Fog signal
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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.
[edit] Marine fog signals
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.
[edit] Railway fog signals
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.
