Foghorns

Audible fog signals have been used in one form or another for hundreds of years, initially simply 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). In the United States, whistles were also used where a source of steam power was available, though Trinity House, the English 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; clockwork systems were also developed for striking bells.

Captain James William Newton claimed to have been the inventor of the fog signalling technique using loud and low notes.

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