SECONDARY PRESSURE WAVES 



387 



theories which have been considered and the reason for its occurrence 

 is somewhat of a puzzle. 



It is probably significant that the anomalous pulse occurs for depths 

 such that the calculated maximum radius of the bubble is very nearly 

 equal to the depth of the charge below the surface. Kirkwood has 

 suggested that at the stage of maximum expansion the bubble sucks in 

 air from the atmosphere above ; further reaction of the gaseous products 

 then evolves energy which is available for the later motion. Photo- 



Fig. 9.8 Peak pressure and positive impulse 4 feet from 0.55 pound TNT 

 charges fired in 24 feet of water. 



graphs of the bubble under these conditions show a funnel shaped region 

 between the top of the bubble and the water surface, which may indi- 

 cate air venting into the interior of the bubble, but conclusive proof of 

 the true reason for the phenomenon has not been found. The critical 

 nature of the effect makes its practical significance somewhat dubious. 

 B. Large charge results. The experimental difficulties in making 

 bubble pulse measurements under controlled conditions present a seri- 

 ous problem in large charge measurement, as discussed in section 9.5. 

 As a result, only meager data have been collected in comparison with 

 small charge results, although a number of scattered observations have 

 accumulated as a by-product of other tests on service weapons.* The 



^ A number of references to such work will be found in the report by Slichter, 

 Schneider, and Cole (103). 



