It should also be
remembered that any improvement made in the efficiency of the hydrophone
will benefit not only the surface ship, but also the submarine, for it
cannot be supposed that under-water craft will be left without these
wonderful submarine ears when their surface destroyers are equipped with
them.

The alliance between the hydrophone and the depth charge is a natural
one. The former instrument enables the surface ship to discover, first,
the presence of a submarine in the vicinity, and, secondly, its
approximate position. At this point its utility _temporarily_ ceases and
that of the depth charge begins. When a surface ship is hot on the track
of a moving submarine she endeavours to attain a position directly over
the top of her quarry, or even a little ahead, and then releases one or
more depth charges according to whether the chance of a hit is good or
only poor.

From this it will be apparent that whereas the hydrophone is the
instrument used for the initial detection of the submarine, and
afterwards for enabling the surface ship to get to close quarters with
her submerged adversary, it is the depth charge with which the attack is
actually made.

This weapon is really a powerful submarine bomb. It consists of several
hundred pounds of very high explosive encased in a steel shell, with a
special firing device which can quickly be set so that the charge
explodes at almost any depth below the surface after being released from
the above-water vessel.

The methods in use during the war for its release from the decks of
surface ships were very diverse, the most usual being for a number of
these weapons to be fitted on slides and held in place by wire slings
which could be released by simply pulling out a greased pin or bolt.

When the depth charge rolled off the stern of the surface ship it sank
to the "set depth" and then exploded like a submarine mine. The result
was a shattering effect exerted through the water for several hundred
feet around. If the submarine was close to the explosion her
comparatively thin plates were nearly always stove-in. When she was over
a hundred feet away, however, the rivets holding her plates together
were often loosened, and the resulting leak frequently compelled her to
come to the surface, where she could be destroyed by gun-fire.

[Illustration: DROPPING DEPTH CHARGES

_British Official Photograph_]

It often happened, however, that neither one nor the other of these