ENGINEERING BEFORE AND AFTER WAR 45 



being greater than that given by any other means of 

 location. A single good set of observations could be re- 

 lied upon to give the position of an enemy gun to about 

 50 yards at 7,000 yards range. It could also be carried 

 on during considerable artillery activity. 



The apparatus for localizing noises transmitted through 

 the ground has been much used for the detection of enemy 

 mining and counter-mining operations. 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 Sig- 

 naling Company, of which Sir William White was one of 

 the early directors, used submerged microphones for de- 

 tecting sound transmitted through the water, and a sub- 

 merged 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 greatest 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, depend- 

 ing on microphones and magnetophones of exceedingly 

 high sensitivity. Among them may be particularly men- 

 tioned the hydrophones devised by Captain Ryan and 

 Professor Bragg, being adaptations of the telephone trans- 

 mitter to work in water, instead of air. These instru- 

 ments, when mounted so as to rotate, are directional, be- 

 ing insensitive to sound waves whose front is perpendicu- 

 lar 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 



