h
the sea splashing and the wind roaring, it was often difficult to hear
anything beyond half-a-mile.

[Illustration: FIG. 6.--Diagram showing essential parts of a portable
hydrophone. _A._ Head and ear pieces, by means of which a trained
listener hears submarine sounds. _B._ Flexible leads to enable an
officer to verify reports from listener. _C._ Battery box, containing
spare set of cells. _D._ Terminals. _E._ Terminals of spare cells. _F._
Flexible armoured electric cable which is lowered over side of ship.
_G._ Metal case protecting the microphone _H_. _H._ Microphone or
delicate receiver of submarine sounds, which is submerged (when
required, but not when ship is moving) to a depth of about 18 feet, as
in small diagram. The sound is detected by the microphone and
transmitted up the cable _F_ and wires _B_ to the ear-pieces _A_.]

In fine weather a submarine could usually be heard at a distance of
about two or three miles. There were, however, many microscopic noises
of the under-seas which were picked up and magnified by this type of
hydrophone. They were called "water noises," and often made it extremely
difficult to differentiate between them and the sound of a moving
submarine at a great distance. Later types were not so prone to these
disturbing influences.

To describe here the different natural and artificial noises heard on a
portable hydrophone is extremely difficult. One general statement can,
however, be made. It is the noise caused by the rapidly revolving
propellers of both surface ships and submarines that is the guiding
factor in the work of detection by submarine sound. A destroyer
travelling at full speed on a calm sea, when heard on a hydrophone
resembles the roar of a gigantic dynamo. The sound does not alter as the
distance between the _stationary_ listening ship and the _fast-moving_
warship increases or decreases; it continues to be a roar or low hum,
according to distance, until it fades out of hearing altogether. The
same statement applies also to a slow-moving cargo steamer, only in this
case the _single_ propeller is revolving very much slower, and, when
listening on a hydrophone about two or three miles distant, each
successive beat of the engines can be distinctly heard.

The simple movement of a vessel's hull through the water cannot be heard
on a hydrophone. Therefore for detecting the presence in the vicinity of
a _sailing_ ship at night or in a thick fog this instrument is quite