Optical telegraph

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A Chappe semaphore tower near Saverne, France

The semaphore or optical telegraph is an apparatus for conveying information by means of visual signals, with towers with pivoting blades or paddles, shutters, in a matrix, or hand-held flags etc. Information is encoded by the position of the mechanical elements; it is read when the blade or flag is in a fixed position. In modern usage it refers to a system of signaling using two handheld flags. Other forms of optical telegraphy include ship flags, Aldis lamps, and Heliographs

Semaphore lines preceded the electrical telegraph. They were faster than post riders for bringing a message over long distances, but far more expensive and less private than the electrical telegraph lines which would replace them. The distance that an optical telegraph can bridge is limited by geography and weather, thus in practical use, most optical telegraphs used lines of relay stations to bridge longer distances.

History

Although passing mention of this idea had been made at many points in history, it was apparently the English scientist Robert Hooke who first gave a vivid and comprehensive outline of visual telegraphy to the Royal Society in a submission dated 1684; in it he outlined many practical details, but his system was never put into practice. Over a hundred years later a French engineer, Claude Chappe and his brothers took up the challenge again and succeeded to cover France with a network of 556 stations stretching a total distance of 4,800 kilometres. It was used for military and national communications until the 1850s.

France

There was a desperate need for swift and reliable communications in France during the period of 1790-1795. It was the height of the French revolution, and France was surrounded by the allied forces of England, Holland, Prussia, Austria, and Spain. The cities of Marseilles and Lyons were in revolt, and the English Fleet held Toulon. In this situation the only advantage France held was the lack of cooperation between the allied forces due to their inadequate lines of communications.

The Chappe brothers in the summer of 1790 set about to devise a system of communication that would allow the central government to receive intelligence and to transmit orders in the shortest possible time. 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. However in the summer of 1792 Claude was appointed Ingénier-Télégraphiste and charged with establishing a line of stations between Paris and Lille, a distance of 230 kilometres. 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.

File:Claude Chappe.jpg
Claude Chappe
File:Chapperégulateur.gif

Paris to Strasbourg with 50 stations was the next line and others followed soon after. By 1824, the Chappe brothers were promoting the semaphore lines for commercial use, especially to transmit the costs of commodities. Napoleon Bonaparte saw the military advantage in being able to transmit information between locations, and carried a portable semaphore with his headquarters. This allowed him to coordinate forces and logistics over longer distances than any other army of his time. However because stations had to be within sight of each other, and because the efficient operation of the network required well trained and disciplined operators, the costs of administration and wages were a continuous source of financial difficulties. Only when the system was funded by the proceeds of its own lottery did costs come under control.

Relative Costs

The semaphore system was cleverly designed, and provided a strategic advantage for France in a difficult time. However, it was almost 30 times more expensive per message than the electric telegraph. Here's a brief breakdown using $US:

Semaphore line, 120 miles (Paris to Lille)

  • 15 towers ($1,500,000)
  • At least 15 full-time operators ($450,000/year).
  • Operates at most ten hours a day.
  • Sends roughly 2 words per minute (1 symbol per minute, at 2 symbols per phrase, using the efficient directors' codebook).
  • Cost to send one word one mile, at 10% interest: $0.0114

Electric Telegraph line, 120 miles

  • At least six full-time operators ($180,000/year)
  • Poles, right-of-way, wires, installation: $15,000/mile, ($1,800,000)
  • Operates 24 hours a day.
  • Sends 15 words per minute (includes breaks for the operators).
  • Cost to send one word one mile, at 10% interest: $0.0003809

Description

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 black movable wooden arms, the position of which indicated alphabetic letters. The Chappe system was controlled by only two handles and was mechanically simple and reasonably rugged. Each of the two arms showed seven positions, and the cross bar connecting the two arms had four different angles, for a total of 196 symbols (7x7x4). Night operation with lamps on the arms was unsuccessful.

To speed 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.

Sweden

At the same time as Chappe, the Swede Abraham Niclas Edelcrantz experimented with the optical telegraph in Sweden. In 1794 he inaugurated his telegraph with a poem dedicated to the Swedish King on his birthday. The message went from the Palace in Stockholm to the King at Drottningholm.

Edelcrantz eventually developed his own system which was quite different from its French counterpart and nearly twice as fast. The system was based on ten collapsible iron shutters. The various positions of the shutters formed combinations of numbers which were translated into letters, words or phrases via codebooks. The telegraph network consisted of telegraph stations positioned at about 10 kilometres from one another.

Soon telegraph circuits linking castles and fortresses in the neighbourhood of Stockholm were set up and the system was extended to Grisslehamn and Åland. Subsequently telegraph circuits were introduced between Gothenburg and Marstrand, at Helsingborg and between Karlskrona and its fortresses. Sweden was the second country in the world, after France, to introduce an optical telegraph network. The Swedish optical telegraph network was restricted to the archipelagoes of Stockholm, Gothenburg and Karlskrona. Like its French counterpart, it was mainly used for military purposes.

Other countries

The system was widely copied by other European states, especially after it was used by Napoleon to coordinate his empire and army. In most states, the postal authorities ran the semaphores lines.

