The fog horn (speaking trumpet or tube, cf. tur.: boru /bo-‘ru/) is a device, which is used in order to warn passing ships of impending danger due to geographical elements such as reefs, shoals and shores, or to passing boats under low visibility conditions in those cases when other visual aids, such as lighthouses, are of no use. The Norwegian-type fog horns were used during the 19th up to mid-20th century; they operated manually through a handle, which rotated by storing air in the device, the release of which under pressure conditions created sound. The basic advantage of this type of horn was that it could produce prolonged loud sound manually.