HAYES— U. S. NAVY MV TYPE OF HYDROPHONE. 379 



plicated if the line of hydrophone receivers is not parallel with the 

 surface and with the center-line of the vessel, or if the sea-bottom 

 is not horizontal. This complication, however, consists largely in 

 the addition of constant terms. Pitching of the vessel or variation 

 of the sea-bottom from a horizontal plane causes the angle ^ to vary 

 somewhat from the ideal conditions cited but, as will be shown later, 

 such variations do not seriously interfere with the determination of 

 depth. 



Both Vertical and Horizontal Angles Determined by MV 



Hydrophone. 



The MV hydrophone measures the angle included between a line 

 passing through the receivers and an intersecting line defining the 

 direction of the approaching sound. From conditions of symmetry 

 the latter line can be rotated around the former without changing 

 the required compensation. Otherwise stated, the same compen- 

 sator setting will serve for sound traversing any element of the 

 conical surface so generated. The angle of the cone is determined 

 always by the compensator setting and if the line of receivers is 

 parallel to the ship's keel in a vertical plane and to the water surface 

 in a horizontal plane, the same compensator scale will serve for 

 determining the direction with respect to the ships keel of sounds 

 traversing either of these planes. 



Errors Introduced by Pitch and Roll Readily Eliminated. 



In general the hydrophone receivers are located along a line par- 

 allel with the ship's keel and at the same depth as the propellers. 

 Under such conditions the compensator measures directly 6, the sup- 

 plement of the sounding angle $ — provided the vessel " rides on an 

 even keel." Rolling of the vessel will not affect the determination 

 of this angle but pitching will. 



The angle of pitch for large vessels is very small except during 

 unusually rough surface conditions and, except at such times, the 

 error introduced by pitching of the vessel need not be considered. 

 It has been found in practice that for large vessels the pitching mo- 

 tion is sufficiently slow to allow the operator to measure the sounding 



