16 



HAYES— DETECTION OF SUBMARINES. 



The minimum number of receivers is three. All of the listening de- 

 vices developed at the Nahant Station except the C-Tube, make use 

 of three receivers located at the vertices of an equilateral or isosceles 

 triangle. Such a device is often called a " 3-Spot." Devices of this 

 character developed at the Naval Experimental Station employ four 

 detectors located at the four corners of a square. Such devices are 

 commonly named " 4-Spots." The " 4-Spot " can still be operated 



Plate IV. Diagram of double-groove compensator. 



if any one receiver becomes damaged but the " 3-Spot " becomes use- 

 less under such conditions. 



An instrument called a compensator has been developed by which 

 the path difference between the two receivers and the ears can be 

 varied. Compensators are made in various forms for dififerent spe- 

 cial purposes. Plate IV. shows the principle of one of the simplest 

 forms. It consists of an upper brass casting with grooves as shown 

 by the full lines. This casting seats on a lower brass plate upon 

 which it can be rotated about a central pivot (0). The lower plate 



