emphasizes the
point that vessels at high speed, when moving under water, may, on
account of the momentum attained, submerge to excessive depths. To
eliminate this tendency, there is a hydrostatic safety system which
automatically causes the discharge of water from the ballast-tank when
dangerous pressures are reached, thus bringing the submersible to a
higher level where the pressure on the hull will not be so severe.
From this it follows that the opportunity of ramming a submersible,
or of sinking it by gunfire, is greatly minimized, since the vessel
can disappear so rapidly.

[Illustration: _Photograph from Underwood & Underwood, N.Y._
INTERIOR OF A SUBMARINE]

A great deal has been attempted with nets. Fixed nets extend across
many of the bodies of water around the British Isles. Their positions,
doubtless, are now very well known to the Germans. The problem of
cutting through them is not a difficult one. Moreover, the hull of the
submersible has been modified so that the propellers are almost
entirely shielded and incased in such a way that they will not foul
the lines of a net. There has also been a steel hawser strung from the
bow across the highest point of the vessel to the stern, so that the
submersible can underrun a net without entangling the superstructure.
Some nets are towed by surface vessels. The process is necessarily
slow, and to be effective the surface vessel must know the exact
location of the submersible. Towing torpedoes or high explosive
charges behind moving vessels has been developed by the Italian Navy,
but the chances of hitting a submersible with such devices are not
very great.

Bomb-dropping from aeroplanes can be practiced successfully under
exceptional conditions only. In view of the fact that such
bomb-dropping is exceedingly inaccurate, and that the charges carried
are relatively small, this form of attack ordinarily would not be very
dangerous for the submersible. Surface craft have also employed large
charges of high explosives, which are caused to detonate by
hydrostatic pistons upon reaching a certain depth. Patrol boats carry
such charges in order to overrun the submersible, drop the charges in
its vicinity, and by the pressure of the underwater explosion crush
its hull. Since the pressure of an underwater explosion diminishes
rapidly as the distance increases from the point of detonation, it
would be necessary to place the explosive charge fairly close to the
hull o