Fog Signals, signals made during foggy weather to prevent collisions, shipwrecks, and other accidents. On board ships it is usual to ring the time-bell at short intervals, and to beat a gong, fire musketry, or produce some other striking sound. The use of such variable signs can but vaguely indicate the whereabouts or the course of a ship, and it has been proposed to adopt a general code for all classes of vessels like that employed in the British navy. Here a bell signifies that a ship is a.t anchor, and a long whistle repeated within two minutes that she is approaching under steam; while a single blast on the foghorn gives notice that she is sailing on the starboard tack, and two blasts together repeated within two minutes that she is on the port tack. For warning ships off a dangerous coast, foghorns and whistles are more efficacious than bells or guns, since a strong wind blowing landwards will prevent the latter from being heard at any distance from the shore. The most powerful foghorn is the "Siren." It is furnished with two discs, each containing twelve radial slits, one of which revolves in front of the other, where a large trumpet of cast iron is attached. The moving discs make 2,800 revolutions per minute, so that the slits are together 33,600 times every minute; on each occasion compressed air is made to pass through them, and afterwards reverberates through the trumpet, producing a powerful sound, sometimes audible at a distance of 10 miles. The fog-signals used on railways consist of detonating caps, which explode when an engine passes over them.