1564 
16 e 
least part of the observed attenuation at very long ranges to 
non-linear reflection. 
15-3 It was pointed out previously that when an acoustic wave 
in water is reflected from a water-air interface, in view of 
the higher sound velocity in the water, reflection is regular 
for all angles of incidence. The reflected wave is undistorted 
but is slightly attenuated due to a very small transmission of 
sound into the air. Even in the extreme case of normal incidence 
this amounts to a decrease of less than 0.01% in amplitude. Thus 
no observable attenuation can be attributed to this cause. 
15.4 Roughness in the sea surface (which prevailed to some 
extent during all the measurements) undoubtedly produces some 
incoherent scattering of the reflected wave due to the essentially 
chaotic character of surface wave motion considered on the scale 
pertinent here- However, the remarkable sharpness of the front 
of all reflected waves measured up to ranges of 5000 feet (see 
Figure 2) makes it clear that scattering due to surface roughness 
is not an important means of producing distortion of the surface 
reflection in these particular measurements. 
Bulk cavitation 
16.1 The phenomenon called by Kennard!’ "pulk cavitation" has 
a most important effect on the structure of the reflected wave. 
If the negative pressure of the reflection is great enough the 
water experiences an insupportable tension and is torn into many 
bubbles, giving rise to a region of cavitation which has a rapid 
