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 foggy conditions. The term is most often used in relation to marine transport. When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to shipping. Description All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, such as the Daboll trumpet, used vibrating plates or metal reeds, a similar principle to a modern electric car horn. Others used air forced through holes in a rotating cylinder or disk, in the same manner as a siren. Semi-automatic operation of foghorns was achieved by using a clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them. History Early fog signals Audible fog signals have been used in one form or another for hundreds of years, initially simply fog bells or gongs struck manually. At some lighthouses, a small cannon was let off periodically to warn away ships, but this had the obvious disadvantage of having to be fired manually throughout the whole period the fog persisted (which could be for several days). Lighthouse windows and lighting apparatus were susceptible to damage depending on the proximity of the explosion. One incident of lax handling of explosives nearby resulted in a concussion that propelled the lighthouse keeper at Fort Amherst, who was seated, to the other end of the room. In the United States, whistles were also used where a source of steam power was available, though Trinity House, the British 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 gun-cotton 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. Stricken bells were developed throughout the