Signals
2 of the 7 encyclopedias on this shelf carry an entry for Signals. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the means by which communications are made to greater distances than can be reached by the human voice. To the eye these are conveyed by flags, lights, etc., and to the ear by guns, steamwhistles, fog-horns, etc. The most complete systems of signaling are those devised to enable ships some distance apart to communicate with each other. Of these the most important ones in the United States navy are the international code, the secret naval code, the wig-wag system, the Ardois night signal code, and a system of wireless telegraphy invent by invented by officers of the navy. The The wig-wag system is the simplest of all, and in most general use when vessels of a fleet are lying in close proximity to each other. It is carried on by means of a small flag of conspicuous color, mounted on a staff some six or seven feet long. This is held by the person sending the message and is waved down, or to the right or left, in accordance with an understood arrangement. The international code, which is used by all vessels at sea, consists of 27 flags, of which there are 19 square ones, 6 pennants and 2 burgees. These flags represent letters, and in order to spell out a message combinations of three or four, having the same meaning in all languages, are hoisted to the top of the signal mast.
Aiton's Encyclopedia (1910)
a system of communicating, especially at sea. In 1902 a new international code was adopted superseding all previous ones. Two swallow-tailed pennants are used for the letters A and B. Five triangular pennants of different colors are used for the letters C to G; nineteen other flags readily distinguished are used to complete the alphabet. It is possible to spell out any word or message by displaying these flags in proper order. A very great number of signals have been agreed upon, however. C displayed alone means, "Yes"; D, "No"; A hoisted over B signifies "Abandon your ship at once." One combination asks whether icebergs have been seen; another, whether water can be spared; another indicates a desire to send a message, etc. Other combinations of two and three flags afford several thousand possible signals, any of which may be read by the use of the international code book with which each ship is provided. A similar system of conveying information is familiarly spoken of in military service as "wig-wagging." In such cases, the utmost secrecy is observed, of course, and the meaning of signals is changed frequently.