889 
not more than 2% in estimating the momentum of shocks of the 
strengths considered here when they are reflected from rigid walls. 
However, since the pistons, unlike rigid walls, recoil, the velocity 
of the fluid behind them may become high. This may be ae large 
as 4x 10° em/sec for a shock of 300 atmosvheres. (The corres- 
ponding material velocity in the wave is only 2 x 10° em/sec). 
As a result, the force acting on a piston is less than the value 
calculated from linear theory by the amount = A. For example, 
instead of (15.3) one has for the force on the viston 
8 
3 es 7 .2 
2Pa-3R Piz. s BS on 
Thice is shown in peragraph 13. of the apnendix under the incom- 
pressive approximation. When the velocity of the piston is 4 x 
10° cm/sec, the Bernoulli term is seen to be 8 atmosvheres, or 
2% of the total pressure. 
VI, SUMMARY 
Piston gauges are generally used in the side-on orien- 
tation. In this position interpretation of the measurements is 
difficult because of diffraction and irregular reflection; and in 
the same position, experimental dispersion is found to be larger 
than in the head-on orientation. Here the theory has been given 
for the head-on orientation. When a piston recoils it sends out 
a rarefaction. The effects of this rarefaction on the motion of 
its generating piston and on the motion of the other pistons in 
the gauge have been calculated (psragraphe 15. and ]18.). These 
=F 95, 2 
