Positions of a railroad semaphore: (A) clear, (B) caution, (C) stop
semaphore [key], device for the visible transmission of messages. The marine semaphore, used by day between ships or between a ship and the shore, consists essentially of a post at the top of which are two pivoted arms. The arms are connected by light gearing to two operating levers. Each letter of the alphabet and each numeral is indicated by a different placing of the arms. The system can also be used by the signalman through motions of his own arms, with or without small flags as indicators. In the railroad semaphore a single projecting arm pivoted at one end and attached to a vertical post is devised to take three positions. Horizontal indicates stop, and vertical, all clear; the inclined position indicates that the locomotive may go ahead under control expecting to be stopped. See signaling.
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