The heliogragh is a device that was used in the British Army for signaling to a distant point. It comprised of a wooden tripod of which each leg was adjustable. The mirror assembly was usually kept safely packed in a stout leather or wooden box.
To use this devise the signaler would normally set it up on high ground. The mirror assembly would be unpacked and fixed to the tripod and secured by a knurled brass knob. A highly polished mirror fitted into a heavy brass ring about six inches across with pins at each side mounted in an inverted U of brass that could swivel on its stem allowed the mirror to nod on its pivots. The back of the mirror was covered by a brass plate with a brass stump and pivot that connected to the Morse key via a brass tube, the insides of which had been tapped at each end with an opposite thread.
The shaft connected to the mirror was a right handed thread. The shaft from the tube to the Morse key had a left handed thread. So if the knurled tube was turned clockwise the top of the mirror would move forward. If the knurled tube turned anti clockwise the mirror top would move backwards.
The mirror has a tiny spot in the middle that has not been silvered so the reflected light always had a black dot in the center.
In the back of the brass plate is a tiny hole.
To set up this signaling device using the sun the signaler had to mark the position of the sun. With the sun to his front the signaler would adjust the legs of the tripod to get a rough fix on the distant signaler. Then he would look through the tiny hole and fine tune the array In front of the mirror is an arm with a 2 inch by 1 inch black board propped up at itís end. There is a 1" slot in the board from top to bottom, across this slot are two white lines. The signaler now adjusts the knurled tube and the Morse key until he now has a small black dot (due to the lack of reflection by the tiny hole) dancing up and down between the two white lines . With a final look through the hole and the slot at the target he flips the slotted board down out of the way and can now signal in Morse by sunlight to his distant observer.
Should the sun be at his back the signaler fits a like mirror to the assembly but it is minus the stump to connect it to the key. This mirror faces the sun that is behind the signaler, but it reflects the light onto the tilting signal mirror As the sun moved so too the signaler had to re-adjust to the target to keep it aligned.
With the advent of radio these devices took a back seat. Lots of people might argue, "ah well radio is not so cumbersome and it can reach a far greater distance" But the enemy can listen in on the messages and sending them in code is useless if the enemy has broken the code. However semaphore with flags and Morse with flag and lamp and heliograph definitely puts an immediate enemy at a disadvantage provided he is over the horizon.
Tom Barker 1999