887 
tensions sufficient to break the water. The conditions near a 
piston gauge avpear quite different. The pistone are of very 
small diameter and relatively thick. It has already been seen 
that the rerefactions which they produce are weak, and in addition 
they fall off ss 1/r. The argument may be made quantitative in 
terme of the Kirkwood criterion (1), which vredicts no cevitation 
¥f, 
(26.1) B25 1s, 
where R 
c 
L 
Pp 
length of piston 
, density of vieton 
0,72, in 8 
or 
n= fob = damping time of piston 
fe 
This criterion is based on the fact that the total pressure at 
the face of the piston does not become negative after the incom- 
pressive eolution has begun to hold. Since this hapnens at the 
time 6 the pressure falls to zero, if it does at all, before 
p’ 
e The time required during the compreseive regime for the 
p° 
preesure to reach zero is oe. The condition for no csvitation, 
therefore, is that 6, < ee The constant G- has been called the 
cavitation time. This test is really only a test for "Taylor 
cavitation", 1.e., oavitation at the liquid piston interface. 
The Kirkwood criterion is admittedly only rough, but it has been 
checked very well by the exneriments reported in reference (14), 
ae 
