as the propellers of a
submarine, can be distinguished from the natural noises of the sea
water.

Now it will at once be apparent that not only does a stationary ship
offer a splendid target for under-water attack, but also it allows a
somewhat humorous game of hide-and-seek to be played between a hunting
vessel and a hunted submarine.

Nearly all U-boats were fitted with a number of hydrophones and
therefore were as well able to receive timely warning of an approaching
surface ship as the surface ship was of the presence of the submarine.
But the surface ship had the advantage of speed.

The result of all this was that when a German submarine heard a surface
vessel approaching she dived to the bottom, if the water was not too
deep or the sea-bed too rocky. Then shutting off her engines she
listened. The surface ship, mystified by the sudden cessation of the
noise she had been pursuing, also waited, and this stagnation sometimes
lasted for hours. Then if the surface ship moved, as she was often
compelled to do in order to avoid drifting with the tide away from the
locality, the submarine moved also, and the one that stopped her engines
first detected the other, but could not catch up to her again without
deafening her own listening appliance. In which case the next move would
probably be in favour of her opponent.

All of this is, perhaps, a little complicated, but a moment's pause for
reflection will make this curious situation clear to the reader. And so
the game went on, with decisive advantage to neither the surface ship
nor the submarine. Darkness usually intervened and put an end to further
manoeuvring, frequently allowing the submarine to escape.

A case of this kind occurred to a vessel, of a certain hydrophone
flotilla, commanded by the author. For over four hours the U-boat eluded
the pursuing surface ships by moving only when they moved and stopping
when they too had stopped, darkness and a rising sea eventually
favouring the escape of the submarine, which, a few hours later, was
able to attack (unsuccessfully) a big surface ship less than thirty
miles distant from the scene.

Nevertheless the hydrophone is a submarine instrument with a brilliant
future. It has already been improved out of all resemblance to its
original self, and more will undoubtedly follow. It is, however, purely
an appliance for the detection of submarines when cruising beneath the
surface, and not a weapon for their destruction.