Illustration: Fig. 218.]

The reflection of heat may be exhibited very prettily with the
experiment represented in Fig. 218. A sheet of bright gilt
paper is rolled up in the shape of a funnel, with the metallic
side inward. Holding the larger end toward a fire, the rays of
heat coming from the fire into the funnel are reflected toward a
central line, and so pass out of the smaller end of the funnel
concentrated. If, now, a bit of phosphorus or a lucifer match be
held a little distance from this end of the funnel it will be set
on fire.

315. =Formation of Dew.=--It is by the radiation of heat that
dew is formed. The earth is constantly radiating heat into space
as well as the sun. In the daytime it receives a great deal more
than it radiates. But at night this is reversed, and the earth is
cooled. The cooled earth condenses the moisture in the air which
is in contact with it, and so the moisture is deposited. If the
weather be very cold this is frozen, and then we have frost instead
of dew. You observe that the dew does not _fall_, though this is
the ordinary expression. Its formation is analogous to the deposit
of moisture which we so often witness in a hot day in summer on the
outside of a tumbler containing cold water. As the cold tumbler
condenses the moisture in the air, so does the earth at night,
it being cooled by radiation, condense the moisture which has
accumulated in the air by evaporation during the heat of the day.

There are some circumstances which have an influence upon the
deposition of dew and frost. Less is deposited under a tree than
outside of it, because all the heat which radiates vertically
upward from under the tree is radiated back again by the tree.
Hence the efficacy of a covering over plants as a defense against
frost. Clouds operate in the same way, and for this reason no dew
or frost is deposited in a cloudy night. Neither is any deposited
in a very windy night, because the moving air promotes evaporation,
and thus prevents the accumulation of moisture.

Dew is deposited in different amounts on different substances.
This is owing to a difference in radiation. Grass and leaves
radiate heat better than earth, and earth better than stone; and
therefore while stones and gravel-walks may be dry or nearly so,
the loose earth may be moist and the grass and leaves thoroughly
wet. So you see that not even the dew, plentiful as it is, is
wasted by the Creator, but is deposited just where it is wanted to
refresh the parched earth and its vegetation.

316. =Gideon's Fleece.=--If you lay a fleece of wool upon the
ground, it is so poor a radiator of heat that no dew will be
deposited upon it, although there may be an abundance of it on the
grass and leaves in its neighborhood. But this was reversed in the
case of Gideon's fleece. The laws of nature were set aside, and the
fleece was wet with dew while all around was dry.

317. =Dew-Point.=--What is called the dew-point of the air is that
degree of temperature to which any substance must be brought down
in order to have dew deposited upon it. This depends upon the
amount of water there is in the atmosphere. The more there is the
higher is the dew-point. When water condenses on a cold tumbler in
a hot day there is much more water in the air, and the dew-point
is higher, than when no moisture is condensed upon the tumbler.
So after a very hot clear day the earth needs not to be much
cooled to produce a deposit of dew, because the air has become so
highly charged with moisture through the evaporation of the earth
under the hot sun. We can very readily at any time ascertain the
dew-point. Take a glass of water, and, having a thermometer in
it, drop into it some pieces of ice, and watch the outside of the
glass. As soon as it begins to be dimmed with moisture look at the
thermometer, and you have the dew-point.

318. =Freezing Mercury.=--Mercury can be frozen by radiation when
the cold is excessively severe, although the thermometer may
indicate a temperature considerably above -39°, the degree at
which mercury freezes. Suppose that in a clear, still night the
temperature of the air is at -20°. In order to freeze the mercury
it must be cooled down 19 degrees below this. Now this can be
done by surrounding the mercury with some good non-conductor, as
charcoal. This cuts off the supply of heat to the mercury, while
it is all the while giving off heat into space by radiation. In
like manner can ice be formed in an atmosphere that is above the
freezing point, and this is often done in warm climates.

319. =Latent Heat.=--You have seen, § 270, that our sensations do
not inform us accurately of the amount of heat in any substance.
The same is also true of the thermometer. This only indicates the
_sensible_ or free heat. There may be a great deal of heat locked
up, as we may say, in the substance, which can be brought out or
made free by some change in the substance. This heat thus locked up
is called _latent_ heat.

[Illustration: Fig. 219.]

320. =Capacity for Heat.=--The more heat a substance can take in
and render latent the greater is its capacity for heat, as it is
expressed. Thus water has a much greater capacity for heat than
mercury. This can be proved by various experiments. Thus, if we
take two vessels just alike, and having, the one a certain quantity
of water in it, and the other the same quantity of mercury, and
expose them to the same degree of heat, it will take much longer
to raise the water to any specified temperature than the mercury.
Why is this, when they are both receiving the same amount of heat?
It is because the water renders a much larger portion of the heat
latent than the mercury does. We can reverse this experiment.
Take these same vessels with their contents raised to the same
temperature, as indicated by the thermometer, and allow them to
cool in the air side by side. The mercury will cool much faster
than the water, because it has much less of latent heat to part
with. The difference in capacity for heat between water, oil, and
mercury may be shown by the experiment represented in Fig. 219. A
pound of water is put into one Florence flask, a pound of oil into
another, and a pound of mercury into a third. They are all heated
to 212°, and are then placed in funnels filled with pounded ice,
the funnels resting in glass jars of the same size. Now in cooling
these fluids down to a certain point, say 32°, different amounts
of the ice will be melted, in the proportions of 100 and 50 and
3. This shows the proportions of latent heat in them which become
sensible or free as their temperatures are lowered.

[Illustration: Fig. 220.]

321. =Relation of Latent Heat to Density.=--The more dense a
substance becomes the less is its capacity for heat. The heat
produced by hammering iron is the latent heat rendered free by
condensation, this lessening the capacity of the iron for heat. The
same thing can be better illustrated in the condensation of a very
compressible substance, as air. In Fig. 220 you have represented a
glass syringe with a closed end. If there be placed in this end a
little bit of cotton wool moistened with ether, and the piston be
forced downward very quickly, the ether will be set on fire. This
is because the compression of the air lessens so much its capacity
for heat that a great deal of its latent heat is made sensible or
free. The heat which is concealed in it in its ordinary state is,
as we may say, fairly squeezed out, as you would squeeze out the
water that is concealed in the interstices of a sponge.

322. =Coldness of Air at Great Heights.=--You learned in § 152 that
the atmosphere is thinner the farther you go from the earth. It is
very thin, therefore, on the summits of high mountains. This is the
chief reason why it is so cold there, for the rarer the air is the
greater is its capacity for heat, and the more of sensible or free
heat therefore can it render latent.

323. =Relation of Latent Heat to the Forms of Substances.=--Whether
a substance shall be in the form of a solid, liquid, or gas depends
upon the amount of heat which is latent in it. If you take a piece
of ice and melt it in a vessel, the ice and the water in the vessel
that comes from the melting of the ice are both at 32° until the
ice is all melted. But all this time heat is being communicated to
the ice and water. What becomes of it? It is all taken up by the
ice as it changes from its solid to its fluid state, and becomes
latent in it. In fact _every particle of ice must have just so
much of latent heat in order to become fluid_. So, also, if water
be heated to the boiling point, 212°, and be kept boiling, the
water will remain at that point till it is all vaporized. All this
time the water is receiving heat, which, instead of raising its
temperature, is becoming latent in the particles as they change
from their liquid to their vaporous state. As I said of the change
from the solid to the liquid state, so here, _every particle of
the liquid must have just so much of latent heat in order to
become aeriform_. Whenever therefore any solid substance becomes
liquid, or liquid becomes aeriform, heat is absorbed and becomes
latent. So, on the other hand, whenever any aeriform substance
becomes liquid, or liquid becomes solid, latent heat is given
out, and becomes free and sensible. The freezing of water, then,
is a source of warmth to the air in its neighborhood--a fact which
is practically made use of when tubs or pails of water are placed
in conservatories to keep plants from freezing; and the thawing
of snow and ice is a source of cold, as is exemplified by the
chilliness of the air occasioned by this process.

324. =Clouds and Latent Heat.=--The water of which clouds are
composed is heavier than air. Why, then, does it remain suspended?
Why is it necessary that it should be collected into drops in order
to have it fall? This question can be answered by looking at the
manner in which clouds are formed. A cloud, I have stated in § 288,
is made up of minute vesicles or bubbles containing air. Now the
air in these bubbles is lighter than the air that is around the
cloud, because it is warmer. But how does it get its heat? In order
to understand this observe what the bubble is made from. It is made
from the water which was in the air in a state of vapor, or in its
aeriform state, for this is the state of water that is evaporated
and dissolved in the atmosphere. But when it forms the vesicle it
goes out of this state and becomes a liquid, for the wall of the
vesicle is a liquid wall, just as the wall of a soap-bubble is. Now
in passing out of the aeriform into the liquid state some latent
heat must be made sensible. What becomes of this sensible heat? It
just heats the air in the vesicle, and so makes it like a heated
air-balloon. So all clouds are collections of innumerable heated
air-balloons, and the reason that some clouds are higher up than
others perhaps is that their balloons have warmer and therefore
lighter air in them.

325. =Freezing Mixtures.=--The intense cold produced by these
mixtures is the result of the change of free or sensible heat into
latent. For example, when salt and snow are mingled together a
melting of the two is quickly produced. In this sudden change of
a solid into a fluid a great quantity of heat must be rendered
latent, and therefore there will be a great loss of sensible heat
by whatever the freezing mixture comes in contact with. The process
here, you see, is the opposite of solidification in relation to
latent heat. A portion of the snow, after melting with the salt,
becomes solid ice. Why is this? It is because it gives up its
sensible or free heat to portions of the snow that are in the
process of melting and are therefore making heat latent.

326. =Cold from Evaporation.=--If you pour a little ether into the
palm of your hand it will rapidly disappear in vapor, producing a
very cold sensation. This sensation occurs because, in the change
of the liquid into the vaporous or aeriform state, some of the
sensible heat of your hand is abstracted to become latent in the
vapor. The evaporation of water also produces cold, though not
as decidedly as ether, because its change into vapor is not so
rapid at ordinary temperatures. We make a practical use of the
evaporation of water in many different ways. Thus we sprinkle water
in a hot day upon the floors of piazzas, steps, etc., that the
evaporation may make much of the sensible heat about our houses
latent. For the same purpose, in hot climates, apartments are often
separated from each other by mere curtains, which are occasionally
sprinkled with water. So the inhabitants of such climates often
cool their beverages by keeping a wet cloth for some time wrapped
around the vessels that contain them. Evaporation is an important
remedy for many cases of disease. For example, if the head be
hot, a steady application of a wet cloth to the forehead, though
a simple remedy, is generally effectual, and sometimes is very
important. Most people make the application in a wrong manner. They
put on several thicknesses of cloth, when a single thickness is the
best, because it will best secure the evaporation, which is the
cause of the relief afforded.

[Illustration: Fig. 221.]

327. =Freezing in the Midst of Boiling.=--It is from the quantity
of heat rendered latent by evaporation that water can be frozen in
the midst of boiling ether; and, paradoxical as it may seem, the
boiling of the ether is the cause of the freezing. The experiment
is performed in this way: Place a test-tube or a little thin vial
with water in it in the midst of some ether in a shallow vessel
under the receiver of an air-pump. On exhausting the air the ether
will boil, evaporation taking place rapidly because the pressure
of the air is taken off from the ether. Now as the ether passes
into vapor it extracts so much free heat from the vial of water
that the water is cooled down to the freezing point, and so becomes
solid. Water can be frozen even by its own evaporation. It is
done in this way: Let a shallow vessel, _b_, Fig. 221, contain
a little water, and the vessel _c_ oil of vitriol or sulphuric
acid. When the air is exhausted, the pressure of air being taken
off from the water, vapor rises from it freely. As the sulphuric
acid has a great attraction for water it absorbs this vapor, and
so vapor continually rises from the water the more rapidly because
what is formed is absorbed, instead of remaining to make pressure
on the water. The result is that this rapid formation of vapor,
requiring that a great quantity of heat should be made latent, at
length abstracts so much heat from the water that remains that this
becomes solid.

328. =Degree of Heat Endurable by Man.=--It was formerly believed
that the human body could not endure with impunity, even for
a short time, a much higher degree of temperature than that
which is met with in hot climates. But in the year 1760 it was
accidentally discovered that a much higher temperature than this
could be endured. An insect was destroying at that time the grain
gathered in some parts of France, and it was found that if the
grain was subjected to a certain high degree of temperature the
insect was killed, and yet the grain was not injured. In trying
some experiments in regard to this matter the experimenters wished
to know the point at which the thermometer stood in a large
oven. A girl attending on the oven offered to go in and mark the
thermometer. She did so, remaining two or three minutes, and the
thermometer was at 260°, that is, 48° above the boiling point of
water. As she experienced no great inconvenience from the heat she
remained ten minutes longer, when the thermometer rose to 76° above
that point. These facts were published, and prompted scientific
men to try other experiments. In England, Dr. Fordyce, Sir Charles
Blagden, and others, went into rooms heated even to 240° and
260°, and remained long enough to cook eggs and steaks, and yet
themselves suffered little inconvenience. The pulse was quickened,
the perspiration was very profuse, but the heat of the body, as
ascertained by putting the thermometer under the tongue the moment
they came out, was scarcely raised at all. The air in which they
were roasted eggs quite hard in twenty minutes, and when it was
applied by a pair of bellows to a steak it cooked it in thirteen
minutes. The question arises, how is it that this high degree of
heat did not produce more effect upon the body? One reason is that
the heat of the air in the immediate neighborhood of the body was
continually reduced by the evaporation of the free perspiration,
sensible heat being thus converted into latent. Another reason
is that the air is not a good conductor, and therefore did not
communicate its heat readily to the body. Dr. Fordyce and his
friends found that they could not touch with impunity any good
conductor, as the metals, and they were obliged to wear upon their
feet some non-conducting substance.

329. =Formation of Ice.=--Before dismissing the subject of heat
I must notice the grand exception which we have to some of the
operations of heat in the formation of ice. Heat generally produces
expansion. But in the case of water this law of expansion is set
aside, and the reverse is established. This is done, however, only
within a small range of temperature, viz., from the freezing point
up the scale about seven degrees. In all degrees above that the
usual expansion by heat takes place. The exception occurs at this
part of the scale for a special purpose, viz., _that water, in
distinction from other substances, shall become more bulky, and
therefore lighter, as it takes the solid form_.

[Illustration: Fig. 222.]

330. =Description of the Process of Freezing.=--In order to make
the process of freezing clear to you I will describe it as it
ordinarily occurs, that is, from the action of cold air upon the
surface of water. The uppermost layer of the water imparts some of
its heat to the air in contact with it. This air rises and colder
air takes its place, which being warmed in its turn rises to make
way for more of the cold air. You have therefore a constant current
of warmed air upward from the water. In the mean time there is a
current of a different character in the water--a downward one.
As fast as the water at the surface parts with heat to the air
it falls, other warm water taking its place, to cool in its turn
and go down. This falling of the cooled water goes on regularly
until a portion of water becomes cooled down to 39°, that is, 7°
above the freezing point. This layer does not sink, but remains
at the surface, for it is lighter than the warmer water below.
This is because the law that heat expands matter is now reversed.
Beyond this point of the thermometer the colder the water is the
lighter it is. As the cooling now goes on from the air coming, as
before, in successive layers to the water, the cooled water at
the surface continually increases. At first it is a mere single
layer of particles, but after a while it is quite a body of cold
water lying on the warmer water below. At length some of it is
cooled down to 32°, the freezing point, and a thin film of ice
now forms. The state of things just at this stage of the process
may be represented by a simple diagram, Fig. 222. Let the line
_a_ represent the film of ice. The space between _a_ and _b_ is
the portion of water cooled down below 39°. The space below _b_
is occupied by the water which is above this temperature. In the
space between _a_ and _b_ the cooler the water is the nearer it
is to the surface. That is, from the line _b_, where the water is
exactly at 39°, as you go upward, the water lessens in temperature,
it being successively 38°, 37°, 36°, etc., till, just in contact
with the film of ice, _a_, it is at 32°. The ice goes on to thicken
gradually by additions below. But it is to be remembered that ice
is a good non-conductor, so that the very first layer of ice makes
the cooling of the water proceed more slowly than before. And the
thicker the ice becomes the slower is the cooling. This secures
against too great a formation of ice.

331. =Why the Above Exception to Expansion by Heat Exists.=--That
we may see the reasons in part for the grand exception to the
general law of expansion by heat which I have illustrated, let us
see what would be some of the results if the exception did not
exist. In that case the process of freezing would be as follows:
The water would communicate its heat from the surface to the air,
as before described, and there would be a constant downward current
of the cooled water. When any portion of the water became cooled
by the air down to 32°, it would become ice, and would sink to the
bottom. And after the process of freezing had once begun, there
would be a continual accumulation of ice at the bottom so long as
the air remained cold enough to cool the water with which it comes
in contact down to 32°.

The result may be stated in the general thus: Freezing would not
begin so quickly as it now does; but when once begun it would prove
very destructive. It would not begin as soon, because the whole
of any body of water must be cooled down to just this side of 32°
before it could begin. This would not take long where the water
is shallow, but it would where it is deep. All shallow bodies of
water, then, would be frozen up quite early in the winter; and as
water is a poor conductor, and thawing must go from above downward,
some of them would not be thawed out again fully till quite into
the next summer, if even then. And where the water is quite deep
ice would at length begin to form, and when formed it would be
exceedingly slow in thawing. In some cases it would never be thawed
with such a body of non-conducting water to guard it against the
warmth above. It is easy to see that the heat of spring and summer
would not thaw out any thing like the quantity of ice that it now
does. The reign of ice and snow on our earth would therefore be
vastly more extensive than now, and what is worse, it would be
extended more and more every year. Under such circumstances there
would be great destruction of both animal and vegetable life. I
will mention, however, but a single item, as it would occupy too
much space to go into this subject with any fullness. In the water
under the ice, which is always above 39°, except that which is
close to the ice when freezing is going on, there is a vast amount
of busy life which would be destroyed if ice were formed at the
bottom, chilling all the water above.

332. =Why the Freezing Point is at 32°.=--If the freezing point
of water were higher than 32°, freezing would occur so early in
the autumn, and the ice and snow would last so late in the spring,
that the season would be too short to raise our supplies of fruits
and grains. If, on the other hand, it were at a lower point, the
earth would not have the protection of its light coat of snow, but
instead would be chilled by rains so cold that barrenness would
be the result. The multitudes of animals, too, that now live so
securely in the water, some of them even with the ice above them,
would all perish with the cold.

333. =Force of Expansion in Ice.=--As ice occupies one seventh
more room than the water from which it is formed, it exerts in its
formation an expansive force which under various circumstances
produces varied and often remarkable results. Of the many
experiments which have been tried to show the force of this
expansion I will mention but one. A bomb-shell was filled with
water, at Montreal, and closed with an iron plug which was driven
in with great force. The plug was thrown a distance of 400 feet by
the expansion when the water froze. This expansion is sometimes
an inconvenience to us, as in bursting water-pipes; but besides
the great service which it does in the earth, already noticed,
it is of service also in loosening the soil, and in supplying it
with requisite ingredients from the rocks by breaking them up and
pulverizing them in small quantities from year to year.




CHAPTER XIV.

LIGHT.


334. =Nature of Light.=--We do not know what light is. There are
two suppositions in regard to it. One is that of Sir Isaac Newton,
called the theory of _emission_. According to this light is a
substance, but so ethereal that it has no weight, and is capable
of passing through various substances of even great density. The
other supposition is what is called the _undulatory_ theory. The
advocates of this, which is now quite generally received, believe
light to consist of undulations, waves, or vibrations in an ether
which is supposed to exist every where, pervading all space and
every substance. You perceive an analogy here to sound, the
vibrating medium in the case of sound, however, being always some
palpable substance--solid, fluid, or aeriform. Heat is supposed,
as stated in § 271, to be a vibration of the ethereal substance,
as light is, though the two vibrations must of course be somewhat
different in character. Any body that is capable of communicating
the light-vibration to this ether is said to be _luminous_.

