250 



25 



Figure 15 - The Dome, 53 feet high, raised by a Charge of 1900 pounds 

 of Amatol, detonated 64 feet below the Surface 



This photograph is from Hilliar, Reference (7), Figure 62. 



the surface, the lesser mass of water under the troughs will be accelerated 

 more violently than the greater mass under the crests, but the difference in 

 the accelerations will be greater during the pressure phase than during the 

 subsequent tension phase because the initial differential motion tends to 

 smooth out the waves or even to reverse them. The initial troughs should 

 thus tend to be thrown up as spray. 



An indirect method of determining whether or not cavitation occurs 

 under the surface is by studying the reflected wave of tension itself. In 

 the absence of cavitation, this should be a reversed replica of the incident 

 wave, reduced somewhat by the greater distance of travel. If, however, cav- 

 itation occurs, only the very short initial part of the tension wave as pro- 

 duced at the surface, containing the rapid drop to the breaking-pressure p^, 

 will continue traveling below the level at which the breaking-front halts. 

 It is readily seen that the lower boundary of the cavitated region should 

 stand still thereafter as a stationary boundary, as described on page 5. 

 For, as noted on page 8, 2p' = p^ + pcv^ when the breaking-front halts, where 

 p' is the positive pressure in the incident wave, and thereafter 2p' < p^ + pcv^ 

 as p' decreases, so that the condition for a stationary boundary as stated 

 on page 7 is met. The tail of the incident wave will be reflected from this 

 boundary as a tension wave in which the pressure is p" = p^ - p'. Thus the 

 total reflected wave as it occurs below the region of cavitation will be 

 qualitatively as sketched at C in Figure 14. 



This conclusion is in general harmony with a series of piezoelec- 

 tric observations reported in 1924 (8). Only relatively small tensions were 



