heliograph

n. device for photographing the Sun; instrument which measures the intensity of sunlight; signaling device which reflects the sun's rays by means of a movable mirror; early type of photoengraving

This article is about the signalling device. For other meanings, see heliograph (disambiguation). A heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send Morse code. The heliograph was a simple but highly effective instrument for instantaneous optical communication over 80km or more in the 19th century. Its major uses were for military and survey work. It was still in use at least until 1935.
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Noun
1. an apparatus for sending telegraphic messages by using a mirror to turn the sun's rays off and on
(hypernym) apparatus, setup
Verb
1. signal by means of a mirror and the using the sun's rays
(hypernym) sign, signal, signalize, signalise

n. heliograph, device for photographing the Sun; instrument which measures the intensity of sunlight; signaling device which reflects the sun's rays by means of a movable mirror; early type of photoengraving

(n.)
An instrument for taking photographs of the sun.
  

(n.)
An apparatus for telegraphing by means of the sun's rays. See Heliotrope, 3.
  

(n.)
A picture taken by heliography; a photograph.