A device that uses mirrors to reflect sunlight and send coded messages across long distances, or the act of sending such messages.
From Greek 'helios' (sun) + 'graphia' (writing). Invented in the 1860s, it became a crucial military communication tool before electricity-based telegraph systems became universal.
The heliograph was so effective that British troops used it to coordinate movements across deserts where there were no telegraph wires—the sun's reflection could be seen up to 30 miles away!
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