foghorn

thumb|right|Two roof-mounted diaphone foghorns at [[Split Rock Lighthouse]] 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. The term is most often used in relation to marine transport. When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to shipping.

==Description== All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, such as the Daboll trumpet, used vibrating plates or metal reeds, a similar principle to a modern electric car horn. Others used air forced through holes in a rotating 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 sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them.