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96 THE SUBMARINE TORPEDO BOAT
fast as in air, or about 4700 feet per second. The principle
of the submarine signalling device is to produce sound
waves under water by means of a sending apparatus on
the one hand, and to detect these waves by means of a
receiving apparatus on the other.
Submarine bell and sounding
mechanism
Submarine sound receiving ap-
paratus fixed to side of hull
The sound waves affect the receiving apparatus by set-
ting up vibrations which are transmitted by a micro-
phone and wires to an ordinary telephone receiver in the
hands of the listener. It is quite possible with this set
of instruments to detect the presence of a ship when still
some distance away, lhe distance and direction of the
ship picked up can be judged fairly closely by the intensity
of the vibrations set up in the microphone. It only
needs the addition of an accurate direction indicator to
reach perfection; in which case the submarine though
blind under water will be pretty well equipped to meet a
submarine adversary.