335. =Sources of Light.=--The chief source of light to our earth
is the sun, which is a permanently luminous body. Then we have
the light of combustion in its various forms. Electricity is
another source of light. Light is sometimes emitted during decay
or putrefaction of some substances. Some animals--as fire-flies,
glow-worms, and phosphorescent animals in the sea--have the power
of emitting light.

336. =Light Moves in Straight Lines.=--Light, like heat and sound,
radiates in straight lines in all directions from its source. We
can see this to be true by admitting rays of light into a darkened
room through small openings in the shutters, the rays making
straight lines across the darkness, as may be seen by the motes
which are flying in the air. The fact is recognized by the marksman
in taking aim, and by the engineer in making his levels. The
carpenter acts upon it when he tests the smoothness of any surface
by letting the light pass along over it to his eye.

[Illustration: Fig. 223.]

[Illustration: Fig. 224.]

337. =Diffusion of Light.=--As light passes in all directions from
any body or point, the farther we go from its source the less
will the light be. If we take any two rays of light, the farther
we trace them from their source the farther are they separated
from each other, and what is true of any two rays is true of all
the rays. It follows that the farther removed any surface is from
a source of light the less light will there be upon it. This
decrease of light in proportion to distance is a perfectly regular
decrease, and it is as the square of the distance; or, in other
words, the intensity of light is inversely as the square of the
distance. Take a screen, Fig. 223, and a candle, placing a square
piece of pasteboard between them at one foot from each. The shadow
on the screen, you see, covers a space four times as large as the
pasteboard. That is, the light that shines on the pasteboard, if
allowed to pass on to the screen, would be diffused over four
times the space, and therefore would have only one-quarter of
the intensity. So if as shown in Fig. 224, the screen be placed
at twice the distance from the pasteboard that the light is, the
shadow will cover a space nine times as large as the pasteboard,
and therefore the light there would have one-ninth of the intensity
which it has where the pasteboard is. Again, it is seen by Fig.
225 that if the screen be placed at the distance of three feet the
intensity of the light is one-sixteenth of that which it is at the
pasteboard. While the distances, therefore, are as 1, 2, 3, 4,
etc., the intensity of the light is _inversely_ as the numbers 1,
4, 9, 16, etc., that is _inversely as the squares of the distance_.

[Illustration: Fig. 225.]

338. =Velocity of Light.=--The velocity of light is so great
that within any ordinary distances it may be considered as
instantaneous. Thus when we measure the distance of a cannon by the
difference between the time of its flash and the report, we do not
reckon the light to consume any time in its passage to the eye. But
when we come to look at objects as distant as the sun and other
heavenly bodies, we reckon in our calculations the time of the
passage of light. It takes light eight minutes to travel from the
sun to us, a distance of ninety-five millions of miles. With the
telescope stars have been seen which have been ascertained to be
at such a distance that it requires over ten years for their light
to come to the earth. Others have been seen which are much farther
off, but their distances have not been absolutely ascertained.
Some have been seen supposed to be at such a distance that the
light coming from them to the eye of the astronomer was a hundred
thousand years in its passage.

[Illustration: Fig. 226.]

339. =Roemer's Observations.=--The velocity of light was first
determined by Roemer, a Danish astronomer, in 1676. It was done
in his calculations and observations of the eclipse of one of
Jupiter's moons. After making the calculation of the time it would
take for the satellite to pass through the shadow of the planet,
he observed its passage, and found that it did not come out from
the shadow as soon as his calculation required by fifteen seconds.
What was the difficulty? If the earth had remained in one spot
from the beginning to the end of the passage of the satellite,
the observation would have come out exactly according to the
calculation. But the earth had moved during this time (about
forty-two hours and a half) the immense distance of 2,880,000
miles. The light of the emerging satellite therefore had to travel
over this additional distance to overtake the earth, and it took
fifteen seconds to do it. If we divide, then, this distance by
15 we get the distance which light travels in a second, which is
192,000 miles. All this can be made clear by the diagram, Fig.
226. Let S be the sun, J Jupiter, and C one of its moons emerging
from its shadow. Let A be the earth as it is when the eclipse of
Jupiter's moon begins. When it emerges the earth has passed to B,
and the light from the satellite has to travel as much farther
to reach it now as B C is longer than A C. Roemer made other
observations with the earth at some other parts of her orbit with
the same result.

[Illustration: Fig. 227.]

340. =Reflection of Light.=--Light, like sound and heat, is
reflected in straight lines when it strikes upon any resisting
substance. We can see this to be the case when it strikes upon
any smooth and plane surface. And it is true of light, as it is
of heat, that the angles of incidence and reflection are equal.
Thus if _c_, Fig. 227, be a reflecting surface, and _b c_ a line
perpendicular to it, then a ray of light, _d c_, will be reflected
in the line _c a_, and the angle of incidence, _d c b_, will be
equal to the angle of reflection, _b c a_.

341. =How we See.=--We see the various objects around us by the
light which is reflected from them. Every point of every surface
that we see reflects rays or vibrations of light to our eyes. Thus
if we see a person there are rays of light reflected into our eyes
from every part of him. These rays form an image of him in the back
part of each eye, and it is by this image that we see him, as will
be explained in full in another part of this chapter. Reflected
light is painting the images of objects in the eye every moment
in great abundance and variety. If a speaker have an audience of
a thousand persons all looking at him, his image is at the same
time in two thousand eyes, and in each of these two thousand images
every motion and every changing expression are faithfully depicted.

[Illustration: Fig. 228.]

342. =Mirrors.=--That reflected light does thus form images of
objects you see in the common mirror. The image formed in it of
any object comes from the light reflected from that object into
the glass. Then in seeing the image light is reflected from it
into the eye, there to form a similar image, though of much less
size. By using two or more mirrors the reflections of the image can
be multiplied, and by some arrangements of them to a very great
extent. That the image appears to be at the same distance beyond
the surface that the object is before it, is owing to the fact that
the reflected rays come from the glass at the same angle that the
incident rays strike upon it. This may be shown from Fig. 228 (p.
263). Suppose _m m'_ is a looking-glass, and an arrow, A B, is
before it. Rays of light come from it at all points to the glass.
We will take only two of these rays at each end of the arrow. The
ray A _g_ will be reflected to the eye at the same angle in the
ray _g o_, and the ray A _f_ will be reflected in the ray _f_ E.
And the reflected rays will have the same rate of divergence as
the incident rays. The same can be shown in regard to rays from B
or any other point on the arrow. Now if the lines _o g_ and E _f_
be extended, they will meet at the point _a_, which is at the same
distance behind the mirror as A is before it. The same thing can be
shown of the rays from B or any other point. Therefore the image of
the arrow will appear to the eye to have the same relative position
behind the glass that the arrow itself has before it.

[Illustration: Fig. 229.]

343. =The Kaleidoscope.=--I have already noticed the multiplication
of the images of objects by using two or more mirrors. In the
kaleidoscope, by a particular arrangement of mirrors, the images
are multiplied, and by changes in the position of the objects the
relative positions of the images are infinitely varied. Fig. 229
will serve to explain the operation of the instrument. Let A B and
B C be two plane mirrors placed at right angles to each other,
and _a_ an object before them. Let I be the position of the eye
looking at the mirrors. The rays _a f_ and _a g_ will be reflected
to I as represented, and the eye will see two images, which appear
to be at _b_ and E. But the ray _a_ K will be reflected to _c_,
and then to I, so that a third image will be seen at _d_. Here is
but a single second reflection, or reflection of an image; but by
placing the mirrors at an angle of 60°, 45°, and 30° the images may
be increased to six, eight, and ten, having a circular arrangement.
In the kaleidoscope two mirrors are placed in a tube at an angle
of 30°, and variously- pieces of glass in the farther end
of the instrument, changing their relative position with every
movement of it, give an endless variety of images symmetrically
arranged.

[Illustration: Fig. 230.]

[Illustration: Fig. 231.]

344. =Curved Mirrors.=--These may be concave or convex. The action
of a concave mirror upon light may be illustrated by Fig. 230. If
parallel rays, as represented, strike upon the mirror they will,
in their reflection, be made to _converge_, or come together, at
the focus, _a_. But suppose the light comes from this focus, the
rays of course _diverging_, or going away from each other; then
the rays, as reflected, will be parallel. If the light or object
be nearer to the mirror than the focus, and the rays of course be
more diverging, then the effect of the mirror will be to lessen the
divergence when the rays are reflected. You see that the tendency
is to make the rays converge. And hence concave reflectors are much
used when it is desired to throw a great amount of light in one
direction. The effect of the concave mirror upon the apparent size
and position of objects placed before it varies with the relation
of their position to the focus. The action of a convex mirror upon
light is the opposite of that of the concave. Its tendency is to
make the rays diverge. Thus (Fig, 231), if parallel rays strike
upon a convex mirror they diverge, as if they came from a focus
behind the mirror, as _b_, as indicated by the dotted lines.

[Illustration: Fig. 232.]

[Illustration: Fig. 233.]

345. =Refraction of Light.=--When light passes from one medium into
another it is bent from its course. This may be illustrated by Fig.
232, in which A B C D is a box, into which a candle, E, is shining.
The candle is so placed that the shadow of the side A C falls at D.
But let the box be filled with water, and now the shadow is removed
to _d_, as if the candle were at _e_. This is because the rays of
light from the candle, in passing from the air into the water, are
bent or refracted so as to take a different direction. Here we have
light passing from a rarer into a denser medium. Let us see now how
it is when light passes from a denser medium into a rarer. This can
be illustrated on Fig. 233. Let the vessel, A B C D, be empty, and
let a coin be placed at O. Let the eye, E, be in such a position
that a straight line, O G E, from the coin to the eye would strike
the side of the vessel a little below the edge, or, in other words,
that the edge of the vessel would prevent the eye from seeing
it. If now, keeping the eye in this position, water be poured in
up to a certain level, say F G, the coin comes into view. This
is because light coming from the coin to L is bent into another
direction, L E, and the coin therefore appears to the eye to be
at K. In this case the refraction is _from_ the perpendicular, P
Q, let down through the point L, where the light emerges from the
denser into the rarer medium. But When light passes from a rarer
into a denser medium the refraction is reversed--it is _toward_ the
perpendicular. It is from this refraction of light that a stick
partly immersed in water appears to the eye to be broken just at
the surface of the water.

[Illustration: Fig. 234.]

346. =Dawn and Twilight.=--The light of the sun, in passing from
space into our atmosphere, is refracted. If it were not we should
have no daylight preceding the rise of the sun, or twilight after
its setting; but light would burst upon the darkness of night
at once when the sun appeared above the horizon, and darkness
would suddenly succeed to the light of day at sunset. As it is,
in the morning the light bends toward us as it strikes across
the atmosphere long before we see the sun, and after the sun has
disappeared from view at evening its light bends toward us in the
same manner. And farther, we really see the sun in the morning
before it gets above the horizon, and in the evening after it has
gone below it. This may be made clear by Fig. 234. Let the central
ball represent the earth. Now as the atmosphere is most dense
near the earth, and is rarer as you go outward from the earth, it
is represented in the figure as having different layers in order
that the operation of the refraction may be more clear to you. The
outermost layer is exceedingly rare, and each layer is more dense
than the previous one as you go in toward the earth. The light
coming from the sun, S, below the horizon into the first layer of
air, instead of passing on straight to _a_, as indicated by the
dotted line, bends toward the earth. Then in entering the second
layer, instead of passing on to _b_, it will be bent or refracted
still more, as this layer is denser; and so on through all the
layers, being refracted in each more than in the previous one. The
result is, that as every object is seen in the direction in which
the rays from it at length reach the eye, the sun, though really
below the horizon, appears to be above it, as represented. The path
of light from the sun, as it passes through the air, is a curved
line. This is because the air, instead of being of uniform density,
lessens in density as we go from the earth. If it were of uniform
density the light would be refracted in straight lines, as in the
experiments in § 345.

347. =Mirages.=--Sometimes inequalities occur in the density
of the lower portions of the atmosphere, causing, of course,
unequal refraction, and producing some strange appearances, termed
_mirages_. For example, at Ramsgate, on the coast of England,
there was seen, at one time, as represented in Fig. 235 (p. 268),
a ship at such a distance that only her topsails were visible;
and above in the air there were two complete images of the ship,
the uppermost being erect and the under one inverted. Captain
Scoresby, in a voyage to Greenland, saw an inverted image of a
ship so well defined that he decided that it was the image of his
father's ship, the _Fame_, which was afterward verified. The ship
itself was at that time at a distance of 30 miles. An incident
in the early history of the author's place of residence may be
cited as an example of mirage. A ship left for England freighted
with a valuable cargo, and having on board a large number of the
best citizens of the colony. Some time after there was immense
excitement in New Haven, because the inhabitants saw, with great
distinctness, what they supposed to be this vessel, at only a
little distance, apparently sailing against the wind. But it soon
disappeared from view, part after part, until the whole was gone.
The ship itself was never heard from, and it was supposed at the
time that this appearance was a manifestation of Providence for
the purpose of informing the colonists what had become of their
friends. But what was seen was undoubtedly the reflected image of
this or some other ship. It is such appearances as these that have
given rise to the stories which have been sometimes told of phantom
ships. Mirages are very common in the extensive deserts in hot
climates, exhibiting to the eye of the traveler various deceptive
appearances, as islands, lakes, etc. In Bonaparte's campaign
in Egypt such an appearance caused whole battalions of thirsty
soldiers to rush forward, supposing at the moment that a plentiful
supply of water was at hand.

[Illustration: Fig. 235.]

The most astonishing instance of mirage of which I have ever heard
is thus narrated: "The cliffs on the French coast are 50 miles
distant from Hastings, on the coast of Sussex, and they are
actually hidden from the eye by the convexity of the earth; that is
to say, a straight line drawn from Hastings to Calais or Boulogne
would pass through the sea. A year or two ago, however, a Fellow of
the Royal Society, who was residing at Hastings, was surprised to
see a crowd of people running to the sea-side. Upon inquiry as to
the cause of this he was informed that the coast of France could
be seen by the naked eye. He immediately went down to the shore to
witness so singular a sight, and there discovered distinctly the
French cliffs extending for some leagues along the horizon, and so
vividly that they appeared to be only a few miles off. The sailors
and fishermen, with whom Mr. Latham walked along the water's
edge, could hardly at first be persuaded of the reality of the
appearance; but as the cliffs gradually became more elevated they
were so convinced that they pointed out to Mr. Latham the different
places they were accustomed to visit--such as the bay and the
wind-mill at Boulogne, St. Vallery, and other places on the coast
of Picardy, even as far as Dieppe, all the French shores appearing
to the English sailors as if they were sailing at a short distance
from them toward the harbors. With the aid of a telescope the
French fishing-boats were plainly seen at anchor; and the different
colors of the land upon the heights, together with the buildings,
were perfectly discernible. The day when this occurred is said to
have been extremely hot, without a breath of wind stirring, and the
phenomenon continued visible in the highest splendor until past
eight o'clock in the evening, having been seen for three hours
continuously."

[Illustration: Fig. 236.]

[Illustration: Fig. 237.]

348. =Visual Angle.=--In order that you may understand the
operation of lenses in relation to vision I must first explain to
you what is meant by the visual angle. In Fig. 236 (p. 270) are
represented arrows of the same size at different distances from the
eye. From the ends of each of the arrows are drawn lines to the
eye. The angle which these lines make in each case as they meet
at the eye is termed the visual angle. Now the apparent size of
an object depends upon the size of this angle. The degrees of the
angles are marked upon the figure. Thus the visual angle of the
nearest arrow is 120 degrees, and that of the second is 60, only
half as large. The first arrow therefore appears twice as large as
the second. For the same reason it appears four times as large as
the third, eight times as large as the fourth, and twelve times as
large as the fifth. The same thing is illustrated in another way in
Fig. 237. Here the arrows _e f_, _g h_, and _i k_ appear to the eye
as large as A B, because they have the same visual angle, and for
this reason make an image of the same size in the eye, as you see
is indicated in the figure. It is hardly necessary to say that what
is true of objects as a whole is true also of any part of them.
Each part, however small, has its visual angle, and this governs
its apparent size.

[Illustration: Fig. 238.]

[Illustration: Fig. 239.]

349. =Lenses.=--Transparent bodies having curved surfaces are
called lenses. There are six kinds, represented in Fig. 238. The
lenses in most common use are the double convex and double concave.
The explanation of the mode in which these act upon light will
sufficiently illustrate the operation of the others. They act
by refraction, the convex collecting the rays, or bringing them
nearer together, and the concave putting them farther apart. You
can at once see, then, that a convex lens by causing the rays
coming from an object to converge more, increases the visual angle,
and therefore makes the object to appear larger than it otherwise
would. This effect is illustrated by Fig. 239. The rays of light
coming from the arrow are made by the lens so to converge as to
meet at _a_, instead of _b_, where they would meet if they did not
pass through the lens. That is, by passing through the lens they
have a larger visual angle, and therefore the object is magnified.
The distance between, _c_ and _d_ shows the size which the arrow
would appear to have to the eye placed at _a_.

[Illustration: Fig. 240.]

350. =Microscopes and Telescopes.=--What has been said of the
action of the convex lens upon the visual angle will serve to
explain the operation of the microscope. This instrument may be
single or compound. The compound microscope has more than one lens,
and is used to magnify very minute objects. Its operation may be
seen by the diagram, Fig. 240. Rays from the object, E F, passing
through the first lens, or object-glass, as it is called, form a
magnified inverted image, G H, which is still more magnified by the
eye-glass, C D. In the telescope we have also convex lenses, but
they are arranged differently from those of the microscope, as the
objects to be magnified are distant.

[Illustration: Fig. 241.]

351. =Magic Lantern.=--This is an instrument by which pictures
made upon slips of glass with coloring substances which allow the
light to pass readily are thrown upon a screen magnified. It is a
metallic lantern, A A, Fig. 241, with a concave reflector, _p q_,
and two convex lenses, _m_ and _n_. At _c d_ is a space between
the lenses into which the pictures are introduced. L is a strong
light, which is in the focus both of the mirror and the lens
_m_. The picture is therefore illuminated strongly by the rays
reflected from the mirror and passed through the lens. The lens _n_
which is movable, is so adjusted as to throw a highly magnified
image of the picture upon the screen. As the image is an inverted
one the pictures must be inserted upside down, that the images
on the screen may be upright. The _solar microscope_ is, in its
essential parts, like the magic lantern, the sun being used as the
illuminator.

[Illustration: Fig. 242.]

[Illustration: Fig. 243.]

352. =Camera Obscura.=--This instrument differs from the magic
lantern in giving us diminished images of objects. An instrument
of this kind can be arranged extemporaneously any where. Thus, if
into a darkened chamber light be admitted through a small opening,
inverted images of any objects in front of the opening will be
formed upon a white screen in the opposite part of the chamber.
Such an arrangement is represented in Fig. 242 (p. 273), C D being
the chamber, L the opening, and _a b_ the image of the object A
B. The images in such a case, however, are faint, because the
opening must necessarily be small, and therefore but few rays,
comparatively, come from the objects. By making the opening larger,
and gathering the rays that enter it with a double convex lens,
we can have well-defined and bright images of objects. Though the
camera obscura may have various forms, I have described what is
essentially the arrangement of the instrument. One form of it, for
sketching either single objects or groups of them in landscapes, is
represented in Fig. 243. Here the rays of light coming from objects
strike upon a mirror, A B, and are reflected through a convex lens,
C D, upon white paper on the bottom, E F, of the box, where the
outlines of the images are traced by the sketcher. The light can
enter only at the opening above, for on the side of the box which
is open there hangs down a curtain on the back of the artist as he
sketches.

[Illustration: Fig. 244.]

