The development of fog signal technology continued apace at the

The development of fog signal technology continued apace at the end of the 19th Century.[4] During the same period an inventor, Celadon Leeds Daboll, developed a coal-powered foghorn called the Daboll trumpet for the American lighthouse service, though it was not universally adopted.[5] A few Daboll trumpets remained in use until the mid 20th century.

All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, like the Daboll trumpet, used vibrating plates or metal reeds, a similar principle to a modern electric car horn. Others utilised air forced through holes in a revolving 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 open the valves admitting air to the horns; each horn was given its own characteristics to help mariners identify their position.[6]

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