886 
25. If the incident preesure is Pe by ordinary scoustic 
theory the velocity of the front face is 
COR) ween he ik 
Po *Pe 
This is correct only for times not exceeding 24/o,; but by 
hypotheeis the gauge action is over by then. The corresponding 
diminution in pressure due to softnese is by (19.2), if e,/°, 
is mede << 1, 
(25.9) 2p ¢'p, 
Pope 
If the block is eteel, the correction 1e avvroximately 3.6% of 
2p,- The expreseion (25.2) ie correct only for a block of in- 
finite radius. It supposes that all parte of the front face move 
inwards at the same velocity in response to the seme pressure. 
This, of course, ig not true, since the pressure, and hence the 
velocity, at the center are grester than the corresponding 
quantities at the edge, where they are diminished by diffraction. 
The expression (25.2) therefore gives an uvper limit for the effect 
of softness. 
V. OTHER EFFECTS \ 
£26. Some digecussion will be given to show that there is no 
cavitation in front of the recoiling pistons. It will then 
apnear that the same is true of the block. Conditions favorable 
for the development of cavitation are found in front of large, 
thin plates, which are given rather high velocities by imninging 
shocks. The retreating pvlates send out rarefactions which cause 
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