353. =The Eye.=--The eye is essentially a camera obscura. It is a
dark chamber in which images are formed upon a screen in its back
part, and the light which comes from objects is admitted through
an opening in front, where there is a double convex lens. That
you may understand the manner in which the images are formed, I
give you, in Fig. 244, a map of the eye. At _a_ is the thick,
strong white coat called the _sclerotic_ coat, from a Greek word
meaning hard. This, which is commonly the white of the eye, gives
to the eyeball its firmness. Into this is fastened in front,
like a crystal in a watch-case, _e_, the _cornea_. The sclerotic
and cornea, you see then, make together one coat of the eye, the
outer one. The cornea is the clear, transparent window of the eye
through which the light enters. Next to the sclerotic coat comes
the _choroid_ coat, which is dark, to prevent too much reflection
back and forth in the eye. Then you have a very thin membrane, _c_,
the _retina_, the screen on which the images are formed. This is
composed chiefly of the fine fibres of the nerve of sight, _d_. To
return to the front of the eye where the light enters--behind the
cornea is the iris, _g g_, which is immersed in a watery fluid,
_f_, called the _aqueous humor_. The light passing through the
cornea and the aqueous humor comes to the crystalline lens, _h_,
which, you see, is a double convex lens. Passing through this and
through a jelly-like substance, called the vitreous humor, which
fills all that large space _i_, it strikes upon the retina, _c_,
where it forms the images of the objects from which it came.

You see now how the eye is like a camera obscura. You have in it
the dark chamber with its screen, the opening through the iris, the
pupil, for the admission of the light, and just behind this opening
the lens for gathering or concentrating the light before it falls
upon the retina. The refraction of the light is not, however, done
wholly by this lens. The projecting cornea, with its contained
aqueous humor, refracts it considerably, for it forms a convex lens.

[Illustration: Fig. 245.]

[Illustration: Fig. 246.]

354. =Distinct Vision.=--In order that vision may be perfectly
distinct, it is necessary that the rays coming from each point of
the object which is seen should, on converging, meet together, or
be brought to a focus on the screen of the eye, the retina. Thus,
in Fig. 245, the rays which come from _a_, the end of the arrow,
meet on the retina at _b_, and those from _c_, the other end,
are brought to a focus at _d_. Now the muscles of the eye have
considerable power in adjusting the eye to objects at different
distances, so as to bring the rays in most cases together exactly
at the retina. They fail to do it with objects that are very near.
You can see that this is so if you bring any object, as your
finger, nearer and nearer to the eye. You will at length find that
you can not see it distinctly. The reason is, that the rays from it
diverge so much that the cornea and lens can not make them converge
enough to meet at the retina. This divergence of rays at different
distances is illustrated in Fig. 246. Suppose that you are looking
at some very minute object. The nearer you bring it to the eye the
better you can see it, till you come to a certain point. There the
rays are so divergent, as you can readily see by the figure, that
the lenses of the eye can not make them converge sufficiently for
distinct vision. Now just here the microscope comes in to help the
eye by causing these divergent rays to come nearer together before
they enter the window of the eye, the cornea.

[Illustration: Fig. 247.]

[Illustration: Fig. 248.]

355. =Near-Sighted and Far-Sighted.=--Some persons have their
eyes so shaped that they can not fully adjust them to objects
at different distances. Thus the near-sighted can see with
distinctness only objects that are near. The reason is that the
rays converge too much, and are brought to a focus before they
arrive at the retina, as represented in Fig. 247. The images
therefore of distant objects are indistinct. If the retina could
in any way be brought forward a little the difficulty would be
obviated. But as this can not be done, concave glasses are resorted
to, which counteract the effect of the too highly refractive power
of the eye. In the far-sighted the difficulty is of an opposite
character. The refractive power is so feeble that when near objects
are viewed the rays are not brought to a focus soon enough, as
seen in Fig. 248. Convex glasses are used in this case, making the
divergent rays of near objects less divergent before they enter the
cornea.

356. =Images in the Eye Inverted.=--The images formed on the
retina are inverted. This can be proved by taking the eye of an
ox and carefully paring off the back of it, leaving little else
than the retina itself. Holding now a candle before the eye,
its image may be seen inverted upon its rear part. The question
arises why it is that we see objects erect when their images on
the retina are inverted. On this point I will quote from my _Human
Physiology_: "It has been supposed by some that we really see
every thing reversed, and that our experience with the sense of
touch, in connection with that of vision, sets us right in this
particular. And this it is supposed is the more readily done from
the fact that our own limbs and bodies are reversed as pictured on
the retina, as well as objects that are around us, so that every
thing is _relatively_ right in position. But if this be the true
explanation, those who have their sight restored after having been
blind from birth should at first see every thing wrong side up, and
should be conscious of rectifying the error by looking at their
own limbs and bodies. But this is not so. The above explanation of
erect vision, and other explanations of a similar character, are
based upon a wrong idea of the office which the nerve performs in
the process of vision. It is not the image formed upon the retina
which is transmitted to the brain, but an impression produced by
that image. The mind does not look in upon the eye and see the
image, but it receives an impression from it through the nerve; and
this impression is so managed that the mind gets the right idea
of the relative position of objects. Of the way in which this is
done we know as little as we know of the nature of the impression
itself."

357. =Single Vision.=--Whenever we see any object with both eyes
there is an image formed in each eye, and impressions go from
both eyes by the optic nerves to the brain. And yet with these
two impressions there is no double vision so long as the two eyes
correspond with each other in situation. This is because the image
in one eye occupies the same place on the retina that the image in
the other eye does. The correspondence is ordinarily perfect, the
two eyes turning always together in the same way, upward, downward,
or laterally, without the least variation. You can observe the
effect of a want of this correspondence by pressing one of the
eyes in some direction with the finger while the other is left
free to move in obedience to the muscles. When this is done every
object appears double, because its image occupies in one eye a
different part of the retina from what it does in the other, and
so two different impressions are carried to the brain. The same
thing occurs in squinting, in which the action of the muscles of
the two eyes does not agree. Ordinarily in squinting there is not
double vision, because the mind has the habit of disregarding the
impressions that come from the defective eye. But when squinting
occurs suddenly from disease there is double vision, for it takes a
little time to form the habit referred to.

358. =Stereoscope.=--The images of objects in the two eyes, though
always similar, are not generally perfectly alike. They are so
only when the object presents a simple surface, as in the case of
pictures. When the object presents two or more surfaces to the
sight the images are more or less unlike. This can be illustrated
in a very simple way. Hold a book up straight before your eyes
with its back toward you. You see the back and both sides. Now if
you shut your right eye you will see with the left the back of
the book and the left side. That is, these two parts of the book
are imaged on the retina of the left eye. By shutting the left
eye it will appear that the image in the right is different, for
you see now with the back the right side of the book. Here you
have the explanation of the stereoscope. In the right side of this
instrument you have the picture of the object as the object itself
would appear to the right eye, and in the left side you have the
picture of it as it would appear to the left eye. Thus, if a book
in the position alluded to above were the object, in the right
picture there should be represented the back together with the
right side of the cover, and in the left the back with the left
side of the cover. The two impressions, carried to the brain by the
optic nerves, give together the impression of a solid book. The
same principles apply to the representation of all solids in the
stereoscope.

[Illustration: Fig. 249.]

359. =Thaumatrope.=--Each impression made upon the optic nerve
by light lasts about the eighth part of a second. No distinct
impressions can be made, therefore, upon the retina unless they
succeed each other with less rapidity than this. If, for example,
in the revolution of a wheel, eight or more spokes pass by one
point in a second, they can not be seen as distinct spokes, but
will be mingled together, producing one continuous impression. So,
too, if a light revolve so as to describe a circle in an eighth
part of a second it will appear to the eye as one unbroken circle
of light. It is this continuous impression on the retina that
makes small objects, as the cars pass swiftly along, appear to run
in long lines along with us. The fact thus developed is made use
of in the contrivance of a toy called the thaumatrope. A picture
is made on each side of a circular card, and whirling the card
around very rapidly by means of two strings fastened to it, the two
pictures are made to mingle together as one. Thus in Fig. 249 are
represented the two sides of such a card, on the one side there
being the picture of a dog, and on the other that of a monkey. When
made to revolve rapidly the monkey will be seen sitting on the back
of the dog.

[Illustration: Fig. 250.]

360. =Light Compound.=--I have thus far spoken of light as if it
were a simple thing. But it is compound. Every ray of white light
has in it seven different colors. That this is so we can prove by
taking a beam of light by itself and dissecting it, as we may say,
or separating it into its seven parts. I will show you how this can
be done. Let D E, Fig. 250, a beam of the sun's light, pass through
a small opening in a shutter into a dark room. The rays will pursue
a straight course, and if a screen be placed at F they will make a
spot of white light. But if a glass prism, A B C, be held in the
position represented the rays will be refracted, and when received
upon the screen M N the light will be separated into seven colors
in the order which is given. The figure thus produced is called the
solar spectrum. Observe why it is that the colors are separated.
It is because they are refracted unequally. If they were equally
refracted the light upon the screen would be white, as before it
was refracted. The violet rays are most refracted, the indigo next,
the blue next, etc., and the red are the least refracted of all.

361. =Proportion of the Colors in Light.=--The colors in light are
not equal in amount. If we divide the spectrum into 360 equal parts
the proportion in the colors will be as follows: red, 45; orange,
27; yellow, 40; green, 60; blue, 60; indigo, 48; violet, 80.

Some suppose that there are really but three simple colors, red,
yellow, and blue, the other colors being produced by a combination
of these. Thus red and yellow will together form orange, and
yellow and blue will form green.

[Illustration: Fig. 251.]

362. =Recomposition of Light.=--After decomposing light by passing
it through a prism we can bring the separated colors together again
and form from them white light. The manner in which this is done is
represented in Fig. 251. The beam of light, after passing through
the prism S A A', instead of proceeding in the direction indicated
by the dotted lines to form the spectrum, is made to pass through
the prism S' B B', placed in a reversed position, and its rays are
refracted so as to assume their original relation, making a white
beam, M. Here the second prism counteracts the effect of the first,
because its position is exactly the reverse.

[Illustration: Fig. 252.]

Newton very justly considered the decomposition and the
recomposition of light as affording the most positive proof that
white light contains all the seven colors. He tried various
experiments to prove the same thing. Thus he mingled together
intimately seven powders having the seven prismatic colors, and
found that the mixture had a grayish-white aspect. He also painted
a circular board with these colors, and found that on whirling it
so rapidly that the colors could not be distinguished the whole
board appeared to be white. In order to have this succeed perfectly
the proportion between the colors must be observed, as in Fig. 252.
A very pretty way of illustrating the composition of light is to
have a top painted in this way. When the top is whirling rapidly
it is white, but as it slackens its motion the seven colors appear.

363. =Colors of Objects.=--The color of any object depends upon the
manner in which it reflects light. Thus, if it be red, it reflects
the red rays of the spectrum, absorbing the other rays; and if it
be green, it reflects the green rays, etc. If it reflect all the
colors together, it is white; and if it reflect none, or almost
none, of the light, it is black.

You can readily see why the color of an object varies with the
kind of light that shines upon it. If an object which is red in
sunlight be exposed to a yellow light, as a yellow flame, or
sunlight that has passed through a yellow- glass or curtain,
it loses its red color, for there are no red rays in the light to
be reflected by it into our eyes. A person exposed to such a light
has a deathlike paleness, the lips and skin losing entirely their
red color. This effect can be witnessed at any time by mixing
alcohol with a little salt on a plate and setting fire to it. You
see in what has been said the reason that, in examining goods in
the evening, especially by candle-light, we find the colors often
differ somewhat from those which they have in the day.

In some substances the colors are changeable with varying
positions, though the light be the same. We see this often in
shells and minerals. We see it also in some fabrics, as changeable
silk. This is owing to the arrangement of the particles, it being
such as to occasion variety in reflection with changes of position.

364. =Colors of the Clouds.=--There is no more gorgeous display of
colors than we sometimes see in the clouds at morning or evening,
especially the latter. These colors are occasioned simply by
refractions and reflections in the minute vesicles (§ 288) of which
the clouds are composed. How simple are the materials, light,
water, and air, and yet how grand and diversified are the results!

[Illustration: Fig. 253.]

[Illustration: Fig. 254.]

365. =The Rainbow.=--In producing the colors of the rainbow the
materials are less even than in producing those of the clouds.
They are only light and water. The colors come from the reflection
and refraction of light in the drops of the falling rain. I will
illustrate the manner in which these reflections and refractions
take place. Take a single drop, represented in Fig. 253. Let S be a
beam from the sun. This entering the drop at A, is refracted, and
passes to B, at the farther side of the drop. Here a portion of
it is lost by its proceeding on in the line B C. The remainder is
reflected to D, and passes to E, being refracted as it thus passes
out into a rarer medium, the air. Here you have a single reflection
and two refractions. But in the second bow, which is sometimes
formed, there are two reflections as well as two refractions, as
represented in Fig. 254. The beam of light, S, from the sun enters
the drop at A, is refracted, and passes to B. Here a portion
proceeds on in the direction B C. The other portion is reflected to
D. Then this is lessened by a part of it proceeding on in the line
D E. What remains is reflected to E. You see here the reason that
the second bow is not so bright as the primary one. In the latter
there is but one reflection in each drop, and therefore there is
but one point where there is loss of light by its passing on out
of the drop; while in the former there are two reflections, and
therefore loss at two points.

[Illustration: Fig. 255.]

366. =Circumstances under which Rainbows are Seen.=--A rainbow is
seen when the spectator stands between the sun and falling rain.
This commonly can not be the case, except in the latter part of
the day. It sometimes, though very rarely, happens that a shower
passes from the east to the west in the morning, and then a rainbow
can be seen in the west. Fig. 255 is intended to show under what
circumstances a rainbow is seen. Let a horizontal line be drawn
from O, the observer, to P, a point directly under the middle point
of the arch. If this line were extended backward from the observer
it would be precisely in the direction of the sun from him. That
is, the sun is directly opposite the middle of the bow. Now if
the drop at A reflect a red ray to the eye of the spectator all
other drops similarly situated in the arch will reflect red rays.
So if B reflect a green ray all other drops similarly situated
will do the same. And so of C, reflecting the violet ray. For the
sake of clearness there are only three reflections represented,
but the same is true of all the seven colors. In the secondary bow
the arrangement of the colors is reversed, the red being at the
inner part of the bow and the violet at the outer part. The double
reflections are manifest in the drops D, E, and F. What I have
described as taking place in a few drops takes place in countless
multitudes of them in forming the bow. As the exact place of the
rainbow depends not only upon the direction of the rays of the sun
but also the position of the spectator, it is clear that no two
spectators see precisely the same bow, for the drops that form it
for the one are not the same drops that form it for the other. This
is very obvious if the two be quite distant from each other; but it
is equally true if they are very near together, although in this
case the bow for the one would be very nearly coincident with the
bow for the other. It is also true that the rainbow of one moment
is not the rainbow of the next, for as the drops that reflect it
are falling drops there must be a constant succession of them in
any part of the bow.

367. =Colors in Dew-Drops and Ice-Crystals.=--We often see
something very analogous to the rainbow in the dew. As the sun
rises, if, with our backs to it, we look at the dew-drops, we
see all the colors of the rainbow glistening every where before
us, as if the grass were filled with gems of every hue. Here we
have the same refraction and reflection in drops of water, and
the resemblance fails only in the regularity of arrangement which
the rainbow presents. We see the same thing also if the ground is
strewed with bits of ice which have fallen from the branches of the
trees, and the sun shines aslant upon them.

368. =Heat and Light.=--We have not yet finished our dissection
of the beam of light, begun in § 360. In the beam of light which
is separated into the seven colors there is heat also; and in the
separation it is found, as represented in Fig. 256, that the rays
of heat are most abundant just beyond the red rays, while they are
very sparing indeed at the other end of the spectrum. The greatest
degree of light is at the boundary between the orange and the
yellow rays.

[Illustration: Fig. 256.]

369. =Chemistry of Light and the Daguerreotype.=--There is a
chemical power also in light, producing every where, quietly but
thoroughly, important effects. The chemical rays are most abundant
at the end of the spectrum opposite to that where the heat-rays
abound. It is these which do the work in Daguerreotyping. In
this art light has been said to be the painter; but this is not
strictly true. Light makes the image of the object, just as it
does in the camera obscura and in the eye, but it has no power to
fasten that image upon the metallic plate. This is done by the
chemical rays, which, like the rays of heat, go along with the
light. Without going into particulars, which will be given in Part
Second, the process of Daguerreotyping is simply this: A metallic
plate is so prepared that the chemical rays of light shall act
upon it sensibly. Then the object to be taken--a person or any
thing else--being before the instrument, a slip of ground glass
is inserted, and when the operator gets the lens so adjusted that
a good image of the object is seen on the glass he takes this out
and puts in its place the metallic plate. Rays of light coming from
the object make the image, and the chemical rays bound up with the
light act upon the plate so as to fix the image there.




CHAPTER XV.

ELECTRICITY.


370. =Origin of the Term.=--The ancients observed that when certain
substances were rubbed together singular phenomena were produced.
One of these substances was amber, and as the Greek name for this
is ηλεκτρον, the power which is thus excited into action has been
called electricity.

[Illustration: Fig. 257.]

371. =Attraction and Repulsion in Electricity.=--One of the most
common effects of electricity is attraction. If we rub a tube or
rod of glass with woolen or silk it will attract light articles,
such as cotton, feathers, lint, etc., so that they will adhere to
it. But repulsion is also an effect of electricity under certain
circumstances. In order that the explanation of these two opposite
effects may be clear to you, I will detail some of the experiments
which exhibit both. Suppose that we have a pith ball, A, Fig. 257,
suspended by a silk thread, B, from a stand, C. I must premise
that silk does not readily let electricity pass over it, or is a
non-conductor, and therefore any electricity communicated to the
pith ball will remain there unless something be brought in contact
with it or very near it. If now you rub a glass tube, thus exciting
electricity upon it, and then bring it near the ball, it will
attract the ball to it, and then in a moment repel it, so that it
will stand off from the tube and retreat from it if you follow the
ball with the tube. Why is this? It is supposed that there is a
subtile fluid on the electrified glass, some of which passes to the
ball as it touches the glass, so that the ball and the glass are
in a similar condition. But the particles of the fluid repel each
other; and this is the reason that the ball is repelled from the
glass as soon as it becomes charged with a part of the electricity
of the glass. For the same reason if two pith balls hanging from
a standard become electrified from a glass tube or rod they will
repel each other, for they are in the same electrical condition.

372. =Vitreous and Resinous Electricity.=--Suppose now that you
rub a rod of sealing-wax with woolen or silk and hold it near a
pith ball which has been electrified from glass. It will attract
the ball. The reason is that an electricity is excited on the
sealing-wax of a different kind from that which is excited on the
glass. The former is called _resinous_, and the latter _vitreous_
electricity. They are supposed to be two fluids, which have a
strong attraction for each other, while, on the other hand, the
particles of either fluid are repellent to each other. It is this
attraction between the two fluids which causes in the case just
stated the sealing-wax to attract the ball to itself. We can
illustrate this attraction in another way. Take two pith balls and
electrify them, the one from glass and the other from sealing-wax.
Brought near together they will attract each other, because they
have two unlike electricities. This, you see, is just the reverse
of the effect produced in an experiment cited at the conclusion
of § 371, in which the electricities were alike in the two pith
balls. Again, if you bring the rubbed sealing-wax near to the ball
electrified from glass, the ball will be attracted, and the same
effect will follow if you bring the rubbed glass near to the ball
electrified from sealing-wax.

