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



position of this vessel relative to the Von Steuben were determined 

 later by triangulation, using the distance covered during the two 

 hours as a base-line. These results checked closely with determi- 

 nations made by dead reckoning and showed that the lightship was 

 never approached closer than 32 nautical miles and that the greatest 

 range at which the bell signals were heard was about 40 miles. 



Thus within a few hours after leaving Hoboken, it was appar- 

 ently demonstrated that navigation could be effectively safeguarded 

 by the MV hydrophone, since by its aid the direction of submarine 

 bell signals was accurately determined at ranges varying from 15 

 to 40 miles and the propeller sounds of near-by vessels heard and the 

 vessels located thereby at ranges sufficient for avoiding collisions. 

 With such information at his disposal, the navigator should be able 

 to take his vessel into or out of port during fog or other conditions 

 of low visibility. 



The next day it was a matter of disappointment to find that the 

 propeller sounds of the Von Steuben could not be heard in depths 

 much greater than 500 fathoms. This result led to the important 

 discovery that the MV hydrophone is able to give a reliable and con- 

 tinuous record of the depth of water beneath a vessel, steaming at 

 any speed, up to depths of approximately three times the length of 

 the vessel and that this record becomes more and more accurate as 

 the depth of water becomes less. 



Only Sound Reflected from t^e Sea-Bottom is Heard on 

 Hydrophones Locatej> Near the Surface. 



The fact that propeller sounds of the Von Steuben could not be 

 heard in deep water led at once to the conclusion that the hydro- 

 phone is affected only by sound that has been reflected from the 

 sea-bottom. Two explanations of this fact are offered. 



Referring to Fig. i, sound from a source such as a propeller (S) 

 reaches the hydrophone receiver (H) by three different paths, 

 namely: 5 — P — H, S — R — H, and S — O — H respectively. If the 

 distance ^S" — H is great with respect to the distance P — R, that is, if 

 the separation of source and receiver is great compared to their 

 submersion below the surface, then the two paths 5' — P — H and 



