1565 
subsequent history of expansion and contraction. The cavitation occurs 
within a space lying near the surface and having a large horizontal 
extente This distorts and attenuates the reflected wave which is 
causing it to an extent depending on the vigor of the cavitation. A 
quantitative theoretical description of the character of the reflected 
wave both during the cavitation and after it emerges into homogeneous 
water is very difficult, involving as it does, wave propagation 
through a medium containing randomly-distributed time-dependent inhomo- 
geneities which have been initiated under conditions not yet adequately 
described. 
16.2 The surface reflections in the present series of experiments 
frequently show the following characteristics: first, a sharp drop, of 
duration approximately equal to the transit time across the gauge; this 
forms the leading side of a brief, but strong negative pressure spike; 
the pressure then rises somewhat and oscillates erratically at a constant 
level for a few tenths of a millisecond and eventually merges into the 
succeeding event. In his theory of bulk cavitation, Kennard assumes that 
cavitation occurs whenever the pressure in the water falls to a character- 
istic fixed "breaking pressure", then instantly becomes equal to a 
characteristic "cavity pressure" which cannot be less than the 
breaking pressure, and finally cavitation disappears instantly when 
the pressure rises above the cavity pressuree Our observations seem 
to support this qualitative picture. Whether or not the breaking 
and cavity pressures are fixed magnitudes however, cannot be 
determined. 
= 89 - 
