Was the concept of messages transmitted with radiowaves conceivable in the 18th century?
No, the concept of transmitting messages using radio waves was not conceivable in the 18th century as the discovery of radio waves itself did not occur until the late 19th century. The understanding and harnessing of radio waves as a means of communication came much later in the late 19th and early 20th centuries.
I don't agree with that at all. Of course it was conceivable, as there was no fundamental difference in man between those two centuries. They simply lacked the knowledge, but they already had the process for acquiring that knowledge.