MARINE SIGNALLING The main difference to be noted between signalling at sea and on land is that in the former case no physical connection, whether by wire or messenger, is possible. One is therefore restricted to visual, sound or wireless methods.
Visual methods, which are normally not effective beyond horizon distances (depending on visibility at the time), fall into two main categories, viz., day and night.
Flags.—By day, coloured flags and pendants have long been employed. These, originally, had purely arbitrary meanings, de pending in many systems on the actual position where they were displayed in a ship. In the middle of the nineteenth century, how ever, standard colours were agreed upon internationally, and an "International" code of signals for use at sea was brought into force. The flags of the code include the twenty-six letters of the alphabet and a "code" pendant. By means of hoists not exceeding four flags, it is possible to make any signal from the International Code or spell out words not contained therein. The code has since been translated into the languages of the principal maritime Powers. A new edition of the International Code of Signals is now in active preparation.
Separate sets of flags and elaborate systems of grouping are in use by most navies to meet their special requirements.
Semaphore.—Visual communication by day can also be carried out by means of the semaphore, a device which exhibits one or more straight arms in certain positions. The early forms of the system were principally used by coast stations and were of the nature of fixed signals. These coast semaphores still exist, chiefly in France, but have been superseded, so far as signalling between ships is concerned, by a moving semaphore system employing only two arms, which are moved to consecutive positions (indicating letters of the alphabet) as neces sary to spell out the message. It can be made by a mechanical semaphore or with the signal man's own arms. International agreement on this system has not been reached, but the alphabet shown here is in force in Great Britain and the United States.
This is an extremely rapid form of communication at short dis tances, and transmission speeds of 25 words a minute or over can be attained by expert signal men.
Flashing.—For longer dis tances by day, most large vessels are now equipped with powerful lamps or searchlights fitted with some form of louvre shutter, enabling the light to be flashed. Using Morse, messages can be spelt out verbatim or code groups signalled as desired. With brilliant lights of this nature com munication can be effected by night between ships at more than horizon distance, provided that weather conditions are favourable. At night, plain and coloured lanterns in various combinations constituted the earliest form of signalling, being superseded even tually by the introduction of flashing in conjunction with the Morse Code. Both by day and at night, the speed of transmis sion by flashing is comparatively slow on account of the physical limitations of the human eye, and a rate of 12 words a minute is seldom exceeded in practice.
Fireworks, e.g., bonfire beacons, Roman candles, rockets, Very's lights, etc., have all been pressed into the service of signalling, but only the three last named still survive and their use is mainly con fined to short messages of a standard nature, such as signals of dis tress, requests for a pilot, etc.