J Ptak Science Books Post 2160
This curious illustration appears in forty-five volume Cyclopedia of Abraham Rees (published 1795-1820), displaying a system for communicating over distances at night. When this part of the Cyclopedia was printed in 1808, the electrical telegraph was still 37 years away from coming into being--45 years from being somewhat well-used. Before this time (visual) communications over long distances at night were limited to just these sorts of means--lighted semaphores, hand-held torches, that sort of thing.
The system outlined in the (first) illustration above shows how a semaphore was articulated to produce telegraphic signals at night, fashioned with arms that had changeable holes in the arms, allowing light through to specify letters. As cumbersome and time-consuming as this might seem, it was about the only way to communicate remotely across distances (and at night)--so to transmit messages over miles there would be a series of installation s such as these on hilltops, transferring the message from one ot the other, until the destination was research. This idea did not look so antiquated until the electric telegraph took over, making it seem as though this fire-and-wood technology was 500 years old. It was that, and older still--but it is the product of revolutionary development that the great discovery can sometimes bring upon instant antiquarianism on whatever it was that was being replaced.
Signaling at sea at night was somewhat different at this time and didn't include anything remotely close to the alphabet. So the rather complex system that we see here (above) is extremely uncommon--it seems also very unwieldy to put into effect. Unfortunately I don't have the text volume that would explain then entire system and implementation, so I'm going to guess that there was a large, powerful light source that was covered by a tight, black, covering tablet that would eliminate nearly all light leakage. The symbols for each letter of the alphabet (and numerals) would be cut out from another tablet that would fit over the face of the light source, placed between the blank and the light. To transmit a letter the user would then simply remove the blank covering tablet to reveal the light broadcast by the hole or slit in the tablet underneath. The blank would then be placed back, a new tablet for a new letter placed underneath, and the process would begin again: blank (dark); letter (light); blank (dark); letter (light), and so on to the end of the message. I guess the distance at which these symbols could be seen would be dependent on light source, atmospheric conditions, ad so on. The way that the letters are made into symbols seems to me very intelligent, so that you distinguish the differences from an appreciable distance. I like it--its an elegant idea. (Well, maybe it didn't work in this manner, but it seems to make sense to me.)
The image of the full sheet: