Etymology
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heliograph (n.)

1848, "instrument for taking photographs of the sun," from helio- "sun" + -graph "something written." Earlier, "a description of the sun" (1706, implied in heliographic). From 1877 as the name of a movable mirror used in signaling. Related: Heliographical.

Heliography (1845) was the word for the product of a type of engraving process by chemical reaction from exposure to sunlight. It also was an early term for what came to be called photography (1840).

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Definitions of heliograph
1
heliograph (v.)
signal by means of a mirror and the using the sun's rays;
2
heliograph (n.)
an apparatus for sending telegraphic messages by using a mirror to turn the sun's rays off and on;
From wordnet.princeton.edu