icate
balance between the weight of the submarine and that of the water
which it displaces. Variations in weights are so important to the
submersible that, as fuel is used, water is allowed to enter certain
tanks to compensate exactly for the loss of the weight of the fuel. To
obtain such an equilibrium, an automatic device controlled by the
pressure of the water, which, of course, varies with the depth, is
used. This device controls the pumps which fill or empty the
ballast-tanks, so as to keep the relation of the submersible to the
water which it displaces constant, under which condition the vessel
maintains a fixed depth. The principle of this mechanism is, of
course, old, and was first embodied in the Whitehead torpedo, which
has a device that can be set so as to maintain the depth at which it
will run practically constant. With the addition of a telescopic
periscope, which can be shortened or extended at will, it will be
possible for the U-boat to lie motionless with only the minute surface
of the periscope revealing her position.

IV

To attack the submersible is a matter of opportunity. It is only when
one is caught operating on the surface, or is forced to the surface by
becoming entangled in nets, that the patrol has the chance to fire
upon it. Against this method of attack, modern submersibles have been
improving their defenses. To-day, they are shielded with armor of
some weight on the superstructure and over part of the hull. They are
also equipped with guns up to five inches in diameter, and, affording,
as they do, a fairly steady base, they can outmatch in gun-play any of
the lighter patrol boats which they may encounter.

One of the important improvements which have been made has resulted in
the increased speed with which they now submerge from the condition of
surface trim. A submersible of a thousand tons displacement will carry
about five hundred tons of water ballast. The problem of submerging is
mainly that of being able rapidly to fill the tanks. On account of the
necessity of dealing with large quantities of water in the ballast
system, the European submersibles are equipped with pumps which can
handle eight tons of water per minute.

Again, the speed which the electrical propulsion system gives the
vessel on the surface greatly increases the pressure which the diving
rudders can exert in forcing the submersible under water. This effect
may be so marked that it becomes excessive, and Sueter