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versus-time gages, pressure values are measured me- 

 chanically, by small pistons moving against springs. 

 In others, the principle of operation is electro-mechan- 

 ical. The pressure wave moves a piston whose motion 

 is opposed by a wire. The wire is thus strained to an 

 extent depending on the pressure, and the wire's elec- 

 trical resistance changes correspondingly. It is this 

 electrical change which is actually recorded and later 

 interpreted in terms of pressure. 



The five thousand pressure gages were placed with 

 great care. The majority were placed topside on target 

 vessels. Many were placed in especially unencumbered 

 positions so that the pressure values recorded would 

 not be invalidated by confusing reflections of the pres- 

 sure wave. (Nearly every part of the ships' super- 

 structures acted as a crude pressure-wave reflector, 

 creating compound, hard-to-interpret, pressure situ- 

 ations.) Gages capable of recording very high pres- 

 sures were mounted on the ships nearest the Zeropoint ; 

 gages of lower range were used on ships farther out. 

 Gages designed to measure relatively low pressures 

 were placed inside turrets, control rooms, and living 

 quarters to show the pressures crews might experience. 

 Even airplanes scheduled to be aloft near the target 

 area were fitted with special gages. 



Nobody expected all the gages to deliver valuable 

 data. A high ''disappointment rate" was anticipated. 

 Some of the gages might fail to start ; others might be 

 sunk with the ships they were on ; others might be dam- 



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