optical telegraph

Strictly, any device used to convey information over long distances by semaphore (The word 'telegraph' roughly translates as 'reading at a distance' from the Greek). The ancient Greeks, Romans and Carthaginians used a simple form of optical telegraphy such as fire beacons atop mountains to warn of impending invasions.

In practice, the phrase usually refers to those machines which stemmed from one invented by Claude Chappe late in the eighteenth century¹ and later superceded by the electrical telegraph. In the 1790's, post-revolutionary France was at war with several of its neighbours. It needed to communicate speedily with its generals.

Monsieur Chappe devised a system which consisted of a central sturdy pole atop which rested a horizontal beam which could be inclined about its pivot (this beam was known as a Regulator). Short arms at the end of the Regulator could also be rotated to eight positions (seperated by 45^o). A system of ropes and pulleys was used to operate the device by the station-master housed below.

92 combinations were used and a codebook was compiled by Chappe with 92 pages and 92 lines on each page (8462 lines in all); each representing a letter, number, word or phrase.

The first optical telegraph line connected Paris and Lille, a distance of 120 miles covered by 15 stations. By 1810 cities as far afield as Venice, Turin and Amsterdam were connected. A message could be sent from Paris to Toulon in 20 minutes, a distance of 500 miles.

The optical telegraph was principally used for military purposes. It was an expensive system to maintain. Stations had to be built every eight miles or so and station-masters employed in each. Later it was used to transmit the results of, and financed by the proceeds of, the National Lottery. By 1844 there were 500 stations in France connecting 29 cities and covering 3000 miles.

The second country to adopt the optical telegraph was Sweden. The scientist Abraham Edelcrantz developed his own system and demonstrated it to the King by transmitting a message from the Royal palace to the Royal residence about seven and a half miles distant. The device was soon linking castles and fortresses throughout Sweden. Gothenburg, Marstrad and Helsingborg were connected to Stockholm. The eastern border was also connected when Sweden went to war with Russia in 1808.

The optical telegraph devised by Edelcrantz used a series of ten shutters. It used a binary code system (shutter opened/shutter closed) and could assume 1024 configurations. This was similar to the telegraph invented by Lord George Murray (later Bishop of St. Davids) in Britain. He used six rectangular shutters on a vertical framework. Each shutter when rotated about the horizontal axis was essentially invisible to the receiver when perpendicular to the frame.

In Britain, London was connected first to Deal in 1796 and later to Portsmouth, Plymouth and Yarmouth. It was used by the Admiralty to co-ordinate the activities at those naval bases. A message sent from London to Portsmouth could be answered in fifteen minutes.

The only optical telegraph of the period developed for commercial rather than military use connected Liverpool to Holyhead in 1827. It was used to signal the departure of ships from port.

Norway and Russia also built optical telegraph communication systems. The Norweigan system employed four flags (2 dark/2 light) and night-time flares. It was used to send signals along the coast from Oslo. Persistent bad weather conditions forced the Norweigans to use a simple system. In 1812, a message sent by optical telegraph summoned a flotilla of the Norweigan navy to join in the Battle of Lyngor with the English. They arrived in time to save the day.

Russia began by linking St.Petersburg to Lake Ladoga and later to Moscow. This line included 149 towers employing 1,908 staff. The position of a wooden boards at right angles to a central pole was the method of communication. To initiate signalling, the transmitting station would hoist a cloth ball to the top of their telegraph pole. They would wait until the receiving station had hoisted their ball before commencing. In poor visibility, a boat or horse would be sent to the next station carrying the message.

In Tasmania the optical telegraph was used for the purpose of tracking escaped convicts from the penal settlements there. This system was in use until 1846.

The death of the optical telegraph came with the Morse electric telegraph. This new-fangled telegraph had greater bandwidth, was cheaper, faster and more efficient. Scientists knew something of electricity all during the era of the optical telescope but had been unable to harness it for the purposes of communication. With that obstacle gone, the optical telegraph quickly disappeared.

¹Edward Hooke in 1684 proposed a method of optical telegraphy to the Royal Society but his idea was not developed.

References
www.telemuseum.se/historia/optel
www.museumnett.no/risormuseum/telgraf