Bishop of Saint David's Lord George Murray, stimulated by reports of the Chappe semaphore, proposed a system of visual telegraphy to the British Admiralty. He employed a large wooden boards on his towers with six large holes which could be closed by shutters. A chain of 15 of these stations was built between London and Deal in 1795 and others followed to Portsmouth, Yarmouth, and Plymouth. Many of the prominences on which the towers were built are known as 'Telegraph Hill' to this day. One restored tower sits on a hill at Chatley Heath in Surrey. As in France the network required lavish amounts of money and manpower to operate and could only be justified as a defence need.

A Chappe telegraph tower in action
A Chappe telegraph tower in action

Once it had proved its success the optical telegraph was imitated in many other countries. Germany began with a line 750 kilometres long between Berlin and Coblenz in 1833, and in Russia, Tsar Nicolas I inaugurated the line between Moscow and Warsaw in 1833; this needed 220 stations maned by 1320 operators. In the United States the first optical telegraph was built by Jonathan Grout. It was a 104 kilometre line connecting Martha's Vineyard with Boston, and its purpose was transmit news about shipping.

Many national services adopted signaling systems different from the Chappe system. For example, Britain and Sweden adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible). In Spain, the engineer Agustín de Betancourt developed his own system which was adopted by that state. This system was considered by many experts in Europe better than Chappe's, even in France.

The semaphores were successful enough that Samuel Morse failed to sell the electrical telegraph to the French government. However, France finally committed to replace semaphores with electric telegraphs in 1846. Note that electric telegraphs are both more private and unaffected by weather. Many contemporaries predicted the failure of electric telegraphs because "they are so easy to cut." The last stationary semaphore link in regular service was in Sweden, connecting an island with a mainland telegraph line. It went out of service in 1880.

Flag semaphore system

Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms) in the maritime world in the early 1800s. Semaphore signals were used, for example, at the Battle of Trafalgar. This was the period in which the modern naval semaphore system was invented. This system uses hand-held flags. It is still accepted for emergency communication in daylight or, using lighted wands instead of flags, at night.

Wig-wag flags

In the 1850s, U.S. Army Major Albert J. Myer, a surgeon by training, developed a system using left or right movements of a flag (or torch or lantern at night), similar to the Morse code of dots and dashes. This is sometimes called the wig-wag method of signaling. More mobile than previous means of optical telegraphy, this code was used extensively by Signal Corps troops on both sides in the American Civil War. (Its first use in battle was by Confederate Lieutenant Edward Porter Alexander at the First Battle of Bull Run in 1861.)

In this code, alphabet letters were equated with three positions of the flag, disk, or light. The flags measured two, four, or six feet square and were generally either red or black banners with white square centers or white banners with red square centers. The disks were 12 to 18 inches in diameter and were made of metal or wood frames with canvas surfaces. Somewhat easier to handle than the flags, they provided a different method for daylight communications. The lights were kerosene lanterns attached to a staff. A second "foot torch" was placed on the ground before the signalman as a fixed point of reference, making it easier for the recipient to follow the lantern's movements.

Each letter consisted of a combination of three basic motions. All began with the flagman holding his device vertically and motionless above his head. The first motion was initiated by bringing the device downward on the signalman's right side and then quickly returning it to its upright position. Motion number 2 involved bringing the device down on the left side and then returning it to the starting position. The third motion required lowering the device in front of the signalman, then restoring it to its vertical position. A flash demo can be found here

Modern semaphore

The newer flag semaphore system uses two short poles with square flags, which a flagperson holds in different positions to signal letters of the alphabet and numbers. The flagperson holds one pole in each hand, and extends each arm in one of seven possible directions. Except for in the rest position, the flags cannot overlap. The flags are coloured differently based on whether the signals are sent by sea or by land. At sea, the flags are coloured red and yellow, while on land, they are red and white.

Characters


Rest position
Numerals
Error
Cancel

A / 1
B / 2
C / 3 / Ack
D / 4
E / 5
F / 6

G / 7
H / 8
I / 9
J / Letters
K / 0

L
M
N
O
P

Q
R
S
T
U

V
W
X
Y
Z

References in fiction

A dramatic episode in A Ship of the Line (one of C. S. Forester's Horatio Hornblower books) involves the destruction of a Napoleonic semaphore station on the coast of France.

One of Dudley Pope's Lord Ramage books, Ramage's Signal, has Ramage's crew seize a Napoleonic semaphore station to send a signal directing a French convoy into a trap. The semaphore, however, is depicted as using flapping panels or shutters rather than the arms of the Chappe system.

The Clacks system in Terry Pratchett's Discworld universe is very similar to the Chappe semaphore, and is probably based upon it. In the books, the Clacks system takes the place of the real world Internet and telephone network.

In Alexandre Dumas' The Count of Monte Cristo the hero uses France's optical telegraph system to trick one of his adversaries into going bankrupt.

An episode of Monty Python's Flying Circus depicted a supposed dramatic production of Wuthering Heights in flag semaphore.

In Jack Vance's SF novel The Blue World, islands in the ocean communicate with "wink machines", which display binary arrays of panels, possibly derived from the system Chappe decided was less effective.

Keith Roberts's Pavane describes an extensive network of semaphores in Britain; the second "measure" of the book is the story of the training and experience of a "signaller".

Ernest Hemingway's "The Sun Also Rises" references a statue in Paris where "the inventor of the semaphore is engaged in doing same" near the Boulevard Raspail.

In the book Winter Holiday by Arthur Ransome, Nancy sends secret messages to the other children by means of a picture in which the people's arm positions represent semaphore letters.

See also

External links

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