Codes and signals
Two groups of five signal flags
The Romans had clever signalling systems. On Hadrian's Wall an alphabetic system was used based on two groups of five flags, which allowed them to send messages letter by letter, and was similar to the system developed in England at the end of the eighteenth century. (The Irishman Richard Lovell Edgeworth is supposed to have invented a telegraph system in order to get the racing results from Newmarket before his bookmaker - but he never developed it!)
A list of codes
The Romans also had a coded system, with which they could send only one of a dozen fixed messages, depending on the time for which they showed a flag. The sender and receiver would have the same code book and identical water clocks, marked perhaps with numbers. To send message VI in the book, raise your flag (or flaming torch at night), wait until the receiver raises a flag to acknowledge, then lower your flag, and raise it again, starting your clock as you raise the flag. When your flag points to VI, lower your flag again. The receiver should have started the clock when the flag went up for the second time, and stopped it when the flag went down; the number VI will reveal the message.
The idea of using codes like this was taken up by the French, also at the end of the eighteenth century.
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Published: 2001-06-01
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