A heliograph () is a solar telegraph system that signals by flashes of sunlight (generally using Morse code from the 1840s) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.
The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th centuries. Its main uses were military, surveying and forest protection work. Heliographs were standard issue in the British and Royal Australian armies until the 1960s, and were used by the Pakistani army as late as 1975. While these solar telegraphs are now obsolete, the daylight signalling mirrors required worldwide in oceangoing lifeboats are often called "heliographs" in Commonwealth nations.
Description
There were many heliograph types. Most heliographs were variants of the British Army Mance Mark V version (Fig.1). It used a flat round mirror with a small unsilvered spot in the centre. The sender aligned the heliograph to the target by looking at the reflected target in the mirror and moving their head until the target was hidden by the unsilvered spot. Keeping their head still, they then adjusted the aiming rod so its cross wires bisected the target.
They then turned up the sighting vane, which covered the cross wires with a diagram of a cross, and aligned the mirror with the tangent and elevation screws, so the small shadow that was the reflection of the unsilvered spot hole was on the cross target. This indicated that the sunbeam was pointing at the target. The flashes were produced by a keying mechanism that tilted the mirror up a few degrees at the push of a lever at the back of the instrument.
If the Sun was in front of the sender, its rays were reflected directly from this mirror to the receiving station. If the Sun was behind the sender, the sighting rod was replaced by a second mirror, to capture the sunlight from the main mirror and reflect it to the receiving station.Manual Of Instruction In Army Signaling 1886 Section III- Apparatus And Method Of Using It. Retrieved on 1 June 2008. Diagrams and instructions for British military heliograph (note British heraldry emblem on cover). The U.S. Army's Signal Corps heliograph used a flat square mirror that did not tilt. This type produced flashes by a shutter mounted on a second tripod (Fig 4).W. N. Millar (1920), Canadian Forestry Service. Methods of Communication Adapted to Forest Protection Google Books. Retrieved on 1 June 2008. pp. 160–181 are devoted to the heliograph, with diagrams of the British, American, and Godwin type.
The heliograph had certain advantages. It allowed long-distance communication without a fixed infrastructure, though it could also be linked to make a fixed network extending for hundreds of miles, as in the fort-to-fort network used for the Geronimo military campaign. It was very portable, did not require any power source, and was relatively secure since it was invisible to those not near the axis of operation, and the beam was very narrow, spreading only per of range. However, anyone in the beam with the correct knowledge could intercept signals without being detected.Kipling, Rudyard A Code of Morals. The Kipling Society website. Retrieved on 1 June 2008.
In the Second Boer War (1899–1902) in South Africa, where both sides used heliographs, tubes were sometimes used to decrease the dispersion of the beam.Major J. D. Harris WIRE AT WAR – Signals communication in the South African War 1899–1902. Retrieved on 1 June 2008. Discussion of heliograph use in the Boer War. In some other circumstances, though, a narrow beam made it difficult to stay aligned with a moving target, as when communicating from shore to a moving ship, so the British issued a dispersing lens to broaden the heliograph beam from its natural diameter of 0.5 degrees to 15 degrees.
The range of a heliograph depends on the opacity of the air and the effective collecting area of the mirrors. Heliograph mirrors ranged from or more. Stations at higher altitudes benefit from thinner, clearer air, and are required in any event for great ranges, to clear the curvature of the Earth. A good approximation for ranges of is that the flash of a circular mirror is visible to the naked eye at a distance of for each inch of mirror diameter, and farther apart seen with a telescope.
The world record distance was established by a detachment of U.S. Army signal sergeants by the inter-operation of stations in North America on Mount Ellen (Utah), and Mount Uncompahgre (Colorado), apart on 17 September 1894, with Army Signal Corps heliographs carrying mirrors only 8 inches (20 cm) on a side.Coe, Lewis The Telegraph: A History of Morse's Invention and Its Predecessors in the United States. Google Books. Retrieved on 1 June 2008.
History
The German professor Carl Friedrich Gauss (1777–1855), of the University of Göttingen developed and used a predecessor of the heliograph (the heliotrope) in 1821. His device directed a controlled beam of sunlight to a distant station to be used as a marker for geodetic survey work, and was suggested as a means of telegraphic communications. This is the first reliably documented heliographic device, despite much speculation about possible ancient incidents of sun-flash signalling, and the documented existence of other forms of ancient optical telegraphy.
For example, one author in 1919 chose to "hazard the theory" that the Italian mainland signals from the capital of Rome that ancient Roman emperor Tiberius (42 B.C.-A.D.37, reigned A.D.14 to 37), watched for from his imperial retreat on the island of Capri. were mirror flashes, but admitted "there are no references in ancient writings to the use of signaling by mirrors", and that the documented means of ancient long-range visual telecommunications was by beacon fires and beacon smoke, not mirrors.
