SCIENCE AND INDUSTRY. 
exceedingly high sensitivity were realized, and after enormous labour, 
ways and means were devised for standardizing their construction 
and their functioning. Hydrophones were constructed and put into 
service which were suitable for use in water of moderate depth, and other 
types were made which could be used in water of great depth. In one 
particular type of instrument modifications and attachments were intro- 
duced which enabled one to detect with it the direction of bearing of a 
source of sound with a fair degree of accuracy. 
But probably the method of determining the direction of a source 
of sound waves in water which has proved to be the best is founded on 
the fact that the sound wave is in the same phase at all points of its 
wave front. Thus, if there were two hydrophones, in themselves non- 
directional, placed in the path of the incoming sound, they can. be used 
for finding the direction of the origin of the sound if the phase difference 
between the sounds received can be detected. There are two ways of 
doing this—the “binaural” method and the “sum and difference” 
method. The binaural method depends on the fact that if the sound 
from one receiver is conveyed to one ear, and that from the second to 
the other ear, the impression is formed that the sound comes from a 
certain direction, and this direction is interpreted by the sensations 
experienced, changes as the phase difference is altered. It can be 
brought to a certain position with respect to the listener—say, to the 
position directly in front—either by rotating the two receivers about 
an axis or by introducing an artificial delay in some form of “ com: 
pensator.” This binaural method, which has great interest from a 
psychological and physiological point of view, has been the subject of 
much work on the part of both British and American scientists, and 
in the anti-submarine campaign it was found to be of very considerable 
service. 
In the “suin and difference” method, the impulses from the two 
receivers are united before reaching the ear, the combined effect observed 
being a maximum when there is no phase difference between the waves 
and a minimum when the phases are in opposition. 
Although an enormous expenditure has been made in time and effort 
to perfect the hydrophone and other listening devices, it is realized that 
such instruments possess an inherent defect. If the submarine can be 
made noiseless in motion, this method of countering it becomes ineffec- 
tive. Even now the range of hearing is not more than 100 yards in the 
case of modern submarines moving at 2 or 8 knots. 
(b) Echo Methods—Owing to the fact that it was found possible 
under certain conditions to render the propulsion of submarines practi- 
cally silent, it became necessary to look in other directions for funda- 
mental methods of detecting them. A system of detection, which is 
full of promise, involves the use of a beam of sound wayes sent out by a 
chasing ship in a manner analogous to the use of a searchlight. With 
such beams of: sound waves, it is possible to sweep the seas, and when 
an object of sound such as a submarine hapnens to come within the 
beam, the sound waves are reflected, and echo effects are obtainable. The 
character of the beam is, of course, determined in large measure by the 
frequency of the waves constituting it. The method has been employed 
with great success, and promises to be a very helpful agent. It can 
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