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This page is about the navigational aid called foghorn, for the Ray Bradbury science fiction short story, see "The Fog Horn". For the cartoon character, see Foghorn Leghorn.

Foghorns near Lizard Point, Cornwall

Foghorns are a navigation aid for mariners. In foggy conditions, when visual navigation aids such as lighthouses are obscured by the weather, foghorns provide an audible warning of rocks, headlands, or other dangers to shipping. The first automated steam-powered foghorn was invented by Robert Foulis of Saint John, New Brunswick, Canada. The first model was installed on Partridge Island in 1859, replacing the less effective bell and cannon which had been formerly used as warnings to ships in fog.

The noise produced by a foghorn is very deep, because deep sounds are audible to human ears farther than higher pitched sounds. It is also very loud so ships a considerable distance away can heed its warning. Legend suggests that Robert Foulis heard his daughter playing piano in the distance on a foggy night, and noticed the low notes were more audible than the higher notes.

All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Many older foghorns, especially those on land, used diaphones to create the audible sound, producing a distinctive, deep and penetrating tone followed by an all-too-audible 'grunt', resulting in the famous two-tone sound that most people associate with foghorns and the sea. Other horns used vibrating plates, similar to a modern electric car horn, or air forced through holes in a revolving cylinder, much in the same manner as a siren.

When diaphone foghorns were still being made, USA lighthouses usually had a two tone horn. Such horns usually sounded an E♭ quarter note followed by an A♭ half or whole note. English-built foghorns were usually single-tone horns, producing a long single note, usually an F or a G. Many Canadian foghorns are single-tone.

Current foghorns are automated. A laser or photo beam is shot out to sea, and if the beam refelcts back to the source, it tells a computer to activate the foghorn.

A Fog signal is a device used in fog to produces an audible warning, and sometimes a visual one too, indicating to a vehicle the presence of a hazard. There are commonly two circumstances where they are used.

Ships and lighthouses warn of their presence in foggy conditions. Until the end of the Nineteenth century this was done variety of means, including gunfire, explosions, bells and steam whistles. However these were gradually replaced by foghorns invented in the 1850s. The replacement was slow in some cases, for example the Fastnet Rock Lighthouse had an explosive fog signal installed as late as 1883 which electrically detonated a small charge of guncotton every five minutes.

Fog signals have also been used on railway lines since the middle of the Nineteenth century to indicate to the driver of a moving train that a broken down train, a working party or some other unforeseen hazard is on the line ahead. They are small explosive detonators or torpedoes which are placed on the track and detonated by the pressure of the wheels of the oncoming train. The loud report of the explosion provides the indication to the driver.

Nautical Portal

• Listen to Golden Gate Bridge foghorn
• a list of foghorn clips from San Pedro, CA.- Foghorn 1 is a good example of the diaphone information from the article above.