Editing Heliograph (section)

==Description==
[[File:Heliograph (1)-2.jpg|thumb|Fig. 2: German heliograph made by R. Fuess in [[Berlin]] (on display at the Museum of Communication in [[Frankfurt]])]]
There were many heliograph types. Most heliographs were variants of the [[British Army]] Mance Mark V version (Fig.1). It used a flat<ref>{{cite journal |last1=Shackleton |first1=William |title=The Testing of Heliograph Mirrors and  the Measurement of Mirrors of Long Focal Length |journal=Transactions of the Optical Society |date=April 1921 |volume=22}}</ref> 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.<ref name=Manual1922>{{cite book|title=Signal Training|volume=III. Pamphlet No. 2. Heliograph, 5-inch, Mark V. 1922.|year=1922|publisher=His Majesty's Stationery Office|location=London|pages=10–13|url=http://www.royalsignals.org.uk/signals.htm}}</ref> 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.<ref name=Manual1922 /> 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.<ref name="Forestry"/><ref name="British">[http://www.smecc.org/heliograph_-_signaling_by_the_sun.htm 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).</ref> The [[United States Army Signal Corps|U.S. Army's Signal Corps]] heliograph used a flat square mirror that did not tilt.<ref name="US1905Manual">{{cite book |last1=Carr |first1=D.J. |title=Manual of Visual Signaling of the US Signal Corps |date=1905 |publisher=Government Printing Office |location=Washington |pages=68 |url=https://books.google.com/books?id=3DtCAQAAMAAJ&pg=PA68&dq=parallel |access-date=16 November 2024}}</ref> This type produced flashes by a [[Shutter (photography)|shutter]] mounted on a second tripod (Fig 4).<ref name="Forestry">W. N. Millar (1920), Canadian Forestry Service. [https://books.google.com/books?id=CI5OAAAAMAAJ 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.</ref>

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 {{cvt|50|ft}} per {{cvt|1|mi}} of range. However, anyone in the beam with the correct knowledge could intercept signals without being detected.<ref name="Boer"/><ref name="Kipling">Kipling, Rudyard [http://www.kipling.org.uk/poems_codeofmorals.htm A Code of Morals]. The Kipling Society website. Retrieved on 1 June 2008.</ref> 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.<ref name="Boer">Major J. D. Harris [http://rapidttp.com/milhist/vol111jh.html 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.</ref> 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.<ref name=1905Handbook>{{cite web|last=Signals|first=Royal|title=The Heliograph|url=http://royal-signals.org.uk/Datasheets/THE_HELIOGRAPH.php|work=Signalling Handbook (1905)|access-date=15 April 2012}}</ref>

The range of a heliograph depends on the opacity of the air and the effective collecting area of the mirrors. Heliograph mirrors ranged from {{cvt|1.5|to|12|in}} 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 {{cvt|20|to|50|mi}} is that the flash of a circular mirror is visible to the naked eye at a distance of {{cvt|10|mi}} for each inch of mirror diameter,<ref name="Jacob1849">{{cite journal |last1=Jacob |first1=W.S. |title=On the Extinction of Light in the Atmosphere |journal=Proceedings of the Royal Society of Edinburgh |date=November 1849 |volume=2 |pages=272–273 |url=https://books.google.com/books?id=OgsFAAAAQAAJ&pg=PA272 |access-date=19 May 2019}}</ref> 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)|Mount Ellen]], ([[Utah]]), and [[Uncompahgre Peak|Mount Uncompahgre]], ([[Colorado]]), {{cvt|183|mi}} apart on 17 September 1894, with Army Signal Corps heliographs carrying mirrors only 8 inches (20 cm) on a side.<ref name="Coe">Coe, Lewis [https://books.google.com/books?id=pzkZPIXm89UC The Telegraph: A History of Morse's Invention and Its Predecessors in the United States]. Google Books. Retrieved on 1 June 2008.</ref>