There are a great many different kinds of signals for the guidance of vessels during fogs, when lights or other visible signals cannot be perceived.

One of the most powerful signals is the siren fog horn, the sound of which is produced by means of a disk perforated by radial slits made to rotate in front of a fixed disk exactly similar, a long iron trumpet forming part of the apparatus. The disks may each contain say twelve slits, and the moving disk may revolve 2,800 times a minute; in each revolution there are of course twelve coincidences between the slits in the two disks'; through the openings thus made steam or air at a high pressure is caused to pass, so that there are actually 33,600 puffs of steam or compressed air every minute. This causes a sound of very great power, which the trumpet collects and compresses, and the blast goes out as a sort of sound beam in the direction required. Under favorable circumstances this instrument can be heard from twenty to thirty miles out at sea.

Fog signals are also used on railways during foggy weather; they consist of cases filled with detonating powder, which are laid on the rails and exploded by the engine when it runs over them.