president's address. 13 



mining opei-ations. Acoustic tubes, microphones, and amplifying valves 

 have been employed to increase the volume of very faint noises. 



For many years before the war the Bell Submarine Signalling Com- 

 pany, of which Sir WiUiam White was one of the early directors, used 

 submerged microphones for detecting sound transmitted through the 

 water, and a submerged bell for sending signals to distances up to one 

 mile. With this apparatus passing ships could be heard at a distance 

 of nearly a mile when the sea was calm and the listening vessel 

 stationary. 



Of all the physical disturbances emitted or produced by a moving 

 submarine, those most easily detected, and at the gi'eatest distance, 

 are the pressure waves set up in the water by vibrations produced by 

 the vessel and her machinery. A great variety of instruments have 

 been devised during the war for detecting these noises, depending on 

 microphones and magnetophones of exceedingly high sensitivity. Among 

 them may be particularly mentioned the hydrophones devised by Captain 

 Eyan and Professor Bragg, being adaptations of the telephone trans- 

 mitter to work in water, instead of air. These instruments, when 

 mounted so as to rotate, are directional, being insensitive to soundwaves 

 whose front is perpendicular to the plane of the diaphragm, and giving 

 the loudest sound when the diaphragm is parallel to the wave front. 



Another preferable method for determining direction is to use two 

 hydrophones coupled to two receivers, one held to each ear. This is 

 called the biaural method, and enables the listener to recognise the 

 direction from which the sound emanates. 



When the vessel is in motion or the sea is rough the water noises 

 from the dragging of the instrument through the water and from the 

 waves striking the ship drown the noises from the enemy vessel, and 

 under such conditions the insti-uments are useless. The assistance of 

 eminent biologists was of invaluable help at this juncture. Experiments 

 were made with sea-lions by Sir Eichard Paget, who found that they have 

 directional hearing under water up to speeds of six knots. Also Professor 

 Keith explained the construction of the hearing organs of the whale, 

 tlie ear proper being a capillary tube, too small to be capable of per- 

 forming any useful function in transmitting sound to the relatively large 

 aural organs, which are deep set in the head. The v/hale therefore hears 

 by means of the sound waves transmitted through the substance of the 

 head. It was further seen that the organs of hearing of the whale to 

 some degree resembled the hydrophone. 



The course now became clear. Hollow towing bodies in the form 

 of fish or porpoises were made of celluloid, varnished canvas, or very 

 thin metal, and the hydrophone suitably fixed in the centre of the 

 head. The body is filled with water, and the cable towing the fish 

 contains the insulated leads to the observer on board the vessel. When 



