The tower
A mast about 4 m high (CNAM) carries the regulator, a jointed beam about 4 to 4.60 m long; at each end pivots an indicator 1 to 2 m long depending on the source, with its counterweight. In the room below, the manipulator, a small model with cranks, drives it all through cables and pulleys and repeats the figure up top. Two spyglasses point at the neighbouring towers.
The code
- Indicators with 7 positions date from the 1798 equipment: 45° apart, folded included; the eighth, in line with the regulator, isn't used. For messages, the regulator is horizontal or vertical. The first codes of 1794 were different.
- 7 × 7 × 2 = 98 figures, minus 6 kept for the service: 92 signals for messages. Other texts count 4 positions of the regulator, that's 196 combinations: it all depends on how you count.
- The vocabulary in use from 1799 has 92 pages of 92 lines, that's 8,464 words or phrases; each word goes as two signals, the page then the line. On the Lille line, only the directors at the two ends had the code book.
The network
A successful trial on 12 July 1793 over some thirty kilometres (Ménilmontant, Écouen, Saint-Martin-du-Tertre); the Paris–Lille line, at first about fifteen towers every 10 to 15 km over 230 km, is in service in 1794 (May to July depending on the source); first official dispatch on 15 August. The lines to Strasbourg and Brest follow in 1798–1799. All start from Paris, in a star. In 1844, 534 stations link 29 towns over more than 5,000 km.
The people and the limits
Two operators per tower at first, often former soldiers or local people, taking turns at noon; an inspector per division of a dozen towers; the directors code and decode. Nothing gets through at night or in fog, and it takes a lot of people: service signals say « fog » or « absent ».
In the 1830s, the Blanc brothers affair shows the weak spot: a bribed employee at the Tours station slips deliberate errors, followed by a cancel signal, into official messages, to let Bordeaux know whether the government bonds were going up or down. The culprits are acquitted for want of a law; the State has one passed.
And after?
The electric telegraph replaces it in the middle of the 19th century: the last aerial lines close between 1852 and 1855, depending on the source. The word stays: the Télégraphe metro station in Paris, and a La Poste stamp in 1993. The Musée des arts et métiers, in Paris, shows a model of a Chappe telegraph (inv. 14583).
To visit Some restored towers can be visited, such as the one at Haut-Barr near Saverne (its mechanism in working order), Saint-Marcan's or Marcy-sur-Anse's. Check the opening times before you go.
The game's booklet The exercise booklet and its rule (the number on both indicators) are made up for the game: they don't reproduce Chappe's real vocabulary. So is the game's « fog » signal: the real one used the regulator set at a slant.