or the use of other defensive means before the
submarine would be able to execute an attack.

At the present moment it would appear that the most dangerous enemy
of the submarine yet discovered is the airplane or the dirigible.
Some figures as to the mortality among submarines due to the efforts
of aircraft have been published in an earlier chapter. The chief
value of aircraft in this work is due to the fact that objects under
the water are readily discernible at a considerable depth when
viewed from a point directly over them. An illustration familiar to
every boy is to be found in the fact that he can see fish at the
bottom of a clear stream from a bridge, while from the shore the
refraction of the water is such that he can see nothing. From the
air the aviator can readily see a submarine at a depth of fifty feet
unless the water is unusually rough or turbid. The higher he rises
the wider is his sphere of vision. With the lurking craft thus
located the airman can either signal to watching destroyers or may
bide his time and follow the submarine until it rises to the
surface, when a well placed bomb will destroy it. Both of these
methods have been adopted with success. For a time the submarines
were immune from this form of attack because of the difficulty of
finding a bomb which would not explode on striking the surface of
the water, thus allowing its force to be dissipated before it
reached the submarine, or else would not have its velocity so
greatly checked by the water that on reaching the submarine the
shock of its impact would not be great enough to explode it at all.
Both of these difficulties have been overcome. The new high
explosives have such power, taken in connection with the fact that
water transmits the force of an explosion undiminished to a great
distance, that many of them exploding at the surface will put out of
action a submarine at a considerable depth. Furthermore bombs have
been invented, which being fired, not merely dropped from an
airplane, will go through the water with almost undiminished
momentum and explode on striking the target, or after a period fixed
by the assailant. Other bombs known as "depth bombs" are fitted with
flanges that revolve as they sink, causing an explosion at any
desired depth.

About the actual achievements of the airplane as a foe to submarines
there hangs a haze of mystery. It has been the policy of the Allied
governments to keep secret the record of submarines