Iga eae Just after impact the 
TAIL ,T overpressure in the region, 
aia fae M,between the shock front,S, 
i Mo and the wall,W, is nearly 
2p, where p is the peak 
HEAD, S 
overpressure before colli- 
rN 
i M 
\ 
Tiiaiea aaa WALL ,W overpressure is slightly less 
—— 6 Indicates Velocity 
of Material and it decreases to zero at 
sion; in the region,M,, the 
than p just in front of S 
the tail, T. Wow when S 
reverses direction the medium in front of it changes 
from M to M,. Therefore, as soon as S has reversed direction’ 
the pressure has increased immediately in front of it and 
behind it by about the same amount (p). At the same time 
the medium Mj, now in front of S, is still travelling 
toward the wall. The net result, in the examples considered 
in this report, is that the velocity of S relative to the 
wall is subsonic in air and supersonic in water (where 
"Sonic" refers to undisturbed fluid). Now as soon as S 
has been reversed by the wall, the total disturbance 
may be regarded as an approximate superposition of two 
waves of finite amplitude. One of these moves toward 
the wall and is bounded by T and W. The other moves 
away from the wall and is bounded by W and 3S. These two 
waves penetrate each other until T reaches the wall and 
