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.