Everyone heard about the telegraph, that old-timey way to transmit messages over long distances. But did you know that before the famous, electric kind of telegraph existed, there were *optical* telegraphs?

If you've read the Count of Monte-Cristo, you may have been confused by a part of the story where telegraph communication is (supposedly) disrupted by fog.

Fog of course can't disrupt an electric telegraph line. But it sure can screw up the operations of optical telegraphs. Let me explain how.

In the pre-industrial world, large empires often struggled with getting accurate and up-to-date information on their frontier territories. This was particularly vital when they were at war, i.e. pretty much all the time.

They set up elaborate post systems, with posthouses with fresh horses so that messengers could bring urgent news as fast as possible. But it could still take days for a message to travel from Bordeaux to Paris at that speed.

There's one thing that's considerably faster than a horse: light.

The Byzantines had an impressive beacon system, with a line of relay stations stretching from the Arab border all the way to Constantinople. News of an invasion could cross the 720 km in just 1h.

The information content of the messages was crude. They couldn't actually use their 720 km/h transmission speed because the meaning of the message depended on the time of day at which it was sent!

But it worked well enough for the Byzantines' purposes.

The fast transmission of more complicated messages would need the invention of telescopes. Aided with a telescope, the human eye can see details far away, and you can use a more sophisticated signaling system than a big dumb fire.

The idea of an optical telegraph system was already floating around in the 17th century, proposed notably by the Brit Robert Hooke. But it was Revolutionary France that first put the idea into practice in the mid 1790s.

The signaling system invented by the Chappe brothers looked like this. The positions of the two wooden arms and the angles between could be changed to make one of 92 symbols. 2 symbols were combined to form a word or phrase (translated by a code book).

They demonstrated the system's effectiveness in and nearby Paris in 1791, with one of the stations being near today's Télégraphe metro station.

The French quickly built out telegraph lines, linking Paris to Lille in 1794, then to Strasbourg in 1798. In 1794, Paris actually had news of a successful battle at the Belgian border a mere hour after it ended!

The optical telegraph is an underrated factor in how successful the French were in their revolutionary wars. Their lines of communication were just much better than those of the opposing powers.

The system kept being expanded in France during the early 19th century. The Swedes loved the idea and built the second most extensive network. But they actually used a more efficient and thus faster signalling system.

The French loved the optical telegraph so much that they were actually late in adopting Morse's electrical telegraph. They eventually relented, won over by the fact that the electric telegraph could work at night... and during foggy days.

The last optical telegraph messages in France were sent in the 1850s, during the Crimean war.

/end

PS: check out these two awesome wiki pages, from which I took most of the information in this thread.

https://en.wikipedia.org/wiki/Optical_telegraph

https://en.wikipedia.org/wiki/Byzantine_beacon_system