Similarly, the story that a shield was used as a heliograph at the ancient famous Battle of Marathon between the Greeks and Persians in 490 B.C. is a modern myth, originating in the 1800s. The ancient historian Herodotus never mentioned any flash. What Herodotus did write was that someone was accused of having arranged to "hold up a shield as a signal". Suspicion grew in the later 1900s, that the flash theory was implausible. The conclusion after testing the theory was "Nobody flashed a shield at the Battle of Marathon".
In a letter dated 3 June 1778, John Norris, High Sheriff of Buckinghamshire, England, notes: "Did this day heliograph intelligence from Dr [Benjamin] Franklin in Paris to Wycombe".Deacon, Richard (1978). The Silent War: a History of Western Naval Intelligence. David & Charles. p. 21. . However, there is little evidence that "heliograph" here is other than a misspelling of "holograph". The term "heliograph" for solar telegraphy did not enter the English language until the 1870s—even the word "telegraphy" was not coined until the 1790s.
Henry Christopher Mance (1840–1926), of the British Government's Persian Gulf Telegraph Department, developed the first widely accepted heliograph about 1869, while stationed at Karachi (now in modern Pakistan) in the then Bombay Presidency of British India. Mance was familiar with heliotropes by their use earlier for the mapping project of the Great Trigonometrical Survey of India (done 1802–1871). The Mance Heliograph was operated easily by one man, and since it weighed about , the operator could readily carry the device and its supporting tripod. The British Army tested the heliograph in India at a range of with favorable results. During the Jowaki Afridi expedition sent by the British-Indian government in 1877, the heliograph was first tested in war.R. W. Burns (2004) Communications: An International History of the Formative Years. Google Books. Retrieved on 2 June 2008. pp. 192-196 discuss the heliograph.
The simple and effective instrument that Mance invented was to be an important part of military communications for more than 60 years. The usefulness of heliographs was limited to daytimes with strong sunlight, but they were the most powerful type of visual signalling device known. In pre-radio times heliography was often the only means of communication that could span ranges of as much as with a lightweight portable instrument.
In the United States military, by mid-1878, a younger Colonel Nelson A. Miles had established a line of heliographs connecting far-flung military outposts of Fort Keogh and Fort Custer, in the northern Montana Territory, a distance of . In 1886, United States Army now General Nelson A. Miles (1839–1925), set up a network of 27 heliograph stations in the Arizona and New Mexico territories of the old Southwest during the extended campaign and hunt for the native Apache renegade chief / guerrilla warfare leader Geronimo (1829–1909).
In 1890, now little-known Major W.J. Volkmar of the U.S. Army demonstrated in the Arizona and New Mexico territories, the possibility of performing communication by heliograph over a heliograph network aggregating in length. The network of communication begun by General Miles in 1886, and continued by unsung and now relatively unknown Lieutenant W. A. Glassford, was perfected in 1889 at ranges of over a rugged and broken country, which was the stronghold of the Apache, Commanche and other hostile native Indian tribes.
By 1887, heliographs in use included the British Mance and Begbie heliographs, and the American Grugan, Garner and Pursell heliographs. The Grugan and Pursell heliographs used shutters, and the others used movable mirrors operated by a finger key. The Mance, Grugan and Pursell heliographs used two tripods, and the others one. The signals could either be momentary flashes, or momentary obscurations..
In 1888, the U.S. Army Signal Corps reviewed all of these devices, as well as the Finley Helio-Telegraph, and finding none completely suitable, developed its own instrument of the U.S. Army Signal Corps heliograph, a two-tripod, shutter-based machine of total weight, and ordered 100, for a total cost of $4,205.. By 1893, the number of heliographs manufactured for the American Army Signal Corps was 133.
The heyday of the heliograph was probably the Second Boer War of the 1890s and early 1900s in South Africa, where it was much used by both the British and the native immigrant Boers. The terrain and climate, as well as the nature of the campaign, made heliography a logical choice. For night communications, the British used some large signal lamps, brought inland on railroad cars, and equipped with leaf-type shutters for keying a beam of light into dots and dashes. During the early stages of the war, the British Army garrisons were besieged in Kimberley, along with the sieges of Ladysmith, and at Mafeking. With land wire telegraph lines cut, the only contact with the outside world was via light-beam communication, helio by day, and signal lamps at night.
In 1909, the use of heliography for forestry protection was introduced by the United States Forestry Service in the western States. By 1920, such use was widespread in the US and beginning in the neighboring Dominion of Canada to the north, and the heliograph was regarded as "next to the telephone, the most useful communication device that is at present available for forest-protection services". D.P. Godwin of the U.S. Forestry Service invented a very portable () heliograph of the single-tripod, shutter plus mirror type for forestry use.