bmersible
is submerged, she employs rotary and not reciprocating prime-movers,
being in consequence relatively quiet when running under water, and
inaudible at any considerable distance; the noises of the vessel
carrying the listening devices are difficult to exclude, as are also
the noises of the sea, which are multitudinous; finally, the
sound-receiving instruments are not highly directive, hence are not of
great assistance in determining the position of the object from which
they are receiving sounds.[3]

To locate the submersible, aerial observation has been found useful.
It is particularly so when the waters are clear enough to observe the
vessel when submerged to some depth, but its value is less than might
be supposed in the waters about the British Isles and Northern
Europe, where there is a great deal of matter in suspension which
makes the sea unusually opaque. The submersible, however, when running
along the surface with only its periscope showing, is more easily
detected by aircraft than by a surface vessel. Behind the periscope,
there is a characteristic small wake, which is distinguishable from
above, but practically invisible from a low level of observation. Many
sea-planes are operating on the other side for the purpose of locating
enemy submersibles and reporting their presence to the surface patrol
craft. In order to overcome the disadvantages of creating the
periscope wake which I have mentioned, it is reported that the Germans
have developed special means to allow the U-boats, when raiding, to
submerge to a fixed depth without moving. To maintain any body in a
fluid medium in a static position is a difficult matter, as is shown
in the instability of aircraft. One of the great problems of the
submersible has been to master the difficulties of its control while
maintaining a desired depth. The modern submersible usually forces
itself under water, while still in a slightly buoyant condition, by
its propellers and by the action of two sets of rudders, or
hydroplanes, which are arranged along its superstructure and which
tend to force it below the surface when they are given a certain
inclination; but should the engines stop, the diving rudders, or
hydroplanes, would become ineffective, and, because of the reserve
buoyancy in the hull, the vessel would come to the surface.

In order to maintain the vessel in a state of suspension under water
without moving, it would be necessary to hold an extremely del