nd comprised the surface warship's
principal means of offence against submarines operating beneath the
surface.

The hydrophone resembles a delicate telephone. It is so constructed that
when the instrument is lowered over the side of a ship into the sea any
noise, such as the movement of a submarine's propellers, can be heard on
deck by an operator listening at an ordinary telephone receiver
connected to the submerged microphone by an electrified wire.

There were many different types of hydrophone in use during the Great
War. So important was this instrument for the work of submarine hunting
that money was spent in millions, and a corps of naval and civil experts
were engaged for several years, bringing it to a state of efficiency.
Each type introduced into the Service was an improvement on its
predecessor, and there were different patterns for the use of almost
each class of vessel. The fast destroyer required a different
instrument to the slow-moving trawler. The motor launch could only
employ successfully a totally different type to the submarine, and, to
add to the difficulties, the German submarines themselves were
generously supplied with similar instruments. The games of
"hide-and-seek" played on and under the seas with the aid of this
wonderful little instrument would have been distinctly amusing had men's
lives--and often those of women and children--not been dependent upon
the issue.

The portable hydrophone, used by some of the smaller and slower vessels
of the auxiliary fleet, consisted of a microphone, or delicate
mechanical ear, carefully guarded by metal discs from accidental damage,
and connected to ear-pieces or ordinary telephone receivers by an
electric wire which passed through a battery. Where the wire came in
contact with the sea water it was heavily insulated and lightly
armoured.

When it was required to use this instrument the vessel was stopped and
the microphone lowered overboard to a depth of about 20 feet. This was
the distance down from the surface at which submarine noises could be
heard most distinctly. The operator on deck or in the cabin then
adjusted the ear-pieces and sat listening for any noises coming through
the water. Although the sea is a far better conductor than air, the
range at which sounds could be heard varied considerably. On a calm day
or night the noise of a ship's propellers could frequently be
distinguished at from five to seven miles; whereas on a rough day, wit