heliogram
/ˈhiːlioʊɡræm/
noun
- A message sent using a heliograph, which uses reflected sunlight to flash signals over long distances.
- The scout decoded the heliogram and reported the news to the captain.
- Before radios, explorers in the desert often relied on a heliogram to communicate across valleys.
- The soldiers sent a heliogram from the mountaintop to warn of the approaching army.