Optical telegraph
An optical telegraph is a line of stations, typically towers, for the purpose of conveying textual information by means of visual signals. There are two main types of such systems: the semaphore telegraph which uses pivoted indicator arms and conveys information according to the direction the indicators point, and the shutter telegraph which uses panels that can be rotated to block or pass the light from the sky behind to convey information.
The most widely used system was the Chappe telegraph, which was invented in France in 1792 by Claude Chappe. It was popular in the late eighteenth to early nineteenth centuries. Chappe used the term télégraphe to describe the mechanism he had invented – that is the origin of the English word "telegraph". Lines of relay towers with a semaphore rig at the top were built within line of sight of each other, at separations of. Operators at each tower would watch the neighboring tower through a telescope, and when the semaphore arms began to move spelling out a message, they would pass the message on to the next tower.
This system was much faster than post riders for conveying a message over long distances, and also had cheaper long-term operating costs, once constructed. Half a century later, semaphore lines were replaced by the electrical telegraph, which was cheaper, faster, and more private. The line-of-sight distance between relay stations was limited by geography and weather, and prevented the optical telegraph from crossing wide expanses of water, unless a convenient island could be used for a relay station. A modern derivative of the semaphore system is flag semaphore, signalling with hand-held flags.
Etymology and terminology
The word semaphore was coined in 1801 by the French inventor of the semaphore line itself, Claude Chappe. He composed it from the Greek elements σῆμα ; and from φορός, or φορά from φέρειν. Chappe also coined the word tachygraph, meaning "fast writer". However, the French Army preferred to call Chappe's semaphore system the telegraph, meaning "far writer", which was coined by French statesman André François Miot de Mélito.The word semaphoric was first printed in English in 1808: "The newly constructed Semaphoric telegraphs have been blown up", in a news report in the Naval Chronicle. The first use of the word semaphore in reference to English use was in 1816: "The improved Semaphore has been erected on the top of the Admiralty", referring to the installation of a simpler telegraph invented by Sir Home Popham. Semaphore telegraphs are also called, "Chappe telegraphs" or "Napoleonic semaphore".
Early designs
Optical telegraphy dates from ancient times, in the form of hydraulic telegraphs, torches and smoke signals. Modern designs of semaphores developed via several paths, often simultaneously.Possibly the earliest was by the British polymath Robert Hooke, who gave a vivid and comprehensive outline of visual telegraphy to the Royal Society in a 1684 submission in which he outlined many practical details. The system was never put into practice.
One of the first experiments of optical signalling was carried out by the Anglo-Irish landowner and inventor, Sir Richard Lovell Edgeworth in 1767. He placed a bet with his friend, the horse racing gambler Lord March, that he could transmit knowledge of the outcome of the race in just one hour. Using a network of signalling sections erected on high ground, the signal would be observed from one station to the next by means of a telescope. The signal itself consisted of a large pointer that could be placed into eight possible positions in 45 degree increments. A series of two such signals gave a total 64 code elements and a third signal took it up to 512. He returned to his idea in 1795, after hearing of Chappe's system.
While Edgeworth was developing his design, William Playfair, a Scottish political economist traveling in Europe in 1794, surreptitiously obtained the design and alphabet of the French system from a fleeing royalist. Playfair, who had numerous connections to British officials, provided a model of the system to the Duke of York, commander of British forces, then based in Flanders, and, according to the Encyclopædia Britannica, "hence the alphabet and plan of the machine came to England."
Prevalence
France
Credit for the first successful optical telegraph goes to the French engineer Claude Chappe and his brothers in 1792, who succeeded in covering France with a network of 556 stations stretching a total distance of. Le système Chappe was used for military and national communications until the 1850s.Development in France
During 1790–1795, at the height of the French Revolution, France needed a swift and reliable military communications system to thwart the war efforts of its enemies. France was surrounded by the forces of Britain, the Netherlands, Prussia, Austria, and Spain, the cities of Marseille and Lyon were in revolt, and the British Fleet held Toulon. The only advantage France held was the lack of cooperation between the allied forces due to their inadequate lines of communication.In mid-1790, the Chappe brothers set about devising a system of communication that would allow the central government to receive intelligence and to transmit orders in the shortest possible time. Chappe considered many possible methods including audio and smoke. He even considered using electricity, but could not find insulation for the conductors that would withstand the high-voltage electrostatic sources available at the time.
Chappe settled on an optical system and the first public demonstration occurred on 2 March 1791 between Brûlon and Parcé, a distance of. The system consisted of a modified pendulum clock at each end with dials marked with ten numerals. The hands of the clocks almost certainly moved much faster than a normal clock. The hands of both clocks were set in motion at the same time with a synchronisation signal. Further signals indicated the time at which the dial should be read. The numbers sent were then looked up in a codebook. In their preliminary experiments over a shorter distance, the Chappes had banged a pan for synchronisation. In the demonstration, they used black and white panels observed with a telescope. The message to be sent was chosen by town officials at Brûlon and sent by René Chappe to Claude Chappe at Parcé who had no pre-knowledge of the message. The message read "si vous réussissez, vous serez bientôt couverts de gloire". It was only later that Chappe realised that he could dispense with the clocks and the synchronisation system itself could be used to pass messages.
The Chappes carried out experiments during the next two years, and on two occasions their apparatus at Place de l'Étoile, Paris was destroyed by mobs who thought they were communicating with royalist forces. Their cause was assisted by Ignace Chappe being elected to the Legislative Assembly. In the summer of 1792 Claude was appointed Ingénieur-Télégraphiste and charged with establishing a line of stations between Paris and Lille, a distance of. It was used to carry dispatches for the war between France and Austria. In 1794, it brought news of a French capture of Condé-sur-l'Escaut from the Austrians less than an hour after it occurred. The first symbol of a message to Lille would pass through 15 stations in only nine minutes. The speed of the line varied with the weather, but the line to Lille typically transferred 36 symbols, a complete message, in about 32 minutes. Another line of 50 stations was completed in 1798, covering between Paris and Strasbourg. From 1803 on, the French also used the 3-arm Depillon semaphore at coastal locations to provide warning of British incursions.
English military engineer William Congreve observed that at the Battle of Vervik of 1793 French commanders directed their forces by using the sails of a prominent local windmill as an improvised signal station. Two of the four sails of the mill had been removed to resemble the arm of the new telegraph.
Chappe system technical operation
The Chappe brothers determined by experiment that it was easier to see the angle of a rod than to see the presence or absence of a panel. Their semaphore was composed of two black movable wooden arms, connected by a cross bar; the positions of all three of these components together indicated an alphabetic letter. With counterweights on the arms, the Chappe system was controlled by only two handles and was mechanically simple and reasonably robust. Each of the two 2-metre-long arms could display seven positions, and the 4.6-metre-long cross bar connecting the two arms could display four different angles, for a total of 196 symbols. Night operation with lamps on the arms was unsuccessful. To speed up transmission and to provide some semblance of security, a code book was developed for use with semaphore lines. The Chappes' corporation used a code that took 92 of the basic symbols two at a time to yield 8,464 coded words and phrases.The revised Chappe system of 1795 provided not only a set of codes but also an operational protocol intended to maximize line throughput. Symbols were transmitted in cycles of "2 steps and 3 movements."
- Step 1, movement 1 : The operator turned the indicator arms to align with the cross bar, forming a non-symbol, and then turned the cross bar into position for the next symbol.
- Step 1, movement 2 : The operator positioned the indicator arms for current symbol and waited for the downline station to copy it.
- Step 2, movement 3 : The operator turned the cross bar to a vertical or horizontal position, indicating the end of a cycle.