The sound signals generally used to guide mariners, especially during fogs, are, with certain modifications, sirens, trumpets, steam-whistles, bell-boats, bell-buoys, whistling buoys, bells struck by machinery, cannons fired by powder or gun cotton, rockets, and gongs.
Gongs.—Gongs are somewhat used on lightships, especially in British waters. They are intended for use at close quarters. Leonce Reynaud, of the French lighthouse service, has given their mean effective range as barely 550 yards. They are of most use in harbors, short channels, and like places, where a long range would be unnecessary. They have been used but little in United States waters. The term "effective range" is used here to signify the actual distance at which, under the most unfavorable circumstances, a signal can generally be heard on board of a paddle-wheel steamer in a heavy sea-way.
Guns.—The use of guns is not so great as it once was. Instances are on record in which they were quite serviceable. Admiral Sir A. Milne said he had often gone into Halifax harbor, in a dense fog like a wall, by the sound of the Sambro fog gun. But in the experiments made by the Trinity House off Dungeness in January, 1864, in calm weather, the report of an eighteen-pounder, with three pounds of powder, was faint at four miles. Still, in the Trinity House experiments of 1865, made in light weather with a light gun, the report was clearly heard seven miles away. Dr. Gladstone records great variability in the range of gun-sound in the Holyhead experiments. Prof. Henry says that a twenty-four-pounder was used at Point Boneta, San Francisco Bay, Cal., in 1856-57, and that, by the help of it alone, vessels came into the harbor during the fog at night as well as in the day, which otherwise could not have entered. The gun was fired every half hour, night and day, during foggy and thick weather in the first year, except for a time when powder was lacking. During the second year there were 1,582 discharges. It was finally superseded by a bell-boat, which in its turn was after a time replaced by a siren. A gun was also used at West Quoddy Head, Maine. It was a carronade, five feet long, with a bore of five and one-quarter inches, charged with four pounds of powder. The gun was fired on foggy days when the Boston steamer was approaching the lighthouse from St. Johns, and the firing was begun when the steamer's whistle was heard, often when she was six miles away, and was kept up as fast as the gun could be loaded, until the steamer answered with its whistle.
The report of the gun was heard from two to six miles. "This signal was abandoned," Prof. Henry says, "because of the danger attending its use, the length of intervals between successive explosions, and the brief duration of the sound, which renders it difficult to determine its direction with accuracy." In 1872 there were three fog guns on the English coast, iron eighteen-pounders, carrying a three pound charge of powder, which were fired at intervals of fifteen minutes in two places, and of twenty minutes in the other. The average duration of fog at these stations was said to be about six hours, and as it not unfrequently lasted twenty hours, each gun required two gunners, who had to undergo severe labor, and the risk of remissness and irregularity was considerable. In 1881 the interval between charges was reduced to ten minutes.
The Trinity House, in its experiments at South Foreland, found that the short twenty-four pound howitzer gave a better sound than the long eighteen-pounder. Tyndall, who had charge of the experiments, sums up as to the use of the guns as fog-signals by saying: "The duration of the sound is so short that, unless the observer is prepared beforehand, the sound, through lack of attention rather than through its own powerlessness, is liable to be unheard. Its liability to be quenched by local sound is so great that it is sometimes obliterated by a puff of wind taking possession of the ears at the time of its arrival. Its liability to be quenched by an opposing wind, so as to be practically useless at a very short distance to windward, is very remarkable.... Still, notwithstanding these drawbacks, I think the gun is entitled to rank as a first-class signal."
The minute gun at sea is known the world over as a signal of distress. The English lightships fire guns to attract the attention of the lifeboat crew when shipwrecks take place in sight of the ships, but out of sight of the boats; and guns are used as signals of approaching floods at freshet times in various countries.
Rockets.—As a signal in rock lighthouses, where it would be impossible to mount large pieces of apparatus, the use of a gun-cotton rocket has been suggested by Sir Richard Collinson, deputy-master of the Trinity House. A charge of gun-cotton is inclosed in the head of a rocket, which is projected to the height of perhaps 1,000 feet, when the cotton is exploded, and the sound shed in all directions. Comparative experiments with the howitzer and rocket showed that the howitzer was beaten by a rocket containing twelve ounces, eight ounces, and even four ounces of gun-cotton. Large charges do not show themselves so superior to small charges as might be expected. Some of the rockets were heard at a distance of twenty-five miles. Tyndall proposes to call it the Collinson rocket, and suggests that it might be used in lighthouses and lightships as a signal by naval vessels.
Bells.—Bells are in use at every United States lightstation, and at many they are run by machinery actuated by clock-work, made by Mr. Stevens, of Boston, who, at the suggestion of the Lighthouse Board, has introduced an escapement arrangement moved by a small weight, while a larger weight operates the machinery which strikes the bell. These bells weigh from 300 to 3,000 pounds. There are about 125 in use on the coasts of the United States. Experiments made by the engineers of the French Lighthouse Establishment, in 1861-62, showed that the range of bell-sounds can be increased with the rapidity of the bell-strokes, and that the relative distances for 15, 25, and 60 bell-strokes a minute were in the ratio of 1, 1-14/100, and 1-29/100. The French also, with a hemispherical iron reflector backed with Portland cement, increased the bell range in the ratio of 147 to 100 over a horizontal arc of 60°, beyond which its effect gradually diminished. The actual effective range of the bell sound, whatever the bell size, is comparatively short, and, like the gong, it is used only where it needs to be heard for short distances. Mr. Cunningham, Secretary of the Scottish Lighthouse Establishment, in a paper on fog signals, read in February, 1863, says the bell at Howth, weighing 2¼ tons, struck four times a minute by a 60 pound hammer falling ten inches, has been heard only one mile to windward against a light breeze during fog; and that a similar bell at Kingston, struck eight times a minute, had been so heard three miles away as to enable the steamer to make her harbor from that distance. Mr. Beaseley, C.E., in a lecture on coast-fog signals, May 24, 1872, speaks of these bells as unusually large, saying that they and the one at Ballycottin are the largest on their coasts, the only others which compare with them being those at Stark Point and South Stack, which weigh 31¾ cwt. and 41½ cwt. respectively. Cunningham, speaking of the fog-bells at Bell Rock and Skerryvore lighthouses, says he doubts if either bell has been the means of saving a single vessel from wreck during fog, and he does not recall an instance of a vessel reporting that she was warned to put about in the fog, or that she ascertained her position in any respect by hearing the sound of the bell in either place. Gen. Duane, U.S.A., says a bell, whether operated by hand or machinery, cannot be considered an efficient fog signal on the sea-coast. In calm weather it cannot be heard half the time at a greater distance than one mile, while in rough weather the noise of the surf will drown its sound to seaward altogether. The use of bells is required, by the International Code, on ships of all nations, at regular intervals during fog. But Turkish ships are allowed to substitute the gong or gun, as the use of bells is forbidden to the followers of Mohammed.
FIG. 1.—COURTENAY'S WHISTLING BUOY.