All that was now wanted was a flag to denote whether the signal hoisted was to be deciphered by the Signal Book or by the Vocabulary Code. For this purpose Popham designed a flag divided diagonally into white and red to be used as a "preparative" or "telegraph" flag, with all signals made in his code. This was hoisted before the message started, and hauled down when it finished. Such were the signal books in use at Trafalgar, and in the hands of a man like Nelson, who did not keep his tactical ideas to himself, but discussed them freely with his captains during the months of watching and preparation, they proved amply sufficient for the purpose. But suppose the books were captured by the enemy; how then? In such an event, which all captains were told to guard against by throwing the books overboard if there was any probability of their ship being captured, the most effective precaution was to change the whole of the signal numbers, both in the general signals and in the vocabulary, but this was a heavy task. A less effective method was to transpose all the flags. It was less effective since, _ex hypothesi_, the enemy had the signal books, they had therefore only to note the colours of the flags which preceded some easily recognised manoeuvre, such as "Make more sail," "Bear up and sail large," to discover what flags were now being used to indicate the signal numbers which they saw against that signal. A little patience and ingenuity would then supply the key to the changes in the flags, whereas if all the signal numbers were irregularly transposed, each signal would have to be re-constituted separately by the enemy. About fifteen months before Trafalgar the 12-gun schooner, 'Redbridge,' commanded by a Lieut. Lemprière, who, to judge by Nelson's remarks about him, was not particularly efficient, was captured by some French frigates off Toulon. Such a small ship, commanded by an officer of such subordinate rank, was not allowed the confidential signal books, but Lemprière had, in common with many other junior officers, obtained a surreptitious copy for himself--one of those little manuscript and hand-painted signal books one sees in museums, or occasionally picks up in second-hand book shops. This book Lieut. Lemprière had neglected to throw overboard, and when, later on, one of Nelson's scouts, looking into Toulon harbour to see if the French were getting on comfortably, found the captured 'Redbridge' just outside, the latter made the signal for the scout ship to anchor; but fortunately the officer in command was a little sharper than the 'Redbridge's' late commander had been, and the net was spread in vain.