the submarines with every appliance
that science can produce. In order to attack the submarine directly
with any weapon, it is necessary first to locate it. This is a problem
presenting the greatest difficulty, for it is by their elusiveness
that the submarines have gained such importance in their war on
trade. They attack the more or less helpless merchant ships, and
vanish before the armed patrols appear on the scene.

Almost every suitable appliance known to physics has been proposed for
the solution of the problem of submarine location and detection. As
the submarine is a huge vessel built of metal, it might be supposed
that such a contrivance as the Hughes induction balance might be
employed to locate it. The Hughes balance is a device which is
extremely sensitive to the presence of minute metallic masses in
relatively close proximity to certain parts of the apparatus.
Unfortunately, on account of the presence of the saline sea-water, the
submersible is practically shielded by a conducting medium in which
are set up eddy currents. Although the sea-water may lack somewhat in
conductivity, it compensates for this by its volume. For this reason,
the induction balance has proved a failure.

But another method of detecting the position of a metallic mass is by
the use of the magnetometer. This device operates on the principle of
magnetic attraction, and in laboratories on stable foundations it is
extremely sensitive. But the instability of the ship on which it would
be necessary to carry this instrument would render it impossible to
obtain a sufficient degree of sensitiveness in the apparatus to give
it any value. The fact that the submersible is propelled under water
by powerful electric motors begets the idea that the electrical
disturbances therein might be detected by highly sensitive detectors
of feeble electrical oscillations. The sea-water, in this case, will
be found to absorb to a tremendous extent the effects of the
electrical disturbance. Moreover, the metallic hull of the submersible
forms in itself an almost ideal shield to screen the outgoing effect
of these motors.

Considerable and important development has been made in the creation
of sensitive sound-receiving devices, to hear the propeller vibrations
and the mechanical vibrations that are present in a submersible, both
of which are transmitted through the water. There are three principal
obstacles to the successful use of such a device: when the su