373. =Franklin's Theory.=--In § 372 is developed the theory now
commonly received in regard to electricity. The theory of Franklin
was different. He supposed that there is but one electric fluid,
and that all bodies are in their usual state charged with a certain
portion of it, some having more than others, according to their
capacity for electricity. While a body is in its usual state there
is no manifestation of electricity. The fluid is in a quiescent
condition, because its particles are prevented from repelling
each other by the attraction which exists between them and the
particles of the substance. But this quiescence can be disturbed
by friction and other causes. Thus if a glass rod be rubbed with
a piece of silk, the natural equilibrium is disturbed, the glass
having an excess and the cloth a deficiency of electricity.
The glass is therefore said to be _positively_ and the cloth
_negatively_ electrified. The equilibrium can be restored in the
case of a positively electrified body by having its excess drawn
off, and in the case of a negatively electrified body by having
its deficiency made up by receiving electricity from other bodies.
Though this theory is discarded, the terms positive and negative
derived from it are retained, being applied to the two fluids[6] or
electricities, and they are often designated by the two signs + and
-.

374. =Upon What the Kind of Electricity Excited Depends.=--It
depends on what a substance is rubbed with whether vitreous or
resinous electricity is excited in it. Thus smooth glass rubbed
with woolen cloth or silk will be positively electrified; while
if it be rubbed upon the back of a cat it will exhibit negative
or resinous electricity. So, also, if a resin, as gumlac or
sealing-wax, be rubbed with silk or woolen cloth, it will be
charged with resinous electricity, but it will be charged with
vitreous or positive if it be rubbed with sulphur. The terms
vitreous and resinous are therefore incorrect, for they are based
upon the idea that one kind of electricity is always excited on
glass, whatever the friction may be made with, and that the other
kind is always excited on resins. The most decided illustration of
the incorrectness of these terms we have in the fact, that while
smooth glass rubbed with silk or woolen cloth becomes charged with
positive (vitreous) electricity, roughened glass rubbed with the
same gives us negative (resinous) electricity. Below I give a table
of substances, any one of which has positive electricity developed
on it when it is rubbed with any substance below it on the list,
and negative when rubbed with any substance above it:

  1. Cat-skin.
  2. Polished glass.
  3. Woolen cloth.
  4. Feathers.
  5. Wood.
  6. Paper.
  7. Silk.
  8. Sealing-wax.
  9. Amber.
  10. Roughened glass.
  11. Sulphur.

375. =Conductors and Non-Conductors.=--Electricity passes over
the surface of some substances very readily; while over others
it moves with very great difficulty, and therefore very slowly
and sparingly. The former are termed conductors, and the latter
non-conductors. As in the case of heat, so with electricity
there are no substances which are wholly non-conducting. The
best of all the conductors are the metals, those least liable
to oxydation being the most perfect. Next come charcoal, water,
living substances, flame, smoke, steam. The best non-conductors
are gumlac and gutta-percha. Then come amber, resins, sulphur,
glass, silk, wool, hair, feathers, cotton, paper. Non-conductors
are sometimes called _insulators_, from the Latin word _insula_,
as they serve to confine electricity within certain bounds, and
prevent its escaping. Thus in the experiments with pith balls,
already cited, the silk threads by which they are suspended prevent
the electricity from escaping from them. So the glass knobs on
which the wires of the telegraph rest are insulators, preventing
the electric fluid from escaping down the poles into the ground.

[Illustration: Fig. 258.]

376. =Electricity Always on the Surface.=--There is a marked
difference between heat and electricity in the manner in which they
are disposed of. Heat pervades all the particles of substances, and
in its conduction spreads through them, while electricity in its
ordinary movements operates altogether on the surface. A hollow
ball, therefore, can contain as much electricity as a solid, and
a hollow conductor of electricity is just as effectual as a solid
one. The following experiment exhibits in a very striking manner
this disposition of electricity to occupy the surface alone: Let
_a_, Fig. 258, be a metallic ball supported by a glass stand,
_b_; and let _c c_ be metallic caps which will just cover the
ball, having non-conducting handles, either glass or gumlac. Now,
after having charged the ball with electricity, let the caps held
by the insulating handles be carefully placed over the ball. On
withdrawing them it will be found that the electricity of the ball
has all passed to the outer surface of these caps.

377. =Electrics and Non-Electrics.=--It will be observed, on
looking over the list of conductors and non-conductors, that among
the non-conductors are those substances in which electricity is
easily excited by friction, such as glass, amber, silk, etc. These
were therefore called electrics. The conductors, on the other hand,
were called non-electrics, it being supposed that electricity could
not be excited with them. But this has been found not to be true.
For example, if a metal be insulated by being placed on a pillar
of glass or of gumlac, so that the electricity, when excited, can
not pass off readily, its generation can be made manifest. It is
probably true that every substance is more or less an electric, it
being difficult to make this manifest in the case of conductors,
because the electricity passes off as fast as it is generated.

378. =Electricity Every Where Active.=--I have said that there is
electricity in all substances, each having its own capacity for it,
but that in the usual condition of substances the electricity is in
a state of equilibrium, and therefore of quiet. We see this quiet
disturbed whenever there is a thunder-storm, when we rub glass or
silk, or a cat's back, or when we work an electrical machine. But
the active state of electricity is not limited to such palpable
demonstrations as these. Electricity is undoubtedly in action every
where and always, although we can seldom appreciate and measure
its action. Wherever there is motion there is a disturbance of
the equilibrium of electricity, and a consequent return to this
equilibrium. And this change from the one state to the other must
be the constant cause of important changes and operations in the
world around us, and in our own bodies. Let us look at some of the
indications of this universality electrical action. The friction
of any electric upon another awakens it. The friction of the belts
upon the drums in cotton factories does it quite freely. Every
stroke of India rubber upon paper as you erase a pencil mark
excites electricity. The blowing of air upon glass does the same.
So, also, does the blowing off of steam from an engine. Electricity
has been excited even upon ice by rubbing it when cooled down to
13° below zero. Experiments upon the air have shown that there is
usually some free electricity in it, the atmosphere being generally
in a positive state, especially when the air is dry and clear.
It is constantly generated from one source and another. It is
generated every where by evaporation. Every gust of wind, causing
friction of the particles of the air upon various substances,
generates it. Motion of every kind probably generates it. Chemical
action, as you will see in another part of this chapter, generates
it every where. It is generated also in the operations of life, and
in some animals there are special organs--electrical batteries--for
the generation of this agent.

[Illustration: Fig. 259.]

379. =Induction.=--A remarkable influence is exerted by an
electrified body upon another body in its usual state when brought
near it, and this influence is called induction. I will illustrate
this by Fig. 259. Let A be a metallic ball standing on a glass
pillar, and charged with positive electricity. Let B be a metallic
cylinder supported upon two glass pillars. Now if A be placed near
B, but not near enough for the electric spark to pass from it to
B, it will destroy the equilibrium of the two electricties in B,
the negative electricity being accumulated at the end near A,
and the positive at the remote end. This is because the positive
electricity in A repels its like in B and attracts the unlike
fluid. You observe that there is a pair of pith balls suspended
at each end of B, and also at the middle. The two balls at the
positive end repel each other because they are charged with the
same electricity, and so with the balls at the negative end. But
the balls hanging from the middle are not affected, because they
are on middle ground between the two electricities. Here is no
communication of electricity from A to B, but only an influence
upon the quiescent balanced electricities of B. Accordingly, if the
surplus electricity of A be discharged by putting the hand or any
good conductor upon it the influence will cease, the equilibrium in
B will be restored, and the pith balls will all hang straight down.
The same effect will be produced if A be withdrawn to a distance
from B, and the influence will be renewed if A be brought near
again.

[Illustration: Fig. 260.]

If instead of one conductor we use two, B and C, Fig. 260, and have
them in contact, we shall have the negative electricity on B and
the positive on C. Now if we withdraw C from B we may have the two
electricities separate, B being charged with the negative and C
with the positive.

[Illustration: Fig. 261.]

380. =Electrical Machine.=--You are now prepared to see how the
common electrical machine operates. There are two kinds--the plate
and the cylindrical. The plate machine, Fig. 261, has at _p_ a
large plate of glass, and at _r_ a rubber which consists of two
brass plates lined with leather which is stuffed, the pressure of
which upon the glass is regulated by a screw. Above this rubber is
a brass ball, _d_, and a brass chain connects the rubber and the
ball with the floor, or, in other words, with the earth. At _c_ is
what is called the prime conductor--a hollow brass cylinder with
rounded ends, having attached to it a rod with points, as seen at
_a_. There is a similar rod attached to it on the other side of the
glass plate. The different parts of the instrument are supported on
glass pillars, _g g g_, standing on a wooden platform. The lower
part of the plate is covered with a case of silk, which, being a
non-conductor, prevents the electricity on the glass from being
lost in the air, and also serves to keep the plate free from dust.
The rubber is covered with an amalgam of tin, zinc, and mercury,
this being found very effectual in exciting electricity. The
operation of the machine is this: As the plate revolves positive
electricity is collected upon the glass, and negative electricity
upon the rubber. The former, as it comes to the points at _a_, goes
to them and passes on by the rods to the prime conductor, while
the latter passes from the rubber by the chain to the earth. The
points at _a_ are of great service in collecting the electricity,
because the fluid is always much more ready to go to points than to
conductors of a blunt shape.

[Illustration: Fig. 262.]

The cylinder machine is represented in Fig. 262, _a a_ being a
glass cylinder, which can be turned rapidly by the multiplying
wheel, _b b_. At _c_ is a piece of silk, and on the rear part of
the cylinder is the rubber. At _d_ is the prime conductor.

381. =Experiments.=--Many experiments may be tried with the
electrical machine. I will cite a few of them:

If pith balls be attached to the prime conductor, as seen in Fig.
261, they will stand out from each other as soon as the machine
is worked, because they are both charged with the same kind of
electric fluid.

Let a small figure with its head covered with hair be placed upon
the prime conductor. As soon as the conductor becomes charged
with electricity the hair stands out, as represented in Fig. 263,
for the same reason that the pith balls diverged in the previous
experiment.

[Illustration: Fig. 263. Fig. 264.]

So, also, if you place on the conductor a figure having attached to
it strips of tissue-paper, they will diverge in the manner shown in
Fig, 264.

[Illustration: Fig. 265.]

Let a metallic plate, _a_, Fig. 265, be suspended by a chain to
the prime conductor, and another plate, _b_, be supported upon a
conducting stand. If figures of paper or pith be placed between
these plates as the machine is worked they will move about briskly
between the plates, being alternately attracted and repelled by the
communication of the electricity.

[Illustration: Fig. 266.]

The experiment represented in Fig. 266 is a very beautiful one.
Let _a b_ be a brass rod with an arch, _g_, by which it can be
suspended from the end of the prime conductor. To this rod are
suspended three bells, the two outer ones by chains, and the middle
one by a silk thread; also two clappers, _d_ and _e_, by silk
threads. The middle bell has a chain, _f_, connecting it with the
table--that is, with the earth. The operation of the apparatus is
this: As soon as the outer bells become electrified they attract
the clappers. These, on touching the bells, receive a portion of
their electricity, and are repelled. They therefore strike against
the middle bell, to which they impart the electricity which they
received from the outer bells. They swing back again then in the
same state that they were in at first, and now are attracted again
by the outer bells. This goes on so long as the electricity is
communicated.

[Illustration: Fig. 267.]

Let there be pasted upon a slip of glass a continuous line of
tin-foil, going back and forth, as represented in Fig. 267, and
let there be a ball, G, connected with one end of, the foil. The
word light is made upon it by cutting out with a sharp knife little
portions of the foil. If now with your finger on one end of the
line of foil at _a_, you present the ball G to the prime conductor,
the electric fluid will run along the whole length of the line from
G to _a_. In doing this the letters are beautifully illuminated, a
spark being produced at each interruption of the line. So rapid is
the passage of the electricity that the whole appears to the eye
simultaneously illuminated.

[Illustration: Fig. 268.]

382. =The Insulating Stool.=--This consists of a wooden top, _a_,
Fig. 268, supported by glass legs, _c c_. It can be made simply by
boring holes in the four corners of a piece of board sufficiently
large to admit the necks of bottles. Many amusing experiments can
be tried with this. A person standing upon it can be highly charged
with electricity by holding a chain connected with the prime
conductor. The hair will rise up as represented in Fig. 263, and he
can give electric shocks to other persons from any part of his body.

[Illustration: Fig. 269.]

383. =Electricity Discharged from Points.=--I have already, in
giving an account of the electrical machine, spoken of the
readiness with which electricity is received by points. It is
discharged from them with equal readiness; so that, if a metallic
point be attached to the prime conductor, the electricity will be
carried off into the air nearly as fast as it is received upon the
conductor. And as it passes off it creates a current in the air as
it strikes upon it. The reaction of the air upon the electrical
currents can be very prettily exhibited with the apparatus
represented in Fig. 269, which consists of a cap, A, resting upon
the point of a rod, and having pointed wires branching out from
it in a wheel-like arrangement. You observe that the points are
all bent one way. If this apparatus be set upright upon the prime
conductor, the wheel can be made to revolve rapidly by working
the machine. As the reaction of the air against the gases issuing
from the rocket makes it rise, so the same reaction against the
electricity issuing from these points causes the circular motion.
If electricity be discharged from a point in a darkened room it
appears like a brush of light, as represented in Fig. 270.

[Illustration: Fig. 270.]

[Illustration: Fig. 271.]

384. =Leyden Jar.=--The Leyden jar, Fig. 271, is so called because
it was contrived at Leyden. It was suggested by an accidental
result of an experiment tried there with the electrical machine.
It consists of a glass jar coated upon the inside and the outside
to near the top with tin-foil, and having a metallic rod passing
through the cork, with one end touching the inner coating, and the
other surmounted by a brass ball or knob. The jar is charged by
holding the knob near to the prime conductor while the machine
is worked. The electricity passes by the metallic rod to the
inside coating of the jar, and accumulates there. This is positive
electricity. In the mean time there is an accumulation of negative
electricity on the outside coating. But how is this? It is by the
repulsion of positive electricity for itself, and its attraction
for its opposite, negative electricity. As you hold the jar in your
hand positive electricity is repulsed from its outside through your
arm earthward, while negative electricity is attracted to it by the
positive which is within. The two fluids get as near to each other
as possible. They are prevented from coming actually together by
the non-conducting quality of the glass. If a slip of tin-foil were
made to connect the inside foil with the outer, there would be no
accumulation of electricity on the inside, for as fast as it passed
from the prime conductor to the inside it would pass out over the
bridge of foil to the outside, and down your arm and body to the
earth.

[Illustration: Fig. 272.]

If there were no communication of the outside with the earth the
jar would not be charged. No electricity would pass to it, because
the positive electricity which is on the outside can not be driven
off, and no negative electricity can be received. To make this
plain, suppose that the jar, _a_, Fig. 272, having a bent rod, is
suspended to the prime conductor, _b_. Here you have the inside
tin-foil connected with the source of positive electricity. But
the outside is insulated. No electricity can pass from it or to
it. It has both positive and negative electricity, but they are in
equilibrium. If there were a preponderance of negative electricity
there, it would attract positive electricity to it as near as
possible, and so the latter would enter the jar from the conductor.
But there is no such preponderance, and so, though a little may
enter--a spark or two--there will not be enough to charge the jar
sensibly, because there is no attraction in that direction. But
bring now another jar, _c_, near to the outside coating of _a_,
and there is a movement at once in the electricities. The positive
electricity has a chance now to pass off from the outside of _a_ to
the inside of _c_, leaving therefore a preponderance of negative
electricity on the outside of _a_, which exerts an attractive
influence on the positive electricity of the conductor drawing it
to the inside of the jar.

[Illustration: Fig. 273.]

385. =Discharge of the Leyden Jar.=--The jar may be discharged by
making a communication between the inside and outside by means of
any conductor. It may be done with the discharging-rod (Fig. 273).
This has two slender metallic rods, with brass knobs at their ends,
and jointed at _a_, so that the knobs can be separated to different
distances. The handle is glass, so that as the electricity passes
through the rods none of it may be communicated to the hand. In
discharging the jar one knob is placed upon the outside foil, and
the other is brought near to the knob of the jar. The two fluids
now rush together from their attraction, and in doing so a bright
flash is produced, going from the knob of the jar to that of the
discharging-rod, and with this a report. You can yourself be the
conductor to discharge the jar. If, having one hand upon the
outside of the jar, you bring the other near its knob, the fluids
meet in you as they do in the discharging-rod, and a shock will be
experienced in proportion to the amount of charge in the jar. Any
number of persons can together receive the same shock. To do this
they must join hands, and the person at one end of the row must
touch the knob of the jar while the person at the other end has his
hand upon the outside.

You may touch either the knob of the jar or the outside coating
_separately_, and the power that is in it remains quiet; but the
moment that you touch both it bursts forth, because a bridge is
made upon which the two fluids can meet.

In a dry air the charge in the jar can be retained for some
time, the communication between the two electric fluids being
very slow through the medium of air. It is otherwise when there
is much moisture in the air, for water is a good conductor. For
this reason, if you let the moisture from your breath come upon
the jar between the outside coating and the rod, the jar will be
discharged soon, though imperceptibly, the moisture making a medium
of communication between the inner and outer electricities.

[Illustration: Fig. 274.]

386. =The Electrical Sportsman.=--In this contrivance, Fig. 274,
the discharge of the Leyden jar is very prettily exhibited. The
jar, _c_, has a rod with two branches. On the end of one of these,
B, are suspended pith balls cut in the shape of birds. On the other
is a knob by which the jar can receive its charge from the prime
conductor. After it is charged it is placed on the stand with its
knob, _b_, near the gun, _a_, of a metallic figure. The suspended
birds, you observe, stand out from each other, because they are
charged with the same fluid, positive electricity, and therefore
are repellent. Now when the chain, _e_, which is connected with
the outside of the jar, is made to touch the foot of the metallic
image, the connection between the inside and outside of the jar is
established. Of course there is an instantaneous flash between _a_
and _b_, and the birds, losing their electricity, fall, and hang
as they did before the jar was charged.

[Illustration: Fig. 275.]

387. =Electrical Battery.=--By combining together a number of
jars, having the insides all connected together, as seen in Fig.
275, with metallic rods, and the outsides connected together in
a similar manner, we have what is termed an electrical battery.
By such an arrangement we can accumulate a large amount of
electricity, which can be discharged in the same way essentially as
in the case of the single jar.

388. =Light of Electricity.=--The light produced by electricity is
not occasioned by any thing like combustion. It depends obviously
upon the resistance which is offered to its passage. Thus when the
electric fluid passes through air from the prime conductor to the
knob of the Leyden jar it causes a flash of light, but when it
arrives at the knob the flash ceases. What is the reason of the
difference? In both cases it has the resistance of the air, for
when it comes to the knob it passes over the _surface_ of the knob
and rod; but in the latter case it is so diffused in its conduction
over the metallic surface that it meets with much less resistance
from the air. By experiments with the air-pump it is found that the
denser the air is the more vivid is the spark; and if electricity
be passed through a glass vessel from which the air has been mostly
exhausted we have the streams of light seen in the aurora borealis,
which are so strikingly in contrast with the vivid flashes of the
lightning. In the experiment, § 381, in which the word light is
made by the passing electricity, we have a striking illustration of
the production of the spark by the resistance of the air. If the
foil were one continuous surface the electricity would be diffused
over it without giving any light. It is only where the electric
fluid has to leap through the air from one portion of foil to
another that the light is seen.

389. =Sound of Electricity.=--The report of electricity is a sort
of crack or snap from the sudden condensation of the air by the
rapid passage of the fluid. The rolling of thunder is occasioned by
the reverberation of the first sound among the clouds. The nearer
the flash is to us the more like a crack is its first sound as it
comes to our ears.

390. =Mechanical Injuries from Electricity.=--When any great amount
of electricity meets in its passage with any imperfect conductor
it does much violence to it. Thus it rends wood, scatters water,
breaks glass, etc. Various experiments have been tried illustrating
the manner in which mechanical injuries result from electricity.
Thus if it be made to pass through a card or several leaves closely
pressed together, there is a burr on each side of such a character
as to show that two forces moving in opposite directions have made
their passage.

