(c) 
(a) 
1039 
obvious that good pictures can be taken under these circumstances. A large edition 
of the conventional flash charge was used and the equipment, including flash charge, 
depth charge and camera was strung out on a line supported by floats. The line was 
stretched out by a small boat, 
Pressure of shock wave determined from shock wave velocity. -- One measurement of 
average shock wave velocity was made by a double exposure technique. Two flash 
charges were employed and set off about 246 hase apart. It was intended to 
obtain this time difference accurately by me4ns of piezoelectric gages strapped 
to the two flash charges and recording on an oscilloscope. This part of the experi- 
ment failed, and the time difference can be estimated only from the length of 
primacord used to delay the second flash charge. Figure 124 shows the shock wave 
at the two positions and a steel scale 15.66 in, long. The shock wave came from a 
third charge to the left of the field of view. The average velocity determined 
from this shot is about 5180 ft./sec., or about 270 ft./sec. greater than acoustic 
velocity for the conditions of the experiment. This corresponds to a pressure of 
about 8,000 lbs./in.2 averaged over the time interval. The average pressure over 
this period as determined from piezgelectric gage measurements under similar 
conditions is about 11,000 lbs./in.~. Considering the inaccuracy of the time 
interval measurement, this agreement seems setisfactory. It is to be noted that 
since the excess velocity (over acoustic) determines the pressure of the shock 
wave, the accuracy of the measurement of the total velocity must be considerably 
greater then the accuracy desired for the pressure to be determined. 
Experiments showing that the apparent positions of the shock wave coincides 
closely with its actual position. -- Three experiments have shown that, under 
the conditions ordinarily employed at UERL, the position of an underweter shock 
wave is within 1/2 in. of the position indicated by its flash photograph. 
In one experiment piezo gages were used to obtain the time interval between the 
time of the photographic flash and the time the shock wave reached a knowm radius. 
From the knovm velocity of the wave it was possible to calculate its true radius 
at the time of the photograph and compare it with the value computed from the 
photographic image. 
The 250 gm sphere of cast pentolite which produced the shock wave to be photo- 
graphed was detonated simultaneously with the initiation of a 46 in. length of 
primacord leading to the flash charge. The time of the photograph was thus about 
185 / sec after the start of the shock wave. A small tourmaline gage fastened 
to the shock wave charge served to turn on the oscillograph spot for a rotating 
drum camera, while a similar gage on the flash charge signalled the time of the 
photograph. A third gage 17 1/4 in. from the shock wave charge noted the arrival 
of the shock wave at this radius. The diagram shows the arrangement used. i 
+= positions of piezo gauges 
Camera 
[ +#e-—___—_—_ 
+18 
Figure 125. 
The radius at the time of the photograph was 14.7 + 0.2 in. as calculated from 
the positions of the gages and the measured time intervals. Values for the 
velocity of sound were taken from "Tables of the Velocity of Sound in Pure Water 
and Sea Water for use in Echo Sounding and Sound Ranging" (Second Edition) by 
D. J. Mathers. These were corrected for the effect of finite pressure in the 
57 15518 
