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A foghorn or fog signal is a device that uses sound to warn vehicles of navigational hazards such as rocky coastlines, or boats of the presence of other vessels, in conditions. The term is most often used in relation to marine transport. When visual navigation aids such as are obscured, foghorns provide an audible warning of , , , or other dangers to shipping.
At some , a small cannon was let off periodically to warn away ships, but this was labor-intensive and dangerous. In the United States, were also used where a source of steam power was available, though Trinity House, the United Kingdom lighthouse authority, did not employ them, preferring an explosive signal.
Throughout the 19th century, efforts were made to automate the signalling process. Trinity House eventually developed a system (the "Signal, Fog, Mk I") for firing a nitrocellulose charge electrically. However, the charge had to be manually replaced after each signal. At Portland Bill, for example, which had a five-minute interval between fog-signals, this meant the horns had to be lowered, the two new charges inserted, and the horns raised again every five minutes during foggy periods.
Clockwork systems were also developed for striking bells. Struck bells were developed throughout the 1800s with the use of a governor, including the use of a large triangle with 4-foot sides in Maine in 1837. Ships were required to carry bells, with an exemption for Turkish ships because Islam forbade the use of bells.
Captain James William Newton in England claimed to have been the inventor of the fog signalling technique using loud and low notes.
The development of fog signal technology continued apace at the end of the 19th century. During the same period an inventor, Celadon Leeds Daboll, developed a coal-powered foghorn called the Daboll trumpet for the American lighthouse service, though it was not universally adopted.Holland, F. R. (1988). America's Lighthouses. Dover. , p. 204. A few Daboll trumpets remained in use until the mid-20th century.
In the United Kingdom, experiments to develop more-effective foghorns were carried out by John Tyndall and Lord Rayleigh, amongst others. The latter's ongoing research for Trinity House culminated in a design for a siren with a large trumpet designed to achieve maximum sound propagation (see reference for details of the Trials of Fog Signals), installed in Trevose Head Lighthouse, Cornwall in 1913. One reporter, after hearing a steam-powered siren for the first time, described it as having "a screech like an army of panthers, weird and prolonged, gradually lowering in note until after half a minute it becomes the roar of a thousand mad bulls, with intermediate voices suggestive of the wail of a lost soul, the moan of a bottomless pit and the groan of a disabled elevator."
One of the first automated fog bells was the Stevens Automatic Bell Striker.
Some later fog bells were placed underwater, particularly in especially dangerous areas, so that their sound (which would be a predictable code, such as the number "23") would be carried further and reverberate through the ship's hull. For example, this technique was used at White Shoal Light (Michigan). This was an earlier precursor to RACON.
The foghorn's musical connection extended beyond its use of pipe-organ technology. In 1982, for example, the Dutch broadcaster VPRO aired a live "foghorn concert" on national radio, relaying the sound of foghorns from Emden, Calais, Nieuwpoort, Scheveningen, Den Helder, Lelystad, Urk, Marken and Kornwerderzand, mixed with studio music by sound artist Alvin Curran.
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