391. =Heat Produced by Electricity.=--Electricity always produces
in its passage some amount of heat, probably by its mechanical
effect. When it is diffused over a large conducting surface the
heat is not sufficient to be observable; but if it be confined to
the surface of a small wire the heat may be sufficient to melt or
even burn it. Various effects can be produced by the heat thus
caused by the passage of electricity. Gunpowder may be exploded
by it. Alcohol and ether may be readily ignited by it, especially
the latter. Gas can sometimes be lighted by pointing the finger to
an opened burner after walking across the room two or three times
briskly, rubbing the feet upon a thick carpet.

[Illustration: Fig. 276.]

392. =Franklin's Discovery.=--It had very early been conjectured
that the electricity produced by the electrical machine is
identical with lightning; but it was reserved for our countryman
Franklin to prove the fact. A tall spire which was being erected
in Philadelphia in 1752 he conceived might be used in his
investigations, but before it was completed the sight of a boy's
kite in the air suggested to him another plan. He made a kite
by stretching a silk handkerchief over a frame, and sent it up
as he saw a thunder-shower rising, his only companion being his
son. Having raised the kite, he attached to the end of the hempen
string a key, and also a silk ribbon, by which he insulated his
apparatus, as seen in Fig. 276. He now watched with much anxiety
the result. A cloud arose, which he supposed, from its appearance,
was well charged with electricity, and yet no effect was seen.
Franklin began to despair; but he soon saw some loose fibres of
the hempen string bristling up, and, applying his knuckle to the
key, received just such a spark as he had often received from the
conductor of an electrical machine. The discovery was made, and
Franklin was at once overcome with emotion at the thought of the
immortality which it would give his name. He felt very much as
Archimedes did when, after making one of his grand discoveries as
he lay in a bath, he went home saying all the way, Εὕρηκα! Εὕρηκα!
The fame of the discovery, made in a manner so simple and yet so
original, spread every where, and prompted to many experiments by
other philosophers. One, Professor Richman of St. Petersburg, fell
a victim to his investigations. While he was attending a meeting of
the Academy of Sciences he heard the sound of distant thunder, and
hastened home to make some observations with an apparatus which he
had erected. While doing this a charge of electricity flashed from
the conducting rod, and piercing his head killed him instantly. His
assistant, who stood near, was struck down, and remained senseless
for some time, and the door of the room was torn from its hinges.

393. =Lightning-Rods.=--It was the discovery of Franklin which
led to the custom of attaching lightning-rods to buildings. The
object of a lightning-rod is to conduct any electricity in a cloud
that may come over the building down into the ground. For this
purpose the rod should terminate in the air in points, as these,
as you saw in § 380, so readily receive the electric fluid. The
rod should be separated from the house by wooden supports, and it
should pass so far into the ground as to have its end in the midst
of continual moisture. The points should be gilt, in order to
preserve from corrosion, or they may be made of silver or platina.
Lightning is very apt to go down in chimneys, as smoke is a very
good conductor; and therefore it is well to have the rods go up by
chimneys, especially if they are to have fire in them during the
summer. Lightning-rods often undoubtedly are of service when there
is no obvious passage of the lightning down them, by quietly and
continuously receiving electricity upon their points, and passing
it down into the earth.

[Illustration: Fig. 277.]

394. =Galvanic or Voltaic Electricity.=--This form or mode
of electricity I will barely notice here, reserving its full
consideration for Part Second, where it appropriately belongs.
The history of its discovery is interesting. The first dawning of
Galvanism is to be found in an experiment noticed by Sulzer, a
citizen of Berlin, in 1767. He states that if a piece of zinc be
placed under the tongue, and a piece of silver upon it, on being
brought in contact a metallic taste is perceived, and a shock is
felt by the tongue. Sulzer attributed the effect to some vibratory
motion occasioned by the contact of the metals, and, satisfied with
this fanciful explanation, pursued the inquiry no farther. The
statement excited but little notice until other facts of a similar
character were brought out in 1790 by Galvani, professor of Anatomy
at Bologna. He observed that the legs of some frogs, which had
been obtained for his invalid wife, were convulsed, when near an
excited electrical machine, on touching the nerves with a knife.
In contrast with the example of Sulzer, he was led to examine the
matter further. He found that the effect was produced when no
electricity was communicated from the machine, by establishing a
connection between the nerves and the muscles by some conductors.
For example, when a strip of zinc was placed in contact with the
nerve which goes to the lower extremities, and a strip of copper in
contact with the legs, on bringing the two together at the other
end the legs would be convulsed, being drawn up, as represented in
Fig. 277 (p. 306). But Galvani did not get at the true explanation.
He supposed this to be an exhibition of animal electricity,
regarding the muscles as being a sort of Leyden jar, the nerve
being the medium of communication with the inside.

[Illustration: Fig. 278.]

395. =Volta's Pile.=--The observations of Galvani awakened much
interest in all scientific minds, and of course there was much of
inquiry, observation, and experiment. Professor Volta, of Pavia,
went a step farther than Galvani toward the true explanation, in
referring the effects to the contact of dissimilar metals, and he
was led by this view of the subject to construct his _pile_ or
battery--called after him the voltaic pile--the object of which
was to produce a much greater amount of electricity than could
be obtained by the contact of only two pieces of metal. The pile
is made of circular pieces of copper, zinc, and cloth, the cloth
being moistened with salt-water. They are arranged as represented
in Fig. 278. First a disk of copper is laid down, then upon this
one of zinc, then one of cloth, and so on in the same order, the
top of the pile ending in a plate of zinc. If you touch one end of
the pile with a moistened finger and the other end with a finger
of the other hand, you will feel a shock like that from a Leyden
jar. The communication between the two ends of the pile may be made
by wires, as seen in the figure. Volta afterward changed this to
the form of a cup battery, the plates of metal being immersed in a
series of cups in a mixture of sulphuric acid and water. There have
been various improvements from time to time, but the arrangement is
in all the different batteries essentially the same. Although Volta
accomplished so much he did not arrive at the truth in full. His
"contact theory," as it is called, so long received as the true
theory, gradually gave way to the true explanation, viz., that the
electricity produced is owing to chemical action.

396. =Difference Between Frictional and Voltaic Electricity.=--The
electricity produced by the friction of the electrical machine is
more intense than that of the voltaic battery. Voltaic electricity,
on the other hand, is much more abundant, and is more continuous
and lasting. As it is therefore more steady and more easily
controlled than frictional electricity, it is used in the working
of the Telegraph.




CHAPTER XVI.

MAGNETISM.


397. =Loadstones.=--It was discovered many centuries ago that a
certain ore of iron has the property of attracting pieces of common
iron or of steel. The fact was probably considered at first as a
mere curiosity, and the world were slow to find out its value. It
is not till quite recently that it has been discovered that in
magnetism we have one of the great forces of the earth; and even
now we know but little probably of the real extent and variety of
its action. New and important discoveries are yet undoubtedly to be
made in regard to the agency and the laws of this mysterious power,
and its connections with the other grand forces of nature. The
terms magnet and magnetism come from the fact that the loadstone
was first found near Magnesia, an ancient city in Asia Minor. This
ore appears in considerable masses in the iron mines of Sweden and
Norway, and also in different parts of Arabia, China, and Siam. It
has occasionally been found in small quantities in England and in
this country.

398. =Attraction of Magnetism.=--The attraction of the magnet
and iron for each other is exhibited in many different ways. If a
magnet be brought near to a heap of iron-filings or needles, it
will have a quantity of them adhering to it as you raise it up. In
the toy fishes of children there is fastened in the head a bit of
iron, which occasions the following of the fishes after the magnet.
In this case you can see very plainly that the nearer the magnet
and the iron are to each other the stronger is the attraction.
Indeed, the attractive influence is governed by the same law in
regard to distance as the common attraction of matter is, viz., it
is inversely as the square of the distance. The attraction also is
mutual here, the iron attracting the magnet as much as the magnet
does the iron.

[Illustration: Fig. 279.]

[Illustration: Fig. 280.]

399. =Poles of the Magnet.=--Every magnet has two poles. It is
about these poles where the chief power resides. For this reason,
if a magnet be rolled in iron-filings, these are collected about
the ends, as represented in Fig. 279. There is a diminution of
attraction from the ends to the middle line, which is called the
_neutral line_. These poles are called north and south poles,
because if a magnet be suspended, or be supported upon a pivot, so
that it can revolve, it will take a north and south direction, one
of its ends invariably pointing toward the north. In Fig. 280 is
represented a magnet supported upon a pivot, C.

[Illustration: Fig. 281.]

400. =Magnetism by Induction.=--The magnet in exerting its
attraction really temporarily makes a magnet of what it attracts.
Actual contact is not necessary to this result. Thus if a large
key be only brought very near to a powerful magnet it will support
small keys, as represented in Fig. 281. When the key is removed
away from the magnet the keys attached to it fall. You see the
analogy to the induction of electricity noticed in § 379. As in
the induction of electricity, so here the two ends of the body in
which the influence is induced are in opposite states. If the end
of the magnet, to which the first key is near or attached, be the
north pole, the end of the key next to the magnet is the south
pole, and its farther end is the north pole. The same is the case
with the small key attached to the end of the large one. And so
if a nail should hang from the small key, and a needle from that,
both of these would have the same polarities. But all this would be
reversed if the large key were attached to the south pole of the
magnet. In this case the upper end of each of these articles would
be the north pole, and its lower end the south pole.

401. =Attraction and Repulsion in Magnets.=--You have seen in
induction that in magnets _like poles repel while unlike attract_.
But this law can be more strikingly illustrated. If a magnet be
placed on a pivot, as in Fig. 280, and another magnet be brought
near it, attraction or repulsion will be manifested according to
the mode of presentation. If a north pole be presented to a north
pole, or a south to a south, repulsion will be the result. But if
a north pole be presented to a south, or a south to a north, then
attraction will be manifested.

[Illustration: Fig. 282.]

402. =Magnetic Curves.=--The polarity of magnetism causes a very
singular arrangement of iron filings when gently agitated upon a
sheet of paper over a magnet, as represented in Figure 282. The
curves which you see have been supposed by some to be occasioned
by the escape of some fluid or influence from the magnet in these
particular directions. But they are owing entirely to the fact that
each bit of filing is polarized by the bit next preceding it in the
row reckoning from the magnet outward, the nearest one in each row
deriving its magnetic state from the magnet itself. This being so,
as the chief power resides in the ends of the magnet, it is easy
to see how such a disposition of the lines of magnetic filings is
effected. These curves may be beautifully and curiously varied by
having several magnets variously arranged under the paper.

403. =Artificial Magnets.=--The power residing in the loadstone
can be communicated readily, as you have seen, to iron and steel.
Though soft iron takes the magnetic influence more readily than
steel, it does not retain it as steel does, and the latter is
therefore used in making artificial magnets. When a magnet imparts
its magnetic influence it loses none of its own power, whether it
be an original loadstone or an artificial magnet. There, are many
ways of imparting magnetism permanently to steel, but I will notice
only two of them. If you wish to magnetize a bar or needle pass one
pole of a magnet from one end of it to the other a considerable
number of times, always in the same direction. A more effectual
way is to take two magnets, and, placing the south pole of one and
the north pole of the other in contact over the middle of the bar
or needle, draw them slowly and steadily apart toward the opposite
ends. This process must be repeated several times.

[Illustration: Fig. 283.]

[Illustration: Fig. 284.]

404. =Horseshoe Magnets.=--One of the most common forms of the
magnet is the horseshoe magnet, Fig. 283. There is a piece of soft
iron attached to the end of this, held there by attraction. This is
called the _armature_. So long as it is suffered to remain there
it is a magnet having its _two_ poles, the north pole + being
attached to the south pole - of the magnet which holds it, while
the reverse is the case with its south pole. The object of the
armature is to preserve the power of the instrument. Indeed it is
found that the exertion of the magnet's power not only preserves
but actually increases it. If you attach, therefore, to a magnet
an armature having a hook, as seen in Fig. 284, you can add to the
weight gradually from day to day, and so considerably augment the
power of the magnet.

405. =Magnetic Needle.=--The magnetic needle is a very small magnet
fixed upon a pivot. As it points north and south it is of great use
to the mariner. The mariner's compass is a round box with such a
needle balanced in it, and having a card on which is drawn a circle
divided into thirty-two parts, as seen in Fig. 285. The original
compass was a rude affair, consisting of a slip of loadstone laid
upon a piece of cork floating in water. The date and place of its
first use are unknown.

[Illustration: Fig. 285.]

406. =Declination of the Needle.=--The declination of the needle is
its deviation from a north and south line. It is in comparatively
few parts of the earth's surface that there is no deviation from
this line to the east or the west. "True as needle to the pole" has
become a proverb, and when it was first uttered it was supposed
to be founded in strict truth; but modern investigation has shown
not only that the needle varies in its pointing in different
localities, but that it varies to some little degree in its
variations. The declination of the needle was first observed by
Columbus in his first voyage of discovery, and it occasioned great
alarm among the sailors, who, as Irving states, "thought the laws
of nature were changing, and that the compass was about to lose its
mysterious power." Notwithstanding these and other observations of
a similar character, no great account was made of the declination
of the needle till the middle of the seventeenth century. But
since that time extensive records of its declinations at different
localities have been made, and tables and charts have been
constructed exhibiting them. These declinations are not constant,
but vary somewhat every day, from the influence, it is supposed, of
the sun upon the earth.

407. =Dip of the Needle.=--It is found that in most parts of the
earth, if a needle be balanced before it is magnetized, and then
be suspended from the same point, it will not be balanced, but
one end will dip downward. This fact was discovered by Norman,
a London optician, in 1576. He found that the dip at London was
toward the north at an angle of 72°. In pursuing the investigation
of this phenomenon it was found that going from the north toward
the equator the dip constantly lessened, until a point was reached
where the needle was horizontal. Then, on going south of this, a
reverse dip occurred, that of the south pole, and the farther
south the needle was carried the greater was the dip. In the north,
Captain Ross in 1832 came to a locality north of Hudson's Bay, in
lat. 70° 5′ N., long. 96° 45′ W., where the magnetic needle, freely
suspended, was in a vertical line. No such locality has yet been
discovered toward the south pole.

408. =The Earth a Magnet.=--You can readily see, from all that has
been stated in regard to the magnetic needle, that the earth is
a magnet, or has that covered up in it which in some way acts as
such. The dip of the needle shows that the two poles of this magnet
are somewhere near the north and south poles of the earth. The
locality which Captain Ross found must be near the north pole of
the magnet in that quarter of the world. The vertical position of
the needle there is analogous to the straight lines of iron filings
which you see in Fig. 282, near the poles of the magnet; and it is
easy also to trace the analogy between the dip of the needle at
different distances from what is called the magnetic equator of
the earth, where the needle is horizontal, and the curves which
you see extending from pole to pole. The different declinations
of the needle and the different intensities of the magnetic force
in different localities corresponding in latitude show that the
magnet in the earth, if there be one, is irregular in shape, or in
some way has its power varied much in differed parts of the earth's
crust.

409. =The Earth as a Magnetizer.=--As the earth is really a magnet,
it might be expected to impart magnetism by induction as other
magnets do. And this is found to be the fact. If you hold a bar
of soft iron in the direction of the dip of the needle it becomes
a magnet, its lower end being the north pole, and its upper the
south. That this is so can be ascertained by bringing a small
magnetic needle near each end. No effect of this kind is produced
when the bar is held horizontally east and west. Lightning-rods,
pokers, upright iron bars in fences, etc., are often found to
be magnetized because they have continued so long nearly in the
required position for magnetization. When a bar of iron has been
magnetized in the manner indicated, its magnetism may sometimes
be fixed by giving it a stroke with a hammer. It is a curious but
inexplicable fact that this vibration of the particles of the iron
should have this effect. But though such vibration helps to impart
magnetism, it is not at all favorable to its retention, for magnets
are always injured by blows or falls, or indeed any rude treatment.
For this reason care is requisite in removing an armature from a
magnet. If pulled off abruptly the power of the magnet is lessened.

410. =Magnetism in Other Substances besides Iron.=--It was formerly
supposed that magnetism was confined to ferruginous substances, but
this has been found not to be true. Various minerals are magnetic,
especially when they have been heated, also some of the precious
stones, and even silica, which enters so largely into some of
the rocks of the earth. And it is supposed by some that future
investigations will show that the influence of magnetism is as
extensive in the earth as that of electricity.

411. =In what Magnetism is Like Electricity.=--Magnetism is like
electricity in several particulars: 1. Its power is on the surface
of bodies. 2. It is of two kinds, north and south, or boreal and
austral, comparing with the positive and negative electricities.
3. The same rule of attraction and repulsion applies to both;
viz., like repel and unlike attract. 4. As electricity can be
communicated by induction, so can magnetism.

412. =In what Magnetism is Unlike Electricity.=--The circumstances
in which magnetism is unlike electricity are chiefly these: 1.
The obvious manifestations of magnetism are to a great extent
confined to one class of substances, the ferruginous, and to but
a portion of them; while electricity makes its manifestations in
connection with all kinds of substances. 2. Magnetism is never
transferred as electricity is from one body to another, but a
body gains rather than loses in imparting magnetic power to other
bodies. 3. The two magnetisms, the boreal and austral, can not be
obtained separately as the two electricities can. If a magnet be
broken in two, each piece will have in it the two magnetisms and
the two poles as the whole did. This is in entire contrast with
the electrical experiment noticed in the last of § 379. 4. There
are no non-conductors to interrupt magnetic influence. If in the
experiments in § 379 a plate of glass or resin were interposed
between A and B, the influence would cease, but it would have no
effect on the induction of magnetism if interposed between a magnet
and a bit of steel or iron.

413. =Electro-Magnetism.=--Though electricity and magnetism differ
so much from each other, yet they have intimate relations, and
it is now the general opinion among scientific men that they are
merely different modes of the same power. Magnetism can produce
electricity, and electricity can produce magnetism. The first
discovery of facts revealing this connection was made by Professor
Oersted of Copenhagen in 1819. Since that time electro-magnetism,
or the production of magnetism by electricity, has been a prominent
subject of observation and experiment. Oersted's first observation
was that a current of electricity passing over a wire near a
magnetic needle affected the position of the needle. He found also
that iron filings would adhere to a wire over which a current of
electricity is passing, just as they do to a magnet, dropping off,
however, as soon as the current ceases to pass. Such facts led to
a great variety of investigations and arrangements of apparatus by
Oersted and others.

[Illustration: Fig. 286.]

414. =Electro-Magnets.=--The most powerful electro-magnets are made
by bending a thick cylindrical bar of soft iron into the form of
a horseshoe, A B, Fig. 286, and coiling around it a copper wire.
The wire must be insulated by being wound with some non-conducting
material, as silk, so that the electric current may pass through
the whole length of the wire. With the instrument thus prepared, if
the two ends of the wire be connected with the poles of a voltaic
battery which is in action, the bar will be magnetized, and will
hold up a heavy weight so long as the electric current is passing
through the wire. Whenever the current is cut off by disconnecting
the wires the weight will fall.

[Illustration: Fig. 287.]

Electro-magnets have been made in this way having such power as
to sustain a weight of four thousand pounds. In Fig. 287 we have
represented an apparatus which exhibits electro-magnetism very
prettily. The soft iron, you see, is in two pieces which when
put together form a ring, _d_, and each piece has a handle. If
the pieces be put together with the coil, _c_, in the position
represented, on connecting the wires P and N with a battery in
action, the adhesion is so strong as to resist a great force; but
as soon as the connection is broken the pieces come apart at once.

[Illustration: Fig. 288.]

