eamer'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 hea