Eating codes for dinner – SIGINT in the age of Semaphore

By Roger Davies / 20 October 2014 / History, Insights, SIGINT

Today, we tend to think that “SIGINT” is something modern and related to interception of radio transmissions.  But SIGINT goes much further back and I’ve mentioned before the “letter interception” stations that Britain had established in the 18th century both in London and in Europe.

But between about 1794 and about 1846 there was extensive use of semaphore-like signalling that occurred on a national level and it is fascinating to look at the SIGINT techniques and opportunities that occurred.

The prime driver of the communications revolution that occurred in the late 1700s was France.  In 1794 Claude Chappe erected a series of semaphore telegraph repeating stations between Paris and Lyon (116 stations in all). Napoleon, being a General who understood more then most its potential, expanded the system dramatically in following ears and it covered a large part of Europe, and French ports in particular.  This is a diagram of the Chappe system:

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Some historians (British ones, anyway) suggest that the French system was based on an idea by the British scientist Robert Hooke, who published a paper with the idea in the 1680s.

Thus signals could be sent hundred of miles almost instantly – daylight and weather conditions permitting.  The British, tracked this development and copied it perhaps belatedly. The Rev Lord George Murray developed a version of the French system in 1796, using a six panelled matrix system shown here.

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Lines of these “shutter stations”were set up between the Admiralty and  key ports, such as Harwich, Chatham and Portsmouth.  The Portsmouth “chain” ran as follows:

– Admiralty House, Whitehall
– Royal Hospital Chelsea
– “The Highland” Putney
– Netley Heath
– Hascombe
– Blackdown
– Beacon Hill, Harting
Portsdown Hill
– The Glacis, Portsmouth

One chain was established between the Admiralty office in London and Plymouth in 1806 enabling a message to be transmitted in about 30 minutes, extraordinary speed compared to previous mechanisms controlled by the speed of a horse. Other nations, including the Danish, also developed similar systems.

Governments also learned the value of intercepting signals.   Like radio signals, it is possible for others who are not intended recipients to receive such messages, by placing themselves in line of sight.  And when the French system of communicating between ports placed the relay stations on high ground at the coast, a ship at sea could often read the signals of the enemy ashore.    Of course to interpret the coded messages they needed to decipher them – and in some ways the best way of deciphering messages is to steal the codes which Capt. Lord Cochrane duly did.  Cochrane understood, of course the importance of not letting the enemy know he had the codes, so left the originals and only took copies.  Thus the British were able to read and decode French semaphore signals for some time afterwards.   This is a detail of a page copied from the French code book sent back to the Admiralty by Cochrane in 1808.

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The similarities between the capture of these codes and the capture of Enigma machines in WW2 are distinct.  Cochrane explained to the Admiralty not just the codes, but also the routine of the signals.  With admirable efficiency the French Navy transmitted messages 4 times a day detailing the number, strength , situation, direction, bearing and distance of all vessels they could see along the coast.  Because the signal stations were also along the coast thus Cochrane in his ship the “Imperieuse” benefited directly from the French data.   Cochrane was an interesting fellow – I’ve written about other aspects of his career elsewhere on another blog, here, and here.

For ship based communication, rather than land, flags were used –  but they were surprisingly unsophisticated until about 1776 and took some decades to develop into an effective system. But the system did become into general use and was used extensively in the run up to the Battle of Trafalgar.

The Royal Navy and the French Navy understood that the capture of the signal codebook could be disastrous and so (rather like U Boat codes 250 years later) in the Royal Navy they were kept with sufficient weight attached to them to cause them to sink rapidly if thrown overboard in an emergency.   Occasionally though the best laid plans were not sufficient.  In 1793, The French Frigate “Cleopatre” was captured by HMS Nymphe, and when the British boarded the found the heroic French captain,  Jean Mullon, dying on the quarterdeck. The dying Captain Mullon reached into his jacket pocket and started to tear up and eat his code book – but in his agony got it wrong and ate his captain’s commission document rather then the code papers which were in another pocket.

It is also true that Napoleon’s espionage was very capable and frequently intercepted British naval and diplomatic communications.

More details are in an excellent book “Most Secret and Confidential – Intelligence in the Age of Nelson” by Steven E Maffeo.

2 comments on “Eating codes for dinner – SIGINT in the age of Semaphore

  1. Edward

    Where can I find a complete illustration of Murray’s shutter semaphore code? Any help is greatly appreciated. Thanks, Ed


    1. Roger Davies Author

      Sorry Ed , can’t help you there!


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