The Heliograph: Morse by Mirrored Sunlight Mirror, tripod, shutter and sun: this guide covers the heliograph's principle, its military networks and its survival among modern experimenters. Quick facts Principle reflected sunlight Range 50-180 miles Code Morse flashes Famous network Arizona, 1880s Killed by radio + cloud The heliograph's security was elegant - a beam you must stand inside to read, leaving no wire to cut and no signal to intercept. Its weakness was the weather. On the clear mornings it was built for, whole theatres of war conversed in light. Free tool and guides: Morse Code Works Morse Code Works - free Morse code tools and guides. Reference chart