I recently got around to reading Lest Darkness Fall, the influential alternative history science fiction novel that inspired Harry Turtledove to switch his studies to Byzantine history and to become an alternative history science fiction novelist. (Perhaps I should read some Turtledove…)
The story is in the style of Mark Twain’s A Connecticut Yankee in King Arthur’s Court — which I finally read last year — only the protagonist is an American archaeologist transported back from Italy in 1939 to Italy after the fall of Rome, while it was under barbarian rule, and the east was under Justinian’s rule. It shares the same progressive tone as Twain’s story.
Anyway, one of the first important inventions our protagonist invents — after brandy, which bankrolls everything else he tries — is a semaphore telegraph that relies on telescopes in towers.
As Kris De Decker of Low-Tech Magazine points out, such a system existed in the 18th century:
Centuries of slow long-distance communications came to an end with the arrival of the telegraph. Most history books start this chapter with the appearance of the electrical telegraph, midway the nineteenth century. However, they skip an important intermediate step. Fifty years earlier (in 1791) the Frenchman Claude Chappe developed the optical telegraph. Thanks to this technology, messages could be transferred very quickly over long distances, without the need for postmen, horses, wires or electricity.
The optical telegraph network consisted of a chain of towers, each placed 5 to 20 kilometres apart from each other. On each of these towers a wooden semaphore and two telescopes were mounted (the telescope was invented in 1600). The semaphore had two signalling arms which each could be placed in seven positions. The wooden post itself could also be turned in 4 positions, so that 196 different positions were possible. Every one of these arrangements corresponded with a code for a letter, a number, a word or (a part of) a sentence.
Every tower had a telegrapher, looking through the telescope at the previous tower in the chain. If the semaphore on that tower was put into a certain position, the telegrapher copied that symbol on his own tower. Next he used the telescope to look at the succeeding tower in the chain, to control if the next telegrapher had copied the symbol correctly. In this way, messages were signed through symbol by symbol from tower to tower. The semaphore was operated by two levers. A telegrapher could reach a speed of 1 to 3 symbols per minute.
The technology today may sound a bit absurd, but in those times the optical telegraph was a genuine revolution. In a few decades, continental networks were built both in Europe and the United States. The first line was built between Paris and Lille during the French revolution, close to the frontline. It was 230 kilometres long and consisted of 15 semaphores. The very first message — a military victory over the Austrians — was transmitted in less than half an hour. The transmission of 1 symbol from Paris to Lille could happen in ten minutes, which comes down to a speed of 1,380 kilometres an hour. Faster than a modern passenger plane – this was invented only one and a half centuries later.
The technology expanded very fast. In less than 50 years time the French built a national infrastructure with more than 530 towers and a total length of almost 5,000 kilometres. Paris was connected to Strasbourg, Amsterdam, Toulon, Perpignan, Lyon, Turin, Milan and Venice. At the beginning of the 19th century, it was possible to wirelessly transmit a short message from Amsterdam to Venice in one hour’s time. A few years before, a messenger on a horse would have needed at least a month’s time to do the same.
This 
