1402 
causing w/3 7p and OQ to decrease. In Figs.9 and 10, QM is 
large and the records show a sharp pressure peak and a decay 
which is definitely exponential. For a smaller Q& (Figs. 1l, 
12, and 13) the positive peak is rounded off so that for the 
shorter durations the exponential nature of the decay is 
completely obscured. For shallow gauge positions the decays 
are even more difficult to reconcile with the usual records. 
Note the double peak on record 57-2 (Fig. 12) and the more 
nearly normal cut-off on 61-1, a subsequent record obtained 
with a different gauge. At such very short durations the finite 
diameter of the gauge must introduce some error and variations 
from gauge to gauge may be responsible for such differences. 
5. High-Pressure Series 
To obtