Heliograph, << HEE lee uh graf, >> was an instrument used to send signals by reflecting sunlight with a mirror or mirrors. Heliographs were used by the armies of several countries during the late 1800’s.
Heliograph equipment varied somewhat from country to country. The heliograph used by the U.S. Army had a mirror with a sighting rod–or else two mirrors–mounted on a tripod. A screen or shutter for interrupting the flashes was mounted on another tripod. If the sun was in front of the sender, its rays were reflected directly from a mirror to the receiving station. The sender used the sighting rod to line the flash up with the receiver. If the sun was behind the sender, its rays were reflected from one mirror to another, and from the second mirror to the receiver. The flash was lined up with the receiver by adjusting the two mirrors. Messages were sent as short and long flashes by opening and closing the shutter. The flashes represented the dots and dashes of the Morse code.
The distance that heliograph signals could be seen depended on the clearness of the sky, the length of uninterrupted sight, and the size of the mirrors used. Under ordinary conditions, a flash could be seen 30 miles (48 kilometers) with the naked eye and much farther with a telescope.
