SCIENTIFIC OFFENSIVE 



Pressure experts cannot talk long about over- 

 pressure without bringing in the Mach Stem, a scien- 

 tific anomaly which makes pressure waves doubly de- 

 structive to houses, ships, or other objects situated just 

 above a large immovable surface. Near the end of the 

 last century an Austrian scientist named Ernst Mach 

 was studying electrical sparks — using apparatus plen- 

 tifully coated with dust and soot. He noticed that 

 whenever a spark was produced, dust was scoured 

 away from a nearby, dust-covered surface. He noticed 

 also that the scoured area was curiously limited in its 

 extent. 



The explanation is now well known. It is worth re- 

 cording here since it had such a great influence on the 

 planning and results of the A-Day explosion at Bikini. 

 When an explosion occurs just above the surface 

 of the ground (or say, just above the surface of a 

 lagoon), the spherical pressure wave spreads in all 

 directions. The part of the wave which strikes the 

 ground is reflected. If the intensity of the direct and 

 reflected waves is not too great, the waves go their sep- 

 arate and easily-predicted ways. But if the pressure 

 waves are very intense, then the whole situation 

 changes : the waves affect one another curiously. Near 

 the ground, in what has come to be called the Mach 

 Stem region, the reflected wave finds itself close on the 

 heels of the direct wave, in fact practically in the direct 

 wave. Now it is a fact that when one intense wave 

 travels more or less within another intense wave, it 



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