415. =Electric Telegraph.=--The most remarkable and useful
application of electro-magnetism we have in the electric telegraph.
As before stated, voltaic electricity is used. This is generated at
the place from which the message is sent, and passes over the wire
to the place where the message is received. There it acts upon soft
iron by passing through coiled wire, producing the modified power
called electro-magnetism. I will make all this plain to you by
describing the machine used in Morse's Telegraph, Fig. 288. W W are
the wires which connect with the station from which the message is
to be received, and these connect with the copper wire coiled round
the horseshoe of soft iron, _m m_. Above the magnet is a lever,
_a l_, which works on a fulcrum at _d_. One end of this lever has
a steel point, _s_, attached to it. At _c_ is an arrangement of
wheel-work, the object of which is to pass along regularly a slip
of paper, _p_, in the direction of the arrows. Observe now how
the apparatus works. When the electric current passes through the
coiled copper wire it makes a magnet of the iron, _m m_. The lever,
_a l_, is therefore attracted at the end, _a_, downward. Of course
the end, _l_, moves upward, bringing the steel point, _s_, against
the paper, where it makes a mark. The length of this mark depends
upon the length of time the electricity is allowed to pass along
the coiled wire, for the moment that it is shut off _m m_ ceases to
be magnetic, the "keeper," _a_, being no longer attracted, moves
upward, and the other end, _l_, of the lever moves downward, taking
the point, _s_, from the paper.

[Illustration: Fig. 289.]

In order to make the marks on the paper of different lengths,
there is a contrivance for regulating the length of time that the
current shall pass through the coiled wire. This contrivance,
called the _signal key_ is represented in Fig. 289. N and P are two
strips of brass connected with the two wires R and M, of which M
comes from the battery. The end of the strip N is raised a little
above the end of P. So long as they do not touch the circuit is not
complete, and no electricity passes. But if the operator press N
down upon P, the circuit is established, and the electricity passes
to the station with which he is in communication, and there acts
upon the apparatus seen in Fig. 287. Now the longer the finger
presses down N upon P the longer will be the mark on the paper at
the distant station. An operator then at New York, for example,
controls by this key the length of the marks made on the paper in
New Haven or any other place with which he is communicating.

You can see then very readily how a telegraphic alphabet can be
constructed by combinations of marks of different lengths agreed
upon to represent different letters and numerals. I give the
alphabet used in connection with Morse's Telegraph:

  A -- ----
  B ---- -- -- --
  C -- -- --
  D ---- -- --
  E --
  F -- ---- --
  G ---- ---- --
  H -- -- -- --
  I -- --
  J ---- -- ---- --
  K ---- -- ----
  L --------
  M ---- ----
  N ---- --
  O -- --
  P -- -- -- -- --
  Q -- -- ---- --
  R -- -- --
  S -- -- --
  T ----
  U -- -- ----
  V -- -- -- ----
  W -- ---- ----
  X -- ---- -- --
  Y -- -- -- --
  Z -- -- -- --

  Numerals.

  1 -- ---- ---- --
  2 -- -- ---- -- --
  3 -- -- -- ---- --
  4 -- -- -- -- ----
  5 ---- ---- ----
  6 -- -- -- -- -- --
  7 ---- ---- -- --
  8 ---- -- -- -- --
  9 ---- -- -- ----
  0 ---------------

One of the most singular and interesting things in the operation
of the telegraph remains to be noticed. In order to have the
electricity work it is necessary to have the connection between
the poles of the battery at the point where the effect is to be
produced. You see this in the experiments represented in Figs. 286
and 287. The same is true of the electro-magnet of the telegraph.
This being so, it was thought at first that it was necessary to
have two wires connecting two communicating stations; but it was
found that only one wire was needed, the earth itself answering the
same purpose as another wire. To make the communication through the
earth effectual, there is at each station a plate of metal, having
a surface of several square feet, buried in the ground, with a wire
running up to the machine.




QUESTIONS.


  [Teachers differ much in their plans of conducting recitations.
  Some are very minute in their questions; while others go to the
  other extreme, and merely name the topics, the pupils being
  expected to give in full what is said upon them. Neither of these
  plans should be adopted exclusively, but the mode of recitation
  should be much varied from time to time. This variety is somewhat
  aimed at in the questions which I have prepared, though in no
  case are the questions as minute as they should occasionally be
  made by the teacher. The numbers refer to the pages.

  It would be well to have the pupils draw many of the figures
  upon the blackboard, and then recite from them. By drawing the
  simplest figures first sufficient skill may be acquired to enable
  the pupil to draw those which are quite difficult.]


CHAPTER I.

13. What is said of the distinction between matter and spirit? What
of Bishop Berkeley's ideas? What of Hume's?

14. What is the origin of the word spirit? What is the relation of
the senses to the spirit? What is said of the effects of matter on
the senses? What are the forms of matter?

15. Illustrate the difference between elastic and non-elastic
fluids. What is said of the union of the particles of a solid? Give
the difference noted between different solids. How does a liquid
differ from a solid? Give particularly what is said of water.

16. What is said of the particles of gaseous substances? What
of the atmosphere? What of the vapor in it? What is said of the
entering of liquids and gases into interstices? What of the
mingling of gases with liquids? Give the illustration in regard to
fishes.

17. What is said of the solution of solids in liquids? What of the
evaporation of water in the air? Illustrate the influence of heat
on the forms of matter. What is said of the thermometer? What of
mercury, water, and iron in relation to the liquid state? What is
said of our knowledge of matter?

18. What was the supposition of Newton about the composition
of matter? What is said of the changes of matter? What are the
imponderable agents, and why are they so called?


CHAPTER II.

19. What is said of variety in the properties of matter? What of
the divisibility of matter?

20. What is said of gold-leaf, and of the wire of gold-lace? What
of the soap-bubble? What of the thread of the silk-worm, and of the
web of the spider? What of solution of blue vitriol? What of odors?

21. What is said of the dust of the puff-ball? What of pollen? What
of the dust rubbed from a moth's wing? What of guano? What of the
glazing of visiting-cards? What of the minuteness of some animals?

22. What is said of the Deity in relation to minute animals? What
is said of substances called porous? What of those in which there
are no pores apparent? What proof is there that all substances have
spaces in them?

23. What is said of the amount of space in gases and vapors? Give
the statement in regard to steam. What is said of solutions of
solids in fluids? What of evaporation? What of the diffusion of
odors in the air?

24. What is the relation of heat to space in matter? Upon what do
density and rarity depend? Explain tenacity. What is said of this
property in gases and liquids?

25. How is the comparative tenacity of substances ascertained?
Give the comparative tenacity of various substances. What animal
substances have great tenacity? What is said of the value of
tenacious substances?

26. What is said of hardness? What of flexibility and brittleness?

27. Give examples of flexible and brittle steel. Explain the actual
difference between them. Explain the tempering of steel.

28. What is said of the annealing of glass? What of Prince Rupert's
drops? What metals are the most malleable? What the most ductile?

29. What is said of the ductility of melted glass? What of the
change of position of particles in making plates and wires of
metals? What of welding? What is said of compressibility? What of
the incompressiblity of liquids?

30. What influence has heat on the bulk of liquids? Illustrate by
the thermometer. What is said of the compressibility of gases? How
does elasticity operate in the case of the India-rubber ball?

31. Give the illustration in regard to jumping. State the
experiment with the ivory ball. What is said of the movements of
particles on each other in elastic substances? What of the degrees
of elasticity in different substances?

32. What is the definition of elasticity? What is said of the
usefulness of the variety of properties in matter?


CHAPTER III.

33. What is meant by extension as a property of matter? Illustrate
the fact that it is an _essential_ quality. What is said of it in
reference to air?

34. Does matter ever penetrate matter? Give the illustration
represented in Fig. 6. Give that represented in Fig. 7.

35. State the arrangement of the diving-bell. Give the comparison
between bullets and needles in relation to penetration. What is
said of solution? What of odors?

36. What is the property of matter called inertia? Give
illustrations of it. Illustrate the fact that matter has no power
to stop its own motion. What is the reason that the popular notion
is that matter is more inclined to rest than to motion?

37. What is said of perpetual motion? Why is it not true of
divisibility that it is an essential property of matter? What is
said of weight?


CHAPTER IV.

38. What is said of the nature of attraction? What was Newton's
idea of it?

39. What is said of attraction in solids? What of its different
modes of action? What is the difference in attraction in the case
of steel and of water? What is said of the freeness with which
particles of a fluid move among each other?

40. Explain Fig. 9. Explain Fig. 10.

41. Give the difference between mercury and water in regard to the
globular form. What if said of drops of water on leaves?

42. What is said of oil in reference to attraction? Describe and
explain the manufacture of shot. What is said of the globular form
of the earth and the heavenly bodies?

43. What is said of crystallization? State the examples cited.
What is said of the crystallization of water? Give and explain the
example of sudden crystallization.

44. What is said of frost-work? What of snow?

45. What is stated in regard to the snow-crystals of the arctic
regions? What is said of order in nature? Why can you not make the
surfaces of broken glass adhere?

46. Explain the cementing of glass. What is said of the adhesion of
pieces of India-rubber? Describe and explain the experiment with
bullets and with balls of lead. How may silver and gold be made to
adhere to iron? What is said of the adhesion of tin and lead? What
of the adhesion of panes of glass?

47. Upon what does the strength of adhesion depend? Illustrate
the agency of heat in promoting adhesion. Give familiar examples
of attraction between solids and liquids. Explain the experiment
represented in Fig. 15.

48. What is said of stems in stagnant water? Explain Figs. 16, 17,
and 18.

49. Explain Fig. 19. Explain the rise of fluids in tubes by Fig.
20.

50. What is meant by _capillary_ attraction? Give familiar examples
of the rising of liquids in interstices.

51. Describe and explain the process of getting out millstones. How
does a blotter differ from writing-paper?


CHAPTER V.

51. What is the attraction of _cohesion_? Give examples of
attraction between masses or portions of matter.

52. Explain the falling of a stone to the ground. Illustrate the
fact that attraction is mutual. Give the illustration of the ship
and boat in full.

53. Illustrate the proportion between the mutual motions of the
attracting bodies. Give the calculation in regard to the motion of
the earth in attracting smaller bodies.

54. What is said of the universality of attraction? Explain the
tides. What is said of the attraction of the moon for the land?
What is the difference between the attraction of cohesion and the
attraction of gravitation? Why is the word gravitation thus used?
What is terrestrial gravitation?

55. Explain Fig. 22. Explain Fig. 23. What is said of substances
suspended in different parts of the earth?

56. Explain Fig. 24. What is said of plumb-lines?

57. What is weight? Give the comparison in regard to muscular
force. What is said of scales and weights? What of using springs in
weighing?

58. What would be the effect on weight if the density of the earth
were increased? In what ways would this be perceived? What is said
of the variation of weight with distance?

59. What is said of the difference of weight on mountains and in
valleys? What of weight in the moon? What of it in the sun? What is
said of the different modes of attraction?

60. Show why attraction of cohesion _seems_ to be different from
gravitation. Show now that it is really not different. What is said
of the experiment with the two bullets mentioned in § 66? What of
the adhesion of liquids to solid substances?

61. What is said of the various results of attraction? Explain
fully why you can pour water from a pitcher easier than from a
tumbler.

62. Explain the operation of the quick movement by which you
prevent water from running down the side of a tumbler in pouring it
out. What is said of dropping from a vial? How is the size of drops
limited? What is said of the movements of drops on window panes?

63. Why are the drops of different liquids different in size? Give
the illustration about chalk. Give that about dust. Explain Figs.
27 and 28.

64. Explain Fig. 29. What is said of the difference in size between
water and land animals?

65. Give the illustration in regard to trees. Give that in regard
to mountains. What is said of the mountains of the moon? What of
those of Jupiter? Give the illustrations in § 93 of transgression
of the principles which have been elucidated.

66. What is the difference between the attraction treated of in
natural philosophy and chemical attraction?


CHAPTER VI.

67. Show what we mean by the centre of gravity by Figs. 30, 31, and
32.

68. Give the definition of centre of gravity, and explain it. What
is shown by Fig. 33?

69. How can we find the centre of gravity of a body? What is said
of scales and steelyards?

70. State what is represented by Fig. 38. Illustrate the fact that
the centre of gravity seeks always the lowest point.

71. Give the illustrations of the rocking-horse, the swing, etc.
What is said of the Laggan stones? Why does an egg lie on its side?

72. Give the illustrations from toys in § 101. Give the
illustrations in § 102.

73. Upon what two things does the stability of a body depend?

74. What is said of the stability of bodies whose shapes are
represented in Figs. 48, 49, and 50? What of that of a round ball?
Why is the pyramid the firmest of all structures?

75. What is the relation of upright position to stability? What is
stated of the tower of Pisa?

76. Give the familiar illustrations in § 105. What is said of the
support of the centre of gravity in animals?

77. What is said of the skill exercised in walking? What of the
mode of walking in a child? What of the motions of the centre of
gravity in walking?

78. What is said of the walking of a man with wooden legs?
Illustrate the management of the centre of gravity in different
attitudes. Describe and explain the way in which one rises from a
chair.

79. State and explain the wager case. What is said of unstable
equilibrium? Give illustrations.


CHAPTER VII.

80. What is said of the phenomena treated of in Hydrostatics? What
are the two characteristics of liquids? What makes a liquid have
a level surface? Give the explanation. Give the comparison of the
shot.

81. What is said of water as a mirror? Show that the surface of a
liquid is not strictly level. If the earth had no elevations of
land why would it have a perfectly globular covering of water?

82. What is a so-called perfectly level surface? What is the
variation per mile from a real level? Describe the spirit-level.
Give the comparison between a trough and a river.

83. What is said of the declivity of rivers? How have some rivers
been made? What is stated in regard to the River Danube?

84. What is stated about the Lake of Geneva? Describe the
arrangement of canal locks.

85. How are canals used for working machinery? Give various
illustrations of the tendency of water to be on a level.

86. Describe the arrangement represented in Fig. 71, and give the
explanation.

87. Describe a foolish man's plan for perpetual motion, and give
the reason of its failure. What is said of ancient and modern
aqueducts?

88. Explain the operation of springs and Artesian wells.

89. Whence comes the name Artesian? What is stated of a well in
Paris? What of the situation of London? Why is the pressure of a
liquid in proportion to its depth? Give the illustrations of this
mentioned in § 122.

90. Explain Fig. 75.

91. What is said about the construction of dams and brewers' vats?
Explain the lateral pressure of liquids. Show the difference
between a liquid and a solid in this respect.

92. Show how the earth's attraction causes the lateral pressure by
Figs. 77 and 78. Give the view presented in § 124.

93. What is said of the proposed ship canal between the
Mediterranean and the Red Sea? Show that pressure in liquids is
equal in all directions.

94. Give the illustrations in § 126. Show that the upward pressure
in a liquid is as the depth, and that this is produced by
gravitation.

95. State the experiment represented in Fig. 82. Give the
experiment with the tube and India-rubber.

96. State the examples given of great effects produced by small
quantities of a fluid. Explain these effects by Fig. 83.

97. Explain Fig. 84.

98. What is the Hydrostatic Paradox, and why is it so called?
Describe and explain the Hydrostatic Bellows.

99. Describe and explain Bramah's Hydrostatic Press.


CHAPTER VIII.

100. Define specific gravity.

101. What is the most obvious way of ascertaining the specific
gravities of different liquids? Explain the sinking of heavy
substances in water. Explain the rising of light substances in
water. Explain what is illustrated in Fig. 87.

102. Explain Fig. 88. Explain Fig. 89.

103. Give the illustrations in § 138: lifting a stone; raising
a bucket; and raising the arm in a bath. Relate the anecdote
of Archimedes. What is said of boats and life-boats? What of
estimating the weight of the load in a canal-boat?

104. What is said of the specific gravity of birds? Of insects? Of
fishes? What of the specific gravity of the human body, and of the
prevention of drowning?

105. Give the reasons why so many are drowned that might easily be
saved?

106. What is stated about children in China? Why does the body of a
drowned person sink? Why does it after a while rise? What is said
about wading in rivers?

107. Explain the manner in which the specific gravity of a solid
may be ascertained? Give the experiment of weighing water. What is
stated of Archimedes and the crown?

108. Describe and explain the hydrometer. Relate the anecdote of
the Chinese. What is said of the selling of milk in Switzerland?

109. What is said of the centre of gravity in floating bodies? Give
the illustrations.


CHAPTER IX.

110. What does pneumatics teach? How can you show that air is
material? How that it has weight? What is its weight compared with
that of water?

111. What is said of the air's being attracted by the earth?
Explain why some things rise and others fall in air. How thick is
the earth's air-covering?

112. How is the height of the atmosphere ascertained? At what rate
does the earth move round the sun? How does it carry along the air
with it? State the influence which gravitation has upon the density
of the air at different heights.

113. Give the comparison of air to wool. What is said of hydrogen
and balloons? In what are gases and liquids alike, and what are the
results of the similarity? What is the amount of pressure of the
atmosphere on each square inch of surface? Give the calculations in
regard to this pressure.

114. Show why the great pressure of the air does not produce
destructive effects. Describe the air-pump.

115. Explain by Fig. 95 the plan and working of the air-pump.

116. State some of the experiments with the air-pump. How can you
prove that air, like water, presses equally in all directions?
State the comparison about the fish.

117. What is said of the Magdeburg hemispheres? Give the experiment
with mercury. Explain the operation of the boy's sucker.

118. Give the statements about sucker-like arrangements in animals.
State the experiment of the bladder and weight. Give the experiment
with the India-rubber bag.

119. State the experiment with the egg. Explain the operation of
the hydrostatic balloon.

120. Explain the operation of the Cartesian image. What is said of
the presence of air in various substances?

121. What is said of the elasticity of air? Describe and explain
the condenser.

122. Describe and explain the gasometer. Show how the air-gun
operates. Explain the pop-gun.

123. Explain the operation of gunpowder. Explain that of steam.
What is said of retardation by condensed air in gunnery?

124. Describe and explain what is represented in Fig. 113. Explain
the collection of gases in the pneumatic trough.

125. Explain the experiment represented in Fig. 115. What is said
of tapping a barrel? What causes the gurgling sound when a liquid
is poured from a bottle? How high a column of water will the
pressure of the atmosphere sustain? How do you find from this the
pressure of the air on every square inch of surface? How high a
column of mercury will the atmosphere sustain?

126. Explain the barometer. Relate the incident given by Dr. Arnot.

127. Why would not a water-barometer answer? What is said of the
barometer as a measurer of heights? How is the boiling point
influenced by the amount of the air's pressure? Give the experiment
with ether.

128. State the experiment with the flask. What would happen to
liquids if the atmosphere were removed from the earth? Explain the
operation of the syphon by Fig. 117.

129. Explain what is represented by Fig. 118.

130. Explain the uses of the syphon. Explain the operation of the
Cup of Tantalus.

131. How are intermitting springs accounted for? Explain the
operation of the common pump.

132. Why does the water rise in the pump? How is sucking done?
Explain the forcing pump.

133. Explain the fire-engine.


CHAPTER X.

134. What is said of the universality of motion? What of attraction
as a cause of motion? What of heat? What of chemical agencies? What
of life?

135. What is meant by saying that action and reaction are equal?
State the illustrations of this truth which are given. Describe
Barker's mill.

136. Give the comparisons to the operation of a spring, of firing
of a cannon, and the throwing of stones from the crater of a
volcano. What is said of the jumping of a man from the ground?

137. What is said of the reaction in the case of a hopping bird?
Illustrate the inertia of matter as shown in the communication of
motion. Give the illustrations of the fact that time is required to
communicate motion to bodies.

138. Give illustrations of inertia as shown in the disposition of
motion to continue.

139. Describe and explain the equestrian feat represented in Fig.
127. What is said of skill in jumping from a moving carriage?
Relate the case in court which is stated.

140. What is said of the course of bodies thrown into the air? What
of a man falling from a mast-head?

141. What is said of the atmosphere as revolving with the earth?
What rapid motions are we subjected to when we speak of ourselves
as at rest? Why are we insensible to these motions?

