ly. As a consequence, early
in 1918 it was found possible to increase the supply very largely, as
many as 30 to 40 per destroyer being carried.

Improvements in the details of depth charges were effected during 1917.
One such improvement was the introduction of a pistol capable of firing
at much greater depths than had been in use before. The result was that
all vessels, whether fast or slow, could safely use the 300-lb. depth
charge if set to a sufficient depth. This led to the abolition of the
Type D* charges and the universal supply of Type D.

In spite of the difficulties of dropping depth charges so close to
submarines as to damage them sufficiently to cause them to come to the
surface, very good results were obtained from their use when destroyers
carried enough to form, so to speak, a ring round the assumed position
at which the submarine had dived. In order to encourage scientific
attack on submarines, a system of depth charge "Battle Practice" was
introduced towards the end of 1917.

It is as well to correct a common misapprehension as to the value of
depth charges in destroying submarines.

Many people held very exaggerated ideas on this subject, even to the
extent of supposing that a depth charge would destroy a submarine if
dropped within several hundred yards of her. This is, unfortunately,
very far indeed from being the case; it is, on the contrary, necessary
to explode the charge near the submarine in order to effect destruction.
Taking the depth charge with 300 lb. weight of explosive, ordinarily
supplied to destroyers in 1917, it was necessary to explode it within
fourteen feet of a submarine to ensure destruction; at distances up to
about twenty-eight feet from the hull the depth charge might be expected
to disable a submarine to the extent of forcing her to the surface, when
she could be sunk by gun-fire or rammed, and at distances up to sixty
feet the moral effect on the crew would be considerable and _might_
force the submarine to the surface.

A consideration of these figures will show that it was necessary for a
vessel attacking a submarine with depth charges to drop them in very
close proximity, and the first obvious difficulty was to ascertain the
position of a submarine that had dived and was out of sight.

Unless, therefore, the attacking vessel was fairly close to the
submarine at the moment of the latter diving there was but little chance
of the attack being successful.

HYDROPHONE