On the 21st of October 1805, the most famous naval battle of the age of sail was about to begin. On board his flagship Victory, Vice Admiral Nelson decided to send a signal to his fleet that was not an order, or a request, but an inspirational message. It was the first time that any Admiral had sent such a signal, and he was only able to do so because of the flexible signalling system that had been provided to him by Captain Home Popham. The thirty two coloured flags were hauled aloft, and “England expects that every man will do his duty” entered the history books.
The signalling system in use that day converted words and phrases into a numbered code. Those codes were then transmitted using ten different flags, one for each digit, grouped together to make the whole number. This could work very well. If one ship wanted to tell another that “I am not acquainted with that harbour”, this could be done with just four flags (2349). The system was good, but not perfect. Nelson actually wanted his signal to begin “England confides...” (i.e. has confidence in). Unfortunately there was no suitable word in the code book, so the more hectoring “England expects...” signal had to be sent instead.
Popham’s system was the latest in a long line of signalling systems, the earliest of which were often very crude. The Royal Navy first introduced a flag-based system during the Anglo-Dutch wars of the 17th century, but it only used five different flags, allowing little more than simple instructions to be sent. No formal system of meaning for a particular flag was imposed centrally, so admirals were left to agree their own simple codes with their captains. As the eighteenth century progressed, these systems became more complex, with the same flag having multiple meanings, depending where it was displayed. Sometimes they could stray into the impractical. It is hard to see how the signal for “break off the action” in one code book, a flag accompanied by a gun fired to leeward, would ever have been noticed amongst all the other cannon being fired in the heat of battle.
In 1790 Admiral Howe issued a much improved code book for his captains. It was his system that first hit on the idea of flags representing numbers, instead of a direct command. This greatly increased the number of words and phrases that could be transmitted. Howe’s system was adopted by the Royal Navy and used throughout the Revolutionary wars, and generally worked well. It would have still been in service at Trafalgar, had the code book behind it not fallen into enemy hands in 1803. It was replaced by Home Popham’s slightly more complex system, with its wealth of additional words and phrases, allowing Nelson to send his famous signal.
An 18th century naval code book is an intriguing window into a vanished era that often raises more questions than it answers. With a limited number of flag combinations available, their creators had to decide which words and phrases to include, and which to leave out. In Nelson’s famous signal, for example, the final word (duty) had to be spelt. This seems surprising, given how all-pervasive the word was in naval correspondence of the time. On the other hand, Popham could find room in his code book for “she is laden with cochineal” (2153), which may reveal where his priorities lay. Captain Popham had several run-ins with authority over financial matters, and was known to be fond of a prize.
Flags remained the main form of inter-warship communication throughout the 19th century, and well into the 20th century. Even the introduction of the ship-borne radio did not result in them becoming obsolete. This was because radio transmissions, unlike lines of flags, reveal the position of the transmitting ship to any potential enemy. This was why HMS Erebus, on another momentous day in history, still used flags to repeat Nelson’s famous Trafalgar message. The signal was sent as she closed with the coast of Normandy and began her programme of shore bombardment, in support of the Allied troops fighting their way ashore on D Day.