Key People:
Claude Chappe
Related Topics:
signal

semaphore, method of visual signaling, usually by means of flags or lights. Before the invention of the telegraph, semaphore signaling from high towers was used to transmit messages between distant points. One such system was developed by Claude Chappe in France in 1794, employing a set of arms that pivoted on a post; the arms were mounted on towers spaced 5 to 10 miles (8 to 16 km) apart. Messages were read by telescopic sightings. Modern semaphores included movable arms or rows of lights simulating arms, displayed from towers and used to signal railroad trains. Semaphore signaling between ships, now largely abandoned, was accomplished by persons who held a small flag in each hand and, with arms extended, moved them to different angles to indicate letters of the alphabet or numbers.

This article was most recently revised and updated by Richard Pallardy.

distress signal, a method by which a ship at sea can summon assistance. Distress signals are fixed by custom and by internationally agreed-on rules of the road at sea. The most important are: (1) visual signals, such as a flame, a red flare, an orange smoke signal, or a square flag displayed with a ball below; (2) sound signals, such as a gun or rocket fired at regular intervals, or a continuous sounding of a fog-signal apparatus; and (3) radio signals such as the Morse group SOS, the international code signal NC, or the spoken word “Mayday” (pronounced like the French m’aider, “help me”), by radiotelephone. Distressed vessels may also actuate alarms of other vessels by a radio signal consisting of a series of 12 four-second dashes or by a radiotelephone signal consisting of two tones alternately transmitted for 30 to 60 seconds.

This article was most recently revised and updated by Amy Tikkanen.