Optical Telegraphs: an early Internet.

Updated: 28 Dec 2001
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From London to Portsmouth is about 60 miles. The year is 1796. What was the shortest time in which you could send a message and get a reply between the two towns?

The answer is fifteen minutes. Not three or four days, with relays of mounted messengers, but fifteen minutes. It was done with the shutter telegraph apparatus shown below:

Above: part of a contemporary drawing of the Admiralty Shutter Telegraph, and the codes for a few of the letters. Note the telescope pointing out of the window of the "officer's cabin", to observe the next station. Unfortunately the artist's grasp of perspective seems to have been a bit feeble.

The first practical telegraph system was inaugurated in France by Chappe in 1794; this was a semaphore or moving-arm type. The stimulus for this was command and control of the French armed forces in the Revolutionary wars. The idea was quickly adopted in Britain, (which was at war with France for almost all this period) where there were clear advantages in rapid communication with the coastal ports where the British Navy was based. After tests a shutter type was adopted, rather than a semaphore, and by the end of 1796 two telegraph lines were in operation.

A shutter telegraph station had six pivoted boards, which could be swivelled by the ropes leading down to the cabin, so they were either visible or edge-on. Six shutters gives a 6-bit binary code, allowing 63 non-zero states to be transmitted. These were allocated as the 26 letters of the alphabet, ten numerals, and some useful preset sentences, such as "Defeat the French Navy immediately".

The average London-Portsmouth message took about fifteen minutes to get there. Data-compression was used in the form of omitting the vowels in common words. The preparatory signal could be sent from London to Deal or Portsmouth, and be acknowledged in two minutes; an early version of the "ping". It is said that a similar ping from London to Plymouth and back- a total distance of ?00 miles- took only three minutes. This really is rather impressive.

The Deal and Portsmouth Lines were completed in 1796; a trial Portsmouth-Plymouth "ping" took 20 minutes.

In 1816 the Shutter telegraph was replaced by a Chappe or semaphore type, trials having convinced the authorities that this system gave better visibility.

Above: a map of the routes of the Admiralty Shutter Telegraph. The semaphore system that replaced it took slightly different routes.

Above: a recent (1950-ish) drawing of the interior of a telegraph station. Two stout fellows haul on the ropes to transmit codes, while the chap to the right receives messages from the next station. This has been drawn as much too close; the actual average distance between stations was about 10 miles. No span exceeded 14 miles.

An optical telegraph such as this is obviously vulnerable to fog and other meteorological difficulties. The builders of the Lines were perfectly well aware of this, and went to considerable lengths to build stations that were as high as possible and clear from local fog conditions. The telegraph was able to work throughout the hours of daylight on at least 200 days per year.

Aug 1794
Semaphore telegraph inaugurated in France by Claude Chappe
Aug 1795
First trials in England
Sept 1795
Surveyor appointed to lay out Lines
Jan 1796
London-Deal Line completed
??? 1796
London-Portsmouth Line completed
May 1806
Plymouth extension completed
June 1808
London-Yarmouth Line completed
May 1814
Shutter Telegraph dismantled
May 1816
Construction of Admiralty Semaphore Telegraph begins
Feb 1845
Electric Telegraph installed London-Portsmouth
Dec 1847
Admiralty Semaphore Telegraph closes
Mar 1849
Admiralty Electric Telegraph completed

The timescale above shows that the new technology was adopted, and a successful system constructed, with quite impressive speed. Never underestimate your ancestors.

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