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Optical telegraph

The optical telegraph preceded the electrical telegraph. It was faster than riding couriers for bringing a message over long distances. Types of optical telegraph are the semaphore, ship flags, smoke signals, and beacons. The distance that an optical telegraph can bridge is limited by geography, the shape of the earth, and the sharpness of the human eye. In practical use, most optical telegraphs used relay leagues to bridge longer distances.

There was in France at the end of the 18th century a complete and working installation of optical telegraph. During the Revolutionary period, a French inventor, Claude Chappe (1763-1805), convinced the Deputies to set up a huge network between major cities. It was used for army and national communications till the 1850s.

A chain of semaphores was built in England during the Napoleonic Wars in the 19th century to warn London of a French invasion. The chain ran from the Admiralty in Whitehall to Portsmouth in the English Channel. As a result there are several locations still called Telegraph Hill. There is a restored example on a hill at Chatley Heath in Surrey.

Throughout Chinese history, smoke signals were used as communication tool during war time. Until these days, the Chinese phrase 烽火 (feng1 huo3, fire signals) is still used as a synonym for war.