Editing Telegraphy (section)

=== Early signalling ===

[[File:20090529 Great Wall 8219.jpg|thumb|A section of the [[Great Wall of China]] built during the [[Ming dynasty]] (1368–1644)]]

Passing messages by signalling over distance is an ancient practice. One of the oldest examples is the signal towers of the [[Great Wall of China]]. By 400&nbsp;BC, signals could be sent by [[beacon|beacon fires]] or [[military drum|drum beats]], and by 200&nbsp;BC complex flag signalling had developed. During the [[Han dynasty]] (202&nbsp;BC{{snd}}220&nbsp;AD), signallers mainly used flags and wood fires—via the light of the flames swung high into the air at night, and via dark smoke produced by the addition of wolf dung during the day—to send signals.<ref>{{Cite book |last=Turnbull |first=Stephen |title=The Great Wall of China 221 BC–AD 1644 |publisher=Osprey |location=Oxford |year=2007 |isbn=978-1-84603-004-8 |page=37}}</ref> By the [[Tang dynasty]] (618–907) a message could be sent {{convert|700|mi|km|order=flip}} in 24 hours. The [[Ming dynasty]] (1368–1644) used [[artillery]] as another possible signalling method. While the signalling was complex (for instance, flags of different colours could be used to indicate enemy strength), only predetermined messages could be sent.<ref>Christopher H. Sterling, "Great Wall of China", pp. 197–198 in, Christopher H. Sterling (ed), ''Military Communications: From Ancient Times to the 21st Century'', ABC-CLIO, 2008 {{ISBN|1851097325}}.</ref> The Chinese signalling system extended well beyond the Great Wall. Signal towers away from the wall were used to give early warning of an attack. Others were built even further out as part of the protection of trade routes, especially the [[Silk Road]].<ref>Morris Rossabi, ''From Yuan to Modern China and Mongolia'', p. 203, Brill, 2014 {{ISBN|9004285296}}.</ref>

Signal fires were widely used in Europe and elsewhere for military purposes. The Roman army made frequent use of them, as did their enemies, and the remains of some of the stations still exist. Few details have been recorded of European/Mediterranean signalling systems and the possible messages. One of the few for which details are known is a system invented by [[Aeneas Tacticus]] (4th century BC). Tacticus's system had water filled pots at the two signal stations which were drained in synchronisation. Annotation on a floating scale indicated which message was being sent or received. Signals sent by means of [[torch]]es indicated when to start and stop draining to keep the synchronisation.<ref name=Woods>David L. Woods, "Ancient signals", pp. 24–25 in, Christopher H. Sterling (ed), ''Military Communications: From Ancient Times to the 21st Century'', ABC-CLIO, 2008 {{ISBN|1851097325}}.</ref>

None of the signalling systems discussed above are true telegraphs in the sense of a system that can transmit arbitrary messages over arbitrary distances. Lines of signalling [[Relay league|relay]] stations can send messages to any required distance, but all these systems are limited to one extent or another in the range of messages that they can send. A system like [[flag semaphore]], with an alphabetic code, can certainly send any given message, but the system is designed for short-range communication between two persons. An [[engine order telegraph]], used to send instructions from the bridge of a ship to the engine room, fails to meet both criteria; it has a limited distance and very simple message set. There was only one ancient signalling system described that ''does'' meet these criteria. That was a system using the [[Polybius square]] to encode an alphabet. [[Polybius]] (2nd century BC) suggested using two successive groups of torches to identify the coordinates of the letter of the alphabet being transmitted. The number of said torches held up signalled the grid square that contained the letter. There is no definite record of the system ever being used, but there are several passages in ancient texts that some think are suggestive. Holzmann and Pehrson, for instance, suggest that [[Livy]] is describing its use by [[Philip V of Macedon]] in 207 BC during the [[First Macedonian War]]. Nothing else that could be described as a true telegraph existed until the 17th century.<ref name=Woods/><ref name=Holzmann/>{{rp|26–29}} Possibly the first alphabetic [[telegraph code]] in the modern era is due to [[Franz Kessler]] who published his work in 1616. Kessler used a lamp placed inside a barrel with a moveable shutter operated by the signaller. The signals were observed at a distance with the newly invented telescope.<ref name=Holzmann/>{{rp|32–34}}