When a squadron of chasers leaves its base for the zone it is to patrol it is split up into two divisions of six boats each and a _division officer_ is in charge of each one. Each chaser is given a certain area to patrol and she works with all the other chasers in her squadron, the shore station and ships at sea. If a U-boat has been sighted at sea, the ship who has picked her up immediately sends a wireless message to the base which in turn informs the commander of the squadron. Should a U-boat venture into one of our zones the chasers get as busy as hornets and scout around until she either slips away or comes to the top to enable her commander to take a look around through his periscope to see if there is a ship in sight worth using a torpedo on. Besides the regular wireless set each submarine chaser is fitted with _a sound conduction_ _signalling system_ and this is used to detect the presence of a U-boat when it is submerged and cannot be seen, though to do this the enemy boat must be near-by. This conduction scheme is very simple and you’ll get me fine as I explain it. [Illustration: Telephone receiver with iron rods] Water, as you know, conducts sound waves to much greater distances than air does. You must have often made the experiment when in swimming of ducking your head under water and listening while another fellow would strike a couple of stones together under water at a distance of thirty or forty feet away from you; 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 reaches the second submarine they impinge on the disk of the microphone when it vibrates; this varies the battery current flowing through the microphone and you hear the dots and dashes in the receiver.