What is Heliograph?Sometimes when you are in open country, you may catch a flash of sunlight on the windshield of a car many miles away.
The windshield acts like a mirror and reflects the light.
You have probably shone a flashlight beam against a wall or ceiling and watched the spot of light.
Cover the mirror or flashlight with your hand, and the light disappears.
Put these two ideas together, and you have one of the oldest ways of communicating in the world—signalling by the light of the sun.
The ancient Greeks signaled to each other in this way.
They used an instrument called a heliograph.
The name comes from the ancient Greek words for “sun” and “writing.”
A heliograph can be seen up to 30 miles (48 kilometers) away on a clear day without a telescope.The modern heliograph is mounted on a tripod, like a camera.
It can turn in any direction.
The mirror flashes when it is directed at the sun and can then be dipped away or covered with a shutter.
If the signaler wants to send a message in a direction away from the sun, a second mirror is used to reflect light on to the first.
SIgnaling by Heliograph, Signal Corps, Fort Omaha, Neb. 1908
Messages in the darkA heliograph can be used only in the daytime when the sun is shining.
But at night, light messages can be sent using an Aldis lamp.
This is a powerful searchlight. It has a shutter that can be moved to block out the light and also make the dots and dashes of Morse code.
How to Make a Heliograph - By R. B. Huey
Excerpt from the book:The heliograph which is used in the army provides a good method of sending messages by the reflection of the sun's rays. In the mountains there are stations from which messages are sent by the heliograph for great distances, and guides carry them for use in case of trouble or accident. The wireless telegraph delivers messages by electricity through the air, but the heliograph sends them by flashes of light.
THE BOY MECHANIC - BOOK 2 - 1000 THINGS FOR BOYS TO DO
PUBLISHED 1915, BY H. H. WINDSOR CHICAGO - POPULAR MECHANICS CO. PUBLISHERS
The main part of the instrument is the mirror, which should be about 4 in. square, set in a wood frame and swung on trunnions made of two square-head bolts, each 1/4 in. in diameter, and 1 in. long, which are firmly held to the frame with brass strips, 1/2 in. wide, and 3 in. long. The strips are drilled centrally to admit the bolts, and then drilled at each end for a screw to fasten them to the frame. This construction is clearly shown in Fig. 1.
Detail of the Parts for Making the Mirror and Sight Rod Which are Placed on a Base Set on a Tripod Top, the Whole being Adjusted to Reflect the Sun's Rays in Any Direction Desired (Fig. 1)
A hole is cut centrally through the backing of the frame and a small hole, not over 1/8 in. in diameter, is scratched through the silvering on the glass. If the trunnions are centered properly, the small hole should be exactly in line with them and in the center.
A U-shaped support is made of wood strips, 3/8 in. thick and 1 in. wide, the length of the uprights being 3-1/2 in. and the crosspiece connecting their  lower ends a trifle longer than the width of the frame. These are put together, as shown in Fig. 2, with small brackets at the corners. A slot, 1/2 in. deep and 1/4 in. wide, is cut into the upper end of each upright to receive the trunnions on the mirror frame. Nuts are turned on the bolt ends tightly, to clamp the standard tops against the brass strips on the mirror frame. The cross strip at the bottom is clamped to the base by means of a bolt, 1-1/2 in. long. The hole for this bolt should be exactly below the peephole in the mirror and run through one end of the baseboard, which is 3/4 in. thick, 2 in. wide and 10 in. long.
At the opposite end of the base, place a sighting rod, which is made as follows: The rod is 1/2 in. in diameter and 8 in. long. The upper end is fitted with a piece of thick, white cardboard, cut 1/4 in. in diameter and having a projecting shank 1 in. long, as shown in Fig. 3. The rod is placed in a 1/2-in. hole bored in the end of the baseboard, as shown in Fig. 2. To keep the rod from slipping through the hole a setscrew is made of a small bolt with the nut set in the edge of the baseboard, as shown in Fig. 4.
The Parts in Detail for Making the Tripods... (Fig. 5, Fig. 6)
The tripod head is formed of a wood disk, 5 in. in diameter, with a hole in the center, and three small blocks of wood, 1 in. square and 2 in. long, nailed to the under side, as shown in Fig. 5. The tripod legs are made of light strips of wood, 3/8 in. thick, 1 in. wide and 5 ft. long. Two of these strips, nailed securely together to within 20 in. of the top, constitute one leg. The upper unnailed ends are spread to slip over the blocks on the tripod top. These ends are bored to loosely fit over the headless nails driven part way into the block ends. One tripod leg is shown in Fig. 6.
...and the Shutter for Flashing the Light,... (Fig. 7, Fig. 8)
The screen, or shutter, is mounted on a separate tripod and is shown in Fig. 7. Cut out two slats, 3/8 in. thick, 2-1/2 in. wide and 6 in. long, from hard wood, and taper both edges of these slats down to 3/16 in. Small nails are driven into the ends of the slats and the heads are filed off so that the projecting ends will form trunnions for the slats to turn on. Make a frame of wood pieces, 3/4 in. thick and 2-1/2 in. wide, the opening in the frame being 6 in. square. Before nailing the frame together bore holes in the side uprights for the trunnions of the slats to turn in. These holes are 1-3/4 in. apart. The frame is then nailed together and also nailed to the tripod top. The shutter is operated with a key very similar to a telegraph key. The construction of this key is shown in Fig. 7. A part of a spool is fastened to a stick that is pivoted on the opposite side of the frame. The key is connected to the slats in the frame with a bar and rod, to which a coil spring is attached, as shown in Fig. 8. Figure 9 shows the positions of the tripods when the instrument is set to flash the sunlight through the shutter. The regular telegraph code is used in flashing the light.
...and Diagram Showing the Location of the Tripods to Direct the Light through the Shutter (Fig. 9)
To set the instrument, first turn the cardboard disk down to uncover the point of the sight rod, then sight through the hole in the mirror and adjust the sight rod so that the tip end comes squarely in line with the receiving station.
When the instrument is properly sighted, the shutter is set up directly in front of it and the cardboard disk is turned up to cover the end of the sight rod.
The mirror is then turned so that it reflects a beam of light with a small shadow spot showing in the center made by the peephole in the mirror, which is directed to fall on the center of the cardboard sighting disk. It will be quite easy to direct this shadow spot to the disk by holding a sheet of paper 6 or 8 in. in front of the mirror and following the spot on the paper until it reaches the disk.
The flashes are made by manipulating the key operating the shutter in the same manner as a telegraph key.