40. For in such an emergency you will not be able to make use either of the voice or of the trumpet to communicate what is necessary, because of the uproar and the tumult, and the sound of the sea, and the crash of ship against ship, the noise of the oars, and above all the clamour of the combatants.
41. Further, let the signal be given by setting the standard upright or by inclining it to the right or to the left, or by moving it twice to the right or left, or by shaking it, or lifting it up, or lowering it or altogether withdrawing it or altering its position, or by varying the appearance of the head by means of devices or colours as was the practice amongst the ancients; for in time of war they gave the signal for battle by raising what was called the Phoinikis (φοινικίδα [Greek: phoinikida]). There was also what was called the "cap" (μαμελαύκιον [Greek: mamelaukion]) raised upon a pole, red in colour, and they had some other signals which were made known in like manner. Perhaps however it would be safer to make your signals by your own hand.
42. And thus, O General, let the exercise of these signals be practised, so that all the officers in command of ships under you may have certain knowledge of all such signs; of the reason why each is made, and when, and how, and may not fail. So that, well familiarised with the signals, they may readily understand them in time of emergency and carry out the orders indicated.
In reading the above paragraphs one cannot fail to be impressed by the profound grasp of the essential requirements of the subject exhibited by the writer. Nothing like it appears again until modern times, and the concluding words might well have been written by Kempenfelt and the other reformers of the Signal System in the British navy at the end of the eighteenth century.
After Leo we meet with nothing further on this subject until the middle of the fourteenth century, and from what we then find it is clear that the art had not only made no progress in the interval, but on the contrary had decidedly deteriorated. Two sets of instructions preserved in the _Black Book of the Admiralty_, to which Sir Travers Twiss assigns a date between 1337 and 1351[345], contain each one flag signal; one for calling a council, the other for notifying the presence of the enemy: