useless. The same drawback applies also to the location of a floating
derelict or iceberg, and restricts the use of the hydrophone to
faithfully reporting the presence of power-driven ships or special sound
signals at a range of a few miles.

[Illustration: FIG. 7.--An improved directional hydrophone fitted
through keel of motor launch. The tube _B_, at the lower extremity of
which is the microphone, can be raised or lowered from _C_, the cabin of
the M.L. This instrument is so arranged that the direction from which
the submarine sound is coming can be simply and quickly ascertained.]

A German submarine heard at a range of about a mile on a calm night
presents a curious sound which almost defies description. Its principal
constituent consists of a "clankety clank! clankety clank!" at first
barely distinguishable from the low swish of the water past the face of
the submerged microphone, then louder when the sound has been
distinguished and the human ear is on the alert. But when this sound was
heard in war there was little time for analysing or noting. It was the
call to action. The microphone was hauled to the surface and the chase
began, a halt being made every half-mile or so for a further period of
listening on the hydrophone. If the sound was louder the commander of
the pursuing vessel knew that he was on the right track, and if the
sound came up from the sea more indistinct the course was changed and a
run of a mile made in the opposite direction, when the vessel was again
stopped and the instrument dropped overboard.

Should this manoeuvre have placed the surface ship in close proximity
to the submarine, one or more _depth charges_ were released, and if the
explosion of these damaged the comparatively delicate hull or machinery
of the under-water craft, she had either to rise to the surface and
fight for her life with her two powerful deck guns, or, if badly
damaged, sink helplessly to the bottom, emitting oil in large quantities
from her crushed tanks.

Before entering upon a description of the depth charge, however, there
is more to say of the hydrophone, which has played such an important
part in the defeat of the U-boats.

When the advantages of this instrument had been fully demonstrated in
the stern trial of war, successful efforts were made to improve upon the
original crude appliances. The "water noises" were reduced and, greatest
improvement of all, the hydrophone was made "directional." By this is