884 
Here m is proportional to the gauge time, and n is proportional 
to the thickness of the block. A graph of 4 against oe is 
shown in Fig. 5. The values of d are seen to be very large. 
The equation (21.1), or Fig. 5, specifies the thickness of a 
rigid block for which the relief pressure due to recoil is not 
more than 1% of the total pressure during the response of the 
gauge, 
22. It is next necessary to realize that the block is 
actually soft rather than rigid. One then arrives at a second 
criterion for an effectively infinite block. Let a = velocity 
‘of sound in the block, and let d = its thickness. A shock wave 
falling on the front face of the block does not return until- 
a time, Oa» has passed, where 
(22.1) Sarsre 
8 SS 
If @,> oP (the resvonse time of the gauge), the gauge does 
not receive a signal from the back face until after it has 
registered. Hence the pellet deformations are the same as they 
would have been if the back face had been at infinity. There- 
fore a soft block is effectively infinite, if 
(22.2) Je = 
23. In the apnendix, paragraph ll], the two criteria, (21.1) 
SAT Bhs. 