142. Follow out in full the comparison of the steamboat. What is
the difference between absolute and relative motion? What is said
of absolute rest?

143. Illustrate the truth that all the motions which are apparent
to the eye are slight differences in the common absolute motions.
What are the obstacles to motion? How is the motion of a stone
thrown upward destroyed? What causes and what opposes its descent?

144. State and explain the experiment with the lead and feather.
Explain the operation of the water-hammer. Show the relation of
bulk to the resistance of liquids and gases.

145. Illustrate the relation of bulk to the motion of solids,
produced by moving gases and liquids.

146. What is said of the opposition of gravitation to water and air
in moving solids? What difference does the presence of obstacles
make in the relation of force to velocity?

147. State the law of the relation of force to velocity, and
illustrate by Fig. 136. What are some of the practical applications
of this law? What is said of the relation of shape to velocity?
What is said of the shape of fishes?

148. What is said of the shapes of boats? What of the management of
the webbed feet of water-fowls? What of the wings of birds? What is
said of friction as an obstacle to motion? What of it as a cause to
motion? Illustrate fully in the case of the wheel.

149. What is said of the friction of liquids in tubes? What is
the effect of sudden turns in pipes? What is the arrangement of
arteries in the heads of grazing animals? Illustrate the difference
of friction in small and large pipes by Fig. 131.

150. What is said of the effect of friction in brooks and rivers?
In what part of a stream does the water move most rapidly? Explain
the formation and breaking of the crest of waves rolling over
a beach. What is said of the velocity of rivers as affected by
friction? Explain the formation of waves.

151. What is it that really advances in the forward movement of a
wave? Give the comparison mentioned. What is said of the height of
waves?

152. What is momentum? Upon what two things does it depend?
Illustrate this dependence. Explain Fig. 133.

153. Give the illustration of the musket-ball and cannon-ball. Give
that of the plank. That of the candle. That of the air.

154. What is said of the expression, quantity of motion? Under what
circumstances may a single impulse produce a great velocity? What
examples have we of this? How is it with the motions that we see
around us? What is said of the fall of bodies to the earth?

155. Give examples from muscular action. Give that of the arrow.
Give that of gunpowder. What is said of the arrest of great
velocities? Give the illustrations in regard to cannon-balls.

156. State and explain the feat of the anvil. Give examples from
common efforts and labors.

157. Explain the communication of motion in the case of elastic
bodies by Figs. 133 and 134. What is said of the reflection of
motion?

158. What is said of the uniformity of motion? What of its
uniformity in velocity? State by what means we calculate as to time.

159. What is said of the sun-dial? What of the hour-glass? What of
Galileo and the pendulums? Explain the operation of the pendulum.

160. Explain Fig. 137. Explain the operation of the gridiron
pendulum by Fig. 138.

161. What is said of the disposition of motion to be straight? Why
is motion never straight, so far as we know? How can we make motion
very nearly straight? Give the illustration of the bullet in full.

162. Give the illustration represented in Fig. 141. What is
compound motion? Illustrate straight compound motion.

163. Explain Fig. 143.

164. Explain what is represented Figs. 144, 145, and 146.

165. Explain Fig. 147. How is curved motion produced? Give the
illustration of the ball and string. What are centrifugal and
centripetal forces?

166. What are these two forces in the revolution of the earth
around the sun? Give various illustrations of the operation of
centrifugal force.

167. What is said of the formation of bends in rivers?

168. Show how eddies and whirlpools are formed. How is the
centrifugal force used in the art of pottery. How in making
window-glass?

169. Describe and explain the operation of the steam-governor.

170. What is said of the agency of the centrifugal force in shaping
the earth?

171. Explain the operation of the apparatus represented in Fig.
154. What are the forces which act on a projectile? What is said of
balls thrown horizontally from cannon with different velocities?

172. Show by Fig. 155 why a ball dropped from the mouth of a cannon
will fall to the ground in the same time that one fired from it
will. By What two forces is a falling body acted upon?

173. Explain Fig. 156. What is the course of a ball dropped from a
railway car or from a mast-head? Give the comparison between the
cannon-ball and the moon.

174. What is said of the velocities of the heavenly bodies?


CHAPTER XI.

174. What are the Mechanical Powers? Why is the term power not
strictly proper?

175. Explain the terms power, weight, and fulcrum. What is said of
the use of the lever? What is the lever of the first kind? What is
said of its force?

176. What is said of scales? What of steelyards?

177. Give examples of the first kind of lever. Show by Fig. 159
that there is no gain of power in this lever.

178. Give the illustration of the see-saw. What is said of
Archimedes's lever?

179. State the analogy between this lever and the Hydrostatic
Bellows, Bramah's Press, etc. What is the lever of the second
kind? Apply the rule of equilibrium to it. Show how the common
wheel-barrow is a lever of this kind.

180. Give other examples of the second kind of lever. What is lever
of the third kind? How does this differ from the other two kinds?
Apply the rule of equilibrium to it.

181. Give examples of the third kind of lever. Show how it acts
at a mechanical disadvantage in the different examples mentioned.
State in full what is said of muscular action.

182. Explain by the figures the operation of compound levers.

183. State the comparison between the lever and the wheel and axle.
What is said of the common windlass?

184. Describe and explain the capstan. What are its chief uses?
What is said of the fusee of a watch?

185. Describe the arrangement of the fixed pulley. What are its
uses?

186. Describe the arrangement of the movable pulley. Show how the
relation of the power to the weight is estimated in the case of
compound pulleys.

187. Explain the mechanical advantage of the inclined plane. Give
examples of it.

188. What is said of roads? Give the comparison of the wedge to
the inclined plane. How is the power of the wedge estimated? Give
examples of the wedge.

189. What is said of the screw? Show by Fig. 180 how the force of
the screw is estimated. What are some of the uses of the screw?

190. Give the estimate of the power of the screw and lever as
used together. How can you show that there are really but three
mechanical powers? What is said of these as composing tools and
machinery? What is said of friction in machinery?

191. What is the first advantage of the mechanical powers which is
mentioned? Give the illustrations. What is the second advantage?
Give the illustrations.

192. What is the third advantage? Give examples. How is the
velocity of motion in machinery usually varied? What is the fourth
advantage? Mention examples. Describe the instrument called a Lewis.

193. What is said of the title by which Aristotle distinguished man
from other animals?


CHAPTER XII.

194. What is sound? What relation has sound to rapidity of
vibration? Mention cases in which the vibration of sounding
bodies is manifest to the sight and touch. What is said of wind
instruments?

195. State the analogy of a sounding body to a pendulum. Describe
the process by which the sensation of sound is produced. Where does
the vibration caused by the sounding body stop in the ear? What is
transmitted from thence to the brain?

196. Give examples of the transmission of sound through various
substances? State the experiment by which it is shown that sound is
not transmitted through a vacuum. What is said of sound at great
heights?

197. How far has the sound of a volcano been heard? If the same
sound were made in space at that distance from the earth why could
not the inhabitants hear it? What is the cause of the noise of
bodies passing through the air? Why do the heavenly bodies, moving
so rapidly, produce no sound? Cite examples showing the different
velocities of sound in different media.

198. What is said of the uniformity of the velocity of sound? Show
how we can measure distances by sound as compared with light in
velocity. Upon what does the loudness of sound depend? Illustrate
this point.

199. What is said of the diffusion of sound? What of its
reflection? What of echoes?

200. What is said of multiplied and mingled reflections of sound?
Explain the operation of whispering galleries by Fig. 187.

201. Explain the operation of the speaking-trumpet. Give other
examples of the concentration of sonorous vibrations.

202. What is the difference between a musical sound and a noise?
What is said of the exact regularity of musical vibrations? How are
different notes produced in stringed instruments? Upon what does
the note depend in wind-instruments?

203. Explain the operation of the organ-pipe represented in Fig.
190. What is said of the notes of bells and of musical glasses?
Explain the mechanism of the human voice.

204. What is harmony? Upon what does it depend? Between what two
notes of the scale is there the greatest harmony? What note next to
the octave harmonizes best with the fundamental note? And what note
next? Show why the second note, in contrast with the octave, is so
discordant with the fundamental note.

205. State the proportions between the numbers of the vibrations in
the different notes. If you know the number of vibrations of the
fundamental note in a second, how may you determine the number of
vibrations in the other notes? What is said of the number of notes
in the diatonic scale? What of the proportionate lengths of strings
for different notes? What is said of tuning instruments?

206. What is meant by saying that a note is too sharp or too flat?
State in full what is said about the mysteries of sound and hearing.


CHAPTER XII.

207. State the experiment of the three vessels, and the inference
from it.

208. What other facts sustain this inference? How did Sir Humphrey
Davy prove that there is heat in ice? What are the two theories of
heat? What is the chief source of heat for the earth? What is said
of the heat of the sun itself?

209. What is said of the universal influence of the heat of the
sun in the earth? What of the heat supplied from within the earth
itself? What of electricity as a source of heat?

210. What is said of chemical action as a source of heat? Give
examples of the production of heat by mechanical action. What is
said of the relations of heat and light?

211. Show the expansive influence of heat by describing the
experiment represented in Fig. 192. Give familiar examples of this
expansion.

212. How can you loosen a stopper stuck fast in a bottle? Give the
anecdote about the _Persia_. Give the statement about the building
in Paris.

213. What is said of the expansion of liquids by heat? How may the
influence of this expansion upon specific gravity be shown?

214. What is said of thermometers? What of the invention of the
thermometer?

215. State the plan of Fahrenheit's thermometer. Give the plans of
other thermometers.

216. Why is Fahrenheit's thermometer, on the whole, the best? What
is said of the expansion of gases by heat? State experiments in
illustration.

217. What is said of balloons? What of the influence of heat on the
atmosphere? Give examples of this influence.

218. Why, in heating apartments, do we have the heat created or
introduced at as low a place as possible? Explain the _draught_
of a chimney. Why does a stove-pipe generally draw better than a
chimney?

219. State the experiment with the candle and the door. What is the
explanation of the occurrence of wind? Explain the land-breeze.

220. Explain the sea-breeze. How are winds affected by the rotation
of the earth?

221. Show by Fig. 201 why the prevailing winds at the equator are
northeast and southeast.

222. Mention the melting points of various substances. What is said
about the natural state of water and other substances? What are the
two modes of changing a liquid into vapor?

223. What is said of the rapidity of evaporation? What of the
solution of water in air? What influence has heat upon the
capability of air to dissolve water? What phenomena illustrate
this? How is it supposed that water rises in air? What fact is in
opposition to this supposition?

224. What becomes of the water that rises in the air? What is said
of the formation of fog and of clouds? Mention the different shapes
of clouds and their names.

226. What is said of the influences that give shape to clouds?

228. State how rain is produced, and explain Fig. 208. How are snow
and hail formed? What is said of vaporization?

229. What influence has pressure upon the formation of vapor? Give
the experiment with ether in illustration. Describe the experiment
represented in Fig. 209. What is said of Papin's digester?

230. What is said of steam? In what consists the power of the
steam-engine? How is the expansive force of the steam in the boiler
estimated? Describe the working of the engine by Fig. 210.

231. What is the difference between high and low pressure engines?
What is said of the communication of heat? How many modes of
communication are there, and what are they? What is the mode called
convection?

232. Give examples of convection. What is the _conduction_ of heat?

233. State the experiment represented in Fig. 211. State the
experiment represented in Fig. 212. What is said of non-conductors
of heat? Give the examples cited.

234. Explain Davy's safety-lamp.

235. Give what is stated in the note about Stephenson and Davy.
What is said of the influence of density on the conduction of heat?
Give the illustration about melting snow.

236. State the experiments which show that liquids are poor
conductors of heat. What is said of air as a non-conductor of heat?
What is said of double windows?

237. What is said of arrangements of the walls of buildings? What
of an arrangements for preventing the spreading of fires in blocks?

238. How are animals in very cold regions protected from the cold?
What is it in their coverings that affords the protection? What
is said of the coverings of quadrupeds that are natives of warm
climates? What of the elephants whose remains are found in Siberia?

239. What changes take place in the coverings of animals carried
from a cold to a warm climate, and the reverse? Why has man no
covering against the cold? Explain the object of clothing. What
is said of articles of clothing? What of loose clothing? What of
coatings of straw put on trees? What of bricks compared with stones?

240. What is said of cocoons? What of buds of plants in winter?
What of snow as a protection of plants?

241. State the arrangement of snow observed in the arctic regions.
State in full what is said of the influence of the conduction of
heat upon sensation.

242. What is meant by the radiation of heat? Give examples of it.
What is said of the connection of heat and light in the rays of the
sun? What is said of heat from a common fire?

243. What is said of the relation between absorption and radiation?
What of the reflection of heat? State the experiment with the
mirrors and the thermometer and flask.

244. Explain the experiment with the ice. Give the experiment with
phosphorus. Give the experiment represented in Fig. 218.

245. Explain the formation of dew. State the analogy of the
tumbler. What is said of the circumstances that influence the
deposition of dew and frost?

246. What is said of different substances in regard to the
deposition of dew? What about Gideon's fleece? What is the
dew-point? How can you ascertain in it?

247. What is said of the freezing of mercury? Explain the
difference between sensible and latent heat. What is said of
capacity for heat? State the experiment represented in Fig. 219.

248. What is the relation of heat to density? Give the
illustrations.

249. What is the reason that the air is so cold on great heights?
What is the relation of heat to the forms of substances? What is
said of the melting of ice? What of the vaporization of water?
State the general conclusion in regard to latent and sensible heat.

250. State in full what is said of latent heat in reference to
clouds. Explain the operation of freezing mixtures.

251. State the examples of the production of cold by evaporation.

252. State and explain the experiment represented in Fig. 221.

253. Give the facts stated in regard to the degree of heat which
man can endure. Give the reasons why the heat did not produce a
greater effect in these cases.

254. What effect does heat produce upon the bulk of substances?
What is said of water as an exception? Describe the process of
freezing as illustrated by the diagram.

256. What would be the process if the exception did not exist?
State what would be the results.

257. What would be the consequence if the freezing point were
above 32°? What if it were below? What is said of the force of
expansion in ice? What are some of the benefits which come from
this expansion?


CHAPTER XIV.

258. What is Newton's theory of light? What is the undulatory
theory? State the analogies to sound and heat. When is a body
luminous? What are the sources of light?

259. How may you see that light moves in straight lines? State
various familiar recognitions of this fact. Illustrate the fact
that the intensity of light is inversely as the square of the
distance.

260. What is said of the velocity of light in regard to ordinary
distances? How long is light coming from the sun to the earth? What
is said of the light coming to us from certain stars?

261. Give the observation of Roemer represented in Fig. 226.

262. What is said of the reflection of light? What of its
reflection in relation to seeing? What of the images formed in
mirrors?

263. Show by Fig. 228 why the image in a mirror seems to be as the
same distance behind it that the object is before it. Explain by
Fig. 229 the operation of the kaleidoscope.

264. Explain the operation of a concave mirror by Fig. 230. Explain
that of a convex mirror by Fig. 231.

265. What is meant by the refraction of light? Illustrate its
reflection in passing from a denser into a rarer medium. Then from
a rarer into a denser.

266. How is the refraction in regard to a perpendicular in the two
cases? Explain dawn and twilight. Explain what is represented in
Fig. 234.

267. What are mirages? Describe the mirage which occurred at
Ramsgate. Describe that seen by Captain Scoresby. Relate the
incident which occurred at New Haven.

268. What is said of mirages in deserts?

269. Describe the mirage of the French coast. Explain what is meant
by the visual angle as illustrated by Fig. 236.

270. Explain Fig. 237. What are lenses? What are the different
kinds?

271. What is the difference of effect in convex and concave lenses?
Explain the effect of a convex lens on the visual angle. What is
said of microscopes and telescopes?

272. Describe and explain the magic lantern. Describe and explain
the camera obscura.

273. Describe the arrangement of a camera for sketching. How is the
eye like a camera?

274. Describe the arrangement of the parts of the eye as mapped in
Fig. 244.

275. Show now particularly how the eye is like a camera. What is
said of the influence of the cornea on the light? Show what is
required for distinct vision, as illustrated in Fig. 245. Show
why it is that objects brought very near the eye are not seen
distinctly.

276. What is said of the microscope? Explain the difficulty in the
near-sighted. In the far-sighted.

277. How can you show that the images of objects in the retina are
inverted? Give in full what is said of explanations of the fact
that we see objects erect notwithstanding this inversion. Explain
single vision.

278. By what simple experiment can you show the explanation of
single vision to be correct? What is said of squinting? Explain the
stereoscope.

279. What is said of distinct impressions on the retina? Explain
the thaumatrope.

280. State in full what is said of the compound nature of light.
Give the proportions of the colors in it. What is said about there
being only three colors?

281. What is said of the recomposition of decomposed light? Give
the illustration of the powder--the circular board--the top.

282. What is said of the colors of substances? What of the
variations of these colors in different lights? What of variations
with varying positions? What of the colors of clouds?

283. Explain the formation of the first rainbow by Fig. 253.
Explain the formation of the second bow by Fig. 254.

284. What is said of the circumstances under which rainbows are
seen?

285. Explain in full the formation of the two bows as illustrated
by Fig. 255. What is said of the bow as seen by different persons,
and at different moments by the same person? What of rainbow hues
in dew-drops and ice-crystals?

286. Give the dissection of light as represented in Fig. 256. What
is said of Daguerreotyping?


CHAPTER XV.

287. What is the origin of the term electricity? What is said of
attraction and repulsion in electricity?

288. What is the supposed explanation of electrical repulsion?
Explain the difference between resinous and vitreous electricity.
What is said of the two supposed electrical fluids? Detail the
illustrations of attraction which are stated.

289. State the theory of Franklin. Explain the use of the terms
positive and negative. Illustrate the fact that the kind of
electricity excited depends on what a substance is rubbed with.

290. What is said of the incorrectness of the terms vitreous and
resinous? What is said of conductors and non-conductors? Why are
non-conductors called insulators?

291. What marked difference is there between heat and electricity?
State the experiment represented in Fig. 258. What is said of
electrics and non-electrics?

292. What is said of equilibrium in electricity, and of its
disturbance? Give in full what is said of the universality of
electricity.

293. State what is said of induction, as illustrated by the
experiment represented in Fig. 259.

294. State the experiment represented in Fig. 260. Describe the
arrangement and operation of the electrical machine represented in
Fig. 261.

295. Describe the cylinder machine. State the experiment with the
pith balls. State that with the head of hair.

296. State the experiment with the tissue-paper. State that with
the dancing figures. State that with the bells.

297. Describe the experiment with the tin-foil. Describe the
insulating stool and the operation of it.

298. What is said of the escape of electricity from points?
Describe the apparatus represented in Fig. 269, and the operation
of it. What is said of the discharge of electricity from a point in
a dark room? Describe the Leyden jar.

299. Explain the operation of the Leyden jar. What would be the
effect of connecting the inside foil with the outer by a strip of
foil? Give in full the experiment represented in Fig. 272, and the
explanation.

300. What is said of the discharge of the jar? How can a large
number of persons take a shock from it together?

301. Explain the effect of moisture upon the charged jar. State the
experiment represented in Fig. 274.

302. What is the electrical battery? What is said of the light
produced by electricity?

303. To what is the report of electricity owing? What is said of
mechanical injuries caused by electricity? What of the heat caused
by it? What effects may be produced by this heat?

304. What was the discovery of Franklin, and how did he make it?

305. Relate the accident which occurred at St. Petersburg. What is
said of lightning-rods?

306. What was Sulzer's experiment? What were Galvani's observations?

307. What is said of the pile of Volta? What of his cup battery and
of other batteries?

308. What difference is there between frictional and voltaic
electricity?


CHAPTER XVI.

308. What are loadstones? Where do they abound? What is said of
discoveries in magnetism? Whence come the terms magnetism and
magnet?

309. What is said of the attraction of magnetism? What law is there
in regard to it? What is said of the poles of a magnet? What of
magnetism by induction?

310. What is said of attraction and repulsion in magnets? Explain
the formation of the curves of iron filings in the experiment
represented in Fig. 282.

311. How may artificial magnets be made? What is said of the
horseshoe magnet and its armature?

312. What is said of the magnetic needle and the mariner's compass?

313. What is the declination of the needle? When was it first
observed? What is said of observations after this? What is said of
the dip of the needle?

314. What is said of the earth as a magnet? What of it as a
magnetizer?

315. What is said of fixing magnetism? What of impairing it? In
what other substances besides iron does magnetism exist? In what is
magnetism like electricity? In what is it unlike it?

316. What relation has magnetism to electricity? What were the
original observations of Oersted in regard to it?

317. Describe the manner of making the most powerful
electro-magnets. Describe the experiment represented in Fig. 287.

318. Show the application of electro-magnetism in the electric
telegraph.

319. What is the contrivance called the signal key? How is the
alphabet of Morse's Telegraph constructed?

320. What is said of the communication through the earth in
telegraphing?




INDEX.

[The numbers refer to the paragraphs]


  Action and reaction equal, 182

  Air, compressibility of, 152

  Air, elasticity of, 161

  Air, density of, dependent on pressure, 158

  Air, pressure of, as affecting the boiling point, 171, 327

  Air, currents in, from heat, 281

  Air, solution of water in, 287

  Air attracted by the earth, 148

  Air in interstices of substances, 160

  Air a non-conductor of heat, 302, 304

  Air-pump, 155

  Air-guns, 164

  Animals, coverings suited to climate, 305

  Archimedes, his lever, 229

  Archimedes, his discovery in the bath, 138

  Archimedes and the crown, 143

  Artesian wells, 120

  Aqueducts, 119

  Atmosphere, pressure of, 154, 168

  Atmosphere, how it moves with the earth, 188

  Atomic theory, 15

  Attraction, nature of, 53

  Attraction, capillary, 72

  Attraction between masses, 73

  Attraction between solids and liquids, 69

  Attraction, Newton's idea of, 54

  Attraction, universality of, 77

  Attraction, variety in results of, 87

  Attraction, opposition between modes of, 88

  Attraction, proportion of the mutual motions of, 76


  Balloons, 149, 280

  Balloon, hydrostatic, 159

  Barometer, 169

  Barker's mill, 182

  Battery, electrical, 387

  Berkeley, his ideas of matter, 2

  Boats, why they float, 139

  Boiling point as affected by the pressure of the air, 171

  Brittleness, 29

  Buds in winter, 308

  Bulk, relation of, to resistance of liquids and gases, 193


  Camera obscura, 352

  Canals, 117

  Capillary attraction, 72, 86

  Cartesian image, 159

  Centre of gravity defined, 96

  Centre of gravity, support of, in animals, 106

  Centre of gravity, movements of, in walking, 107

  Centre of gravity in relation to attitudes, 108

  Centre of gravity in floating bodies, 145

  Centrifugal force, 213

  Centripetal force, 213

  Chimneys, draught of, 282

  Clothing, object of, 306

  Clouds, how formed, 288

  Clouds, colors of, 364

  Clouds, shapes of, 289

  Clouds, latent heat in, 324

  Cocoons, constructed for winter, 307

  Colors of objects, 363

  Colors of the clouds, 364

  Cohesion, strength of, 68

  Cohesion, what is essential to it, 66

  Cold, absence of heat, 270

  Cold at great heights, cause of, 322

  Cold from evaporation, 326

  Compressibility of substances, 35

  Compressibility of air, 152

  Condenser, 162

  Conductors of heat, 298

  Conductors of electricity, 375

  Conduction of heat, 297

  Conduction, relation of density to, 300

  Conduction in liquids, 301

  Conduction, influence of, on sensation, 310

  Convection of heat, 296

  Coverings of animals suited to climate, 305

  Crystallization, 62


  Daguerreotype, 369

  Dams, 122

  Davy's safety-lamp, 299

  Dawn explained, 346

  Density, 24

  Dew, formation of, 315

  Dew-drops, colors in, 367

  Dew-point, 317

  Diatonic scale, 267

  Draught of chimneys, 282

  Drowning, how it can often be prevented, 141


  Earth, globular form of, 61

  Earth, roundness of, shown, 113

  Earth, attraction toward centre of, 80

  Earth, thickness of its air-covering, 150

  Earth, shape of, influenced by centrifugal force, 218

  Earth, rotation of, affecting winds, 284

  Elasticity, 39

  Elasticity, definition of, 42

  Electricity, origin of the term, 370

  Electricity, attraction and repulsion in, 371

  Electricity, vitreous and resinous, 372

  Electricity, galvanic, 394

  Electricity, universality of, 378

  Electricity, discharge of, from points, 383

  Electricity, sound of, 389

  Electricity, heat produced by, 391

  Electricity, mechanical injuries by, 390

  Electrics and non-electrics, 377

  Electrical battery, 387

  Electro-magnetism, 413

  Electro-magnets, 414

  Electric telegraph, 415

  Equestrian feat, 185

  Equilibrium, stable and unstable, 109

  Evaporation, 286

  Evaporation, cold produced by, 326

  Expansion by heat, 274

  Expansion, exception to it, 330, 331

  Extension, a property of matter, 44

  Eye, description of, 353

  Eye, images in, inverted, 356


  Feathers, how a non-conductor of heat, 305

  Fire-engine, 179

  Flexibility, 29

  Flies, feet of, 157

  Force, relation of, to velocity, 194

  Forcing-pumps, 178

  Franklin, his theory of electricity, 373

  Franklin, his discovery, 392

  Freezing, process of, described, 330

  Freezing point, why at 32°, 332

  Freezing in the midst of boiling, 327

  Freezing mercury, 318

  Freezing mixtures, 325

  Friction, 196-198

  Friction in machinery, 245

  Frost, 44

  Fur, how a non-conductor of heat, 305

  Fur, how altered by climate, 305

  Fusee of a watch, 238


  Galvanism, 394

  Gases, movements of the particles in, 10

  Gases, space in, 21

  Gases, compressibility of, 38

  Gasometer, 163

  Glass, annealing of, 32

  Glass-making, centrifugal force used in, 216

  Governor, steam, 217

  Gravitation, 79, 86

  Gravitation, limiting size, 90

  Gravitation, action of, on solids in a liquid, 136

  Gravity, specific, defined, 135

  Gridiron pendulum, 210

  Gunnery, retardation by condensed air in, 166


  Hail, how produced, 290

  Hardness, 28

  Harmony, 266

  Hearing, mysteries of, 269

  Hearing, trumpet, 262

  Heat, nature of, 271

  Heat, sources of, 272

  Heat, relations of, to forms of matter, 13

  Heat, relation of, to bulk of liquids, 37

  Heat, relation of, to light, 273

  Heat producing expansion, 274, 275, 279

  Heat, communication of, 295

  Heat, convection of, 296

  Heat, conduction of, 297

  Heat, radiation of, 311

  Heat, reflection of, 314

  Heat, connection of, with light, 312

  Heat, production of, by electricity, 391

  Heat, degree of, endurable by man, 328

  Heat, latent, 319

  Heat, capacity of different substances for, 320

  Heat, relation of, to density, 321

  Heat, relation of, to forms of substances, 323

  Heat and cold, 270

  Heights measured by barometer, 170

  Hume, his ideas of matter, 3

  Hydrometer, 144

  Hydrostatics, what it teaches, 110

  Hydrostatic balloon, 159

  Hydrostatic bellows, 132

  Hydrostatic paradox, 131

  Hydrostatic press, 133


  Ice, formation of, 329

  Ice, force of its expansion, 333

  Ice-crystals, colors in, 367

  Icebergs, centre of gravity in, 145

  Impenetrability, 45

  Imponderable agents, 16

  Inclined plane, 241

  Induction in electricity, 379

  Induction in magnetism, 400

  Inertia, 48

  Inertia shown in the communication of motion, 183

  Inertia shown in disposition of motion to continue, 184

  Insulating stool, 382


  Jupiter, mountains in, 92


  Kaleidoscope, 343


  Laggan stones, 100

  Land-breeze, 283

  Lenses, 349

  Lever of first kind, 224

  Lever, no gain of power in it, 227

  Lever of second kind, 231

  Lever of third kind, 232

  Leyden jar, 384, 385

  Life-boats, 139

  Light, nature of, 334

  Light, sources of, 335

  Light, diffusion of, 337

  Light, velocity of, 338

  Light, reflection of, 340

  Light, refraction of, 345

  Light moves in straight lines, 336

  Light shown to be compound, 360

  Light, colors in, 360

  Light, recomposition of, 362

  Light, chemistry of, 369

  Lightning-rods, 393

  Liquids, movability of their particles, 9, 57

  Liquids, incompressibility of, 36

  Liquids, attraction in, 56

  Liquids, globular shape of drops of, 58

  Liquids, cause of level surface of, 111

  Liquids, pressure of, in proportion to depth, 121

  Liquids, lateral pressure of, 123

  Liquids, pressure of, equal in all directions, 125

  Liquids, upward pressure as depth, 127

  Liquids, in what like gases, 153

  Liquids, friction of, in tubes, 197

  Liquids, friction of, in streams, 198

  Liquids, expansion of, by heat, 275

  Loadstones, 397


  Machines not sources of power, 222

  Machinery, friction in, 245

  Magic lantern, 351

  Magdeburg hemispheres, 156

  Magnetism, origin of term, 397

  Magnetism, attraction of, 398

  Magnetism, repulsion of, 401

  Magnet, poles of, 399

  Magnets, artificial, 403

  Magnets, horseshoe, 404

  Magnetic needle, 405

  Malleability, 28

  Man a tool-making animal, 247

  Mariner's compass, 405

  Matter distinguished from spirit, 1

  Matter, effects of, on senses, 6

  Matter, forms of, 7

  Matter, nature of, 14

  Matter, constitution of, 14

  Matter, variety in properties of, 17

  Matter, divisibility of, 18, 51

  Matter, minute division of, 19

  Matter, pores and spaces in, 22

  Matter, relation of heat to spaces of, 23

  Matter, inertia of, 48

  Matter, impenetrability of, 45

  Matter equally inclined to rest and motion, 50

  Mechanical powers, real advantages of, 246

  Mercury, freezing of, 318

  Microscopes, 350

  Microscopical animals, 19

  Mirages, 347

  Mirrors, 342

  Mirrors, curved, 344

  Moth's wing, 19

  Motion, causes of, 181

  Motion, universality of, 180

  Motion, absolute and relative, 191

  Motion, obstacles to, 192

  Motion, reflection of, 206

  Motion, compound, 212

  Motion, curved, 213

  Motion in orbits, 221

  Motion disposed to be straight, 211

  Motion, that of falling bodies projectile, 220

  Motion and rest, 189

  Momentum, 201

  Mountains, size limited, 92


  Nature, order in, 65

  Near-sightedness, 355

  Needle, magnetic, 399

  Needle, declination of, 406

  Needle, dip of, 407

  Noise, how it differs from musical sound, 263

  Non-conductors of heat, 298

  Non-conductors of electricity, 375

  Notes, musical, how produced, 264


  Odors, minuteness of their particles, 19

  Order in nature, 65


  Papin's digester, 292

  Pendulum, 208-210

  Perpetual motion attempted, 118

  Persia, accident to, 274

  Pisa, tower of, 104

  Pitchers, why they have lips, 89

  Pneumatics, what it teaches, 146

  Pneumatic trough, 167

  Pop-guns, 164

  Pottery, centrifugal force used in, 216

  Powder, explosive force of, 165

  Prince Rupert's drops, 33

  Projectiles, 219, 220, 221

  Pulleys, 239

  Pumps, 176

  Pumps, forcing, 178


  Radiation of heat, 311

  Radiation, its relation to absorption, 313

  Rain, how it is caused, 290

  Rainbow, 365, 366

  Rarity, 24

  Reaction equal to action, 182

  Reflection of heat, 314

  Reflection of light, 340

  Reflection of motion, 206

  Reflection of sound, 260

  Remora, 157

  Rest, merely relative, 189

  Rivers, 115, 116

  Rivers, bends in, 215

  Roemer, observations of, in regard to the velocity of light, 339

  Rope-dancers, skill of, in managing centre of gravity, 109


  Safety-lamp, 299

  Scales, 98, 225

  Screw, 243

  Sea-breeze, 283

  See-saw, a lever, 228

  Sensation, influence of the conduction of heat on, 310

  Shape, relation of, to velocity, 195

  Shot, manufacture of, 60

  Signal key, 415

  Silk-worm, thread of, 19

  Size limited by gravity, 90

  Sluice-gates, 122

  Snow, how produced, 290

  Snow, crystals of, 64

  Snow a protection to plants, 309

  Solids, constitution of, 8

  Solids, attraction in, 39

  Solution, 12, 22

  Sound, what it is, 248

  Sound, sensation of, how produced, 252

  Sound, velocity of, 256

  Sound, transmission of, 253

  Sound, loudness of, 258

  Sound, diffusion of, 259

  Sound, reflection of, 260

  Sound, concentration of, 262

  Sound, none produced by the heavenly bodies, 255

  Speaking-trumpet, 262

  Specific gravity defined, 135

  Specific gravity of animals, 140

  Specific gravity of the human body, 141

  Specific gravity of solids, how ascertained, 142

  Specific gravity of liquids, how ascertained, 144

  Spectrum, 360

  Spider, web of, 19

  Spirit distinguished from matter, 1

  Spirit, origin of word, 4

  Spirit-level, 114

  Springs, 120

  Stability of bodies, 103-105

  Steam transparent and invisible, 293

  Steam, expansive force of, 165

  Steam-engine, 294

  Steel, flexible and brittle, 30

  Steel, tempering of, 31

  Steelyards, 98, 225

  Stereoscope, 358

  Suckers, 157

  Sucking, explanation of, 167

  Suction, 177

  Sulzer's experiment, 394

  Sun as a source of heat, 272

  Surface, relation of, to movability, 193

  Syphon, 172


  Tantalus, cup of, 174

  Telegraph, 415

  Telescopes, 350

  Tenacity, 25

  Tenacity, comparative, of substances, 26

  Tenacious substances, value of, 27

  Thermometer, 37, 276

  Thermometer, Fahrenheit's, 277

  Tides, 78

  Tubes, friction of liquids in, 197

  Twilight explained, 346


  Unison, 268

  Up and down, explanation of, 81


  Vapor, influence of pressure upon the formation of, 292

  Vaporization, 291

  Velocity, relation of, to force, 194

  Velocity, relation of, to shape, 195

  Velocities, great, how produced, 203

  Velocities, great, how arrested, 204

  Vibration of sounding bodies, 249

  Vision, distinct, what necessary to, 354

  Vision, why single, 357

  Vision, why erect, 356

  Visual angle, 348

  Voice, how produced, 265

  Voltaic electricity, 394

  Volta's pile, 395


  Walrus, feet of, 157

  Water, crystallization of, 63

  Water man's first mirror, 112

  Waves, how formed, 199

  Waves, height of, 200

  Wedge, 242

  Weight, 52, 82-85

  Wheel and Axle, 235

  Wheel-barrow a lever, 231

  Whispering galleries, 261

  Windlass, 236

  Windows, double, 303

  Winds, 283

  Winds as affected by the rotation of the earth, 284


THE END.




FOOTNOTES:

[1] I take the following from Dr. Arnot:

The reasons that so many people are drowned in ordinary cases, who
might easily he saved, are the following:

1. Their believing that continued exertion is necessary to keep the
body from sinking, and hence their generally assuming the position
of a swimmer, in which the face is downward, and the whole head
must be kept out of the water to allow of breathing. Now as a man
can not retain this position without continued exertion, he is soon
exhausted, even if a swimmer, and if not, the unskillful attempt
will scarcely secure for him even a few respirations. The body
raised for a moment by exertion above the natural level, sinks as
far below when the exertion ceases; and the plunge, by appearing
the commencement of a permanent sinking, terrifies the unpracticed
individual, and renders him an easier victim to his fate.

2. From a fear that water entering by the ears may drown as if
it entered by the nose or mouth, a wasteful exertion is made to
prevent it; the truth being, however, that it can only fill the
outer ear, or as far as the membrane of the drum, and is therefore
of no consequence.

3. Persons unaccustomed to the water, and in danger of being
drowned, generally attempt in their struggle to keep their hands
above the surface, from feeling as if their hands were tied while
held below; but this act is most hurtful, because any part of the
body kept out of the water, in addition to the face, which must
be so, requires an effort to support it which the individual is
supposed at the time incompetent to afford.

4. Not having reflected that when a log of wood or a human body
is floating upright, with only a small portion above the surface,
in rough water, as at sea, every wave in passing must cover the
head for a little time, but will again leave it projecting in the
interval. The practiced swimmer chooses this interval for breathing.

5. Not knowing the importance of keeping the chest as full of air
as possible; the doing which has nearly the same effect as tying
a bladder of air to the neck, and without other effort will cause
nearly the whole head to remain above the water. If the chest be
once emptied, and if from the face being under water the person can
not inhale again, the body is then specifically heavier than water,
and will sink.

[2] This is true except when the tube is so small that capillary
attraction exerts considerable influence.

[3] I was once consulted in regard to a smoking stove. It was an
open Franklin stove, the pipe of which went through a fire-board
into a monstrous chimney. I recommended that a pipe with a knee
should extend from the pipe of the stove a little way up the
chimney. The expedient was successful, because but a small body of
air, that in the pipe, needed to be heated to establish an upward
current.

[4] As in the case of many other inventions, so here the same
idea was originated and put to practical use by more minds than
one. George Stephenson, who from being a common engine-wright in
a colliery rose step by step till he invented the locomotive,
constructed a lamp which illustrated in another way the same
principle as the lamp of Davy does--in other words, he invented
another safety-lamp. But this does not in the least detract from
the glory which the invention has given to the name of Davy, for
each acted independently of the other. In Davy's case, it is to
be remarked, there was a long course of scientific reasoning and
investigation which led him at length to the invention, the record
of which is exceedingly interesting. No invention or discovery is
made without thought, though accident may suggest the thought; but
here is an invention which, without any suggestion by accident, was
evolved by laborious and long-continued thought, proceeding step by
step to its conclusion.

[5] I will mention here a contrivance that I once adopted for a
small conservatory, which I wished to keep warm from the heat of a
room to which it was adjoining. In each space of the window-frames
were put two panes of glass, there being nearly half an inch of
space between them. In this way I secured all the benefit of double
windows with less expense and a less cumbrous arrangement. In
mentioning this contrivance now and then, casually, I have found
that a few others have thought of it, and adopted it with the same
success that I did.

[6] We are in entire ignorance of the nature of electricity, and we
use the term _fluid_ merely as a matter of convenience.




  TRANSCRIBER'S NOTE

  Italic text is denoted by _underscores_.

  Bold text is denoted by =equal signs=.

  Obvious typographical errors and punctuation errors have been
  corrected after careful comparison with other occurrences within
  the text and consultation of external sources.

  Except for those changes noted below, all misspellings in the text,
  and inconsistent or archaic usage, have been retained. For example:
  farther, further; India rubber, India-rubber; every where;
  catechetical; incloses; tricksy; pervious; motal; enrobes; subtile.

  Pg 79, 'gravity made made him' replaced by 'gravity made him'.
  Pg 157, 'if non elastic' replaced by 'if non-elastic'.
  Pg 264, 'the divergency when' replaced by 'the divergence when'.
  Pg 297, '283. Electricity' replaced by '383. Electricity'.
  Pg 321, the Questions begin with number '13' and has not been changed.
  Pg 331, the number '174' is used twice and has not been changed.
  Pg 335, 'with phosporus' replaced by 'with phosphorus'.





End of the Project Gutenberg EBook of Science for the School and Family,
Part I. Natural Philosophy, by Worthington Hooker

*** 