What is a heliograph?

Heliograph, literally "sun writing," is a method of signaling that makes use of the sun's rays, reflecting them with a mirror or mirrors. The chief use of the heliograph was in military operations, though it have often been used for other than military purposes, as, for instance, by the United States Geological Survey in triangulation. A clear atmosphere is the prime requisite in long-distance signaling with the heliograph, which consists of a mirror or system of mirrors mounted on a tripod and fitted with a shutter that keeps the rays of the sun from the mirror at the will of the operator. The Morse system of dots and dashes is usually employed to operate the heliograph.

At the present time, signal mirrors are still included in survival kits for emergency signaling to search and rescue aircraft.