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travels unusually fast. The result is that the reflected 

 wave tends to catch up with — and even coalesce with 



— the direct wave. Actually the combined wave is no 

 longer spherical, but nearly cylindrical or stem-like. 

 All this happens, of course, only in the limited region 

 where the two waves are initially fairly close together 



— i.e., near the ground. 



The scientific result is that near the ground the 

 pressure is now far greater than could be produced by 

 the direct or reflected waves singly. 



The tactical implications predicted for A-Day were 

 ominous. The experts knew that a large fraction of the 

 great armada of target vessels would find themselves 

 in the Mach Stem region; what had been a scientific 

 curiosity might mean destruction for many warships. 

 Interest on this subject thus rose to an especially high 

 pitch. 



Mere interest, of course, was not enough. Pressure 

 gages were needed. Some of the simplest yet most 

 effective gages were those devised by Dr. W. G. Penney, 

 a British scientist. Back in 1945 when he was work- 

 ing at Los Alamos, he made the daring guess that the 

 humble tin can would prove to be one of the best "in- 

 struments" for measuring the terrific pressures pro- 

 duced by atomic bomb explosions. He soon showed his 

 guess to be correct. He demonstrated that whenever a 

 five-gallon gasoline can was partially crushed by a 

 sudden pressure wave, the degree of crushing depended 

 on the exact intensity of the pressure wave. For ex- 



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