Foghorn

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.

When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to shipping.

[2] 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.

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.

[3] 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.

From the early 20th century an improved device called the diaphone, originally invented as an organ stop by Robert Hope-Jones[9] and developed as a fog signal by John Northey of Toronto, became the standard foghorn apparatus for new installations worldwide.

[12] Since automation of lighthouses became common in the 1960s and 1970s, most older foghorn installations have been removed to avoid the need to run their complex machinery, and have been replaced with electrically powered diaphragm or compressed air horns.

In many cases, modern navigational aids, including GPS, have rendered large, long-range foghorns completely unnecessary, according to the International Association of Lighthouse Authorities.