and yet you could hear them click as they struck each other as plainly as you could in air a couple of feet away.
Now, signalling between submerged submarines or a submarine and a chaser is carried on on exactly this same principle, that is by the conduction of sound waves through the water. To do this kind of wireless signalling each submarine has a high-frequency sound producing apparatus, or oscillator as it is called, attached to the hull. It consists of a diaphragm, or disk, that is set into very rapid vibration by means of an electromagnet, just as the diaphragm of a telephone receiver is made to vibrate by its electromagnet.
The disk, or diaphragm, which is very much larger than that of a telephone receiver, sets in the water and when it is made to vibrate by closing the circuit with the key it sends out trains of sound waves to considerable distances through the water.
The other submarine, or chaser, is fitted with a like disk which is fixed to a microphone, or telephone transmitter, and to this a battery and telephone receiver is connected. When the high frequency sound waves from one submarine