Fog-signals, audible warnings used on board ships, on the sea-coast, or on railways, during fogs and mists, or at any other time when lights or ordinary daylight-signals are not available.
The commonest fog-signal on shipboard is the ringing of the ordinary time-bell at frequent intervals, or striking the anchor with a hammer, together with the occasional discharge of musketry and heavy guns. These are adopted, to prevent collisions, when ships are overtaken by a fog in the British Channel, or other places where shipping is abundant. The blowing of a horn, the beating of a drum, an empty cask, or a gong, and various other unusual sounds are also adopted. Steamers and sailing-vessels at anchor sound bells at intervals not exceeding two minutes; sailing-vessels under weigh sound a fog-horn, and steamers sailing their whistle. These sounds, however, only indicate rudely the position of the ship, and not the direction in which she is sailing. Many plans have been devised for a code of signals, by which the directions north, south, &c. might be indicated by the varying length of each sound, or the intervals between the sounds of a fog-horn or whistle.
British ships of war, when under steam, sound a prolonged whistle, at intervals of not less than two minutes. When under sail, one blast with the fog-horn indicates that the vessel is on starboard tack; two blasts in succession, that she is on the port tack; these are to be repeated at intervals of not less than two minutes. When at anchor the bell is sounded instead of horn or whistle. It is very desirable that some general code of signals of this kind should be adopted for the merchant service as well as the navy, and that its recognition by the marine of all other nations should be procured. The admiralty have such a code for the direction of a fleet of ships of war in thick weather, but its application is limited to the navy. See SIGNALS.
Fog-signals from the shore are very desirable, especially on a dangerous coast. Bells and guns have both been used for the purpose, but when a strong wind is blowing in towards the shore their sound is heard only at a very short distance out to sea. Consequently steam-whistles, and fog-horns sounded by compressed air, are being employed in their stead. Fog-sirens, producing a sound after the manner described in the article SIRENE, are now largely in use, and are audible at a distance of from two to ten miles.
The fog-signals used on railways are small cases charged with detonating powder, and laid upon the rails. They explode loudly when the wheel of an advancing train comes upon them. They are used not merely in fogs, but in all cases of danger from obstruction of the line, or in other cases of urgency when a train has to be stopped without delay. Station-masters and railway police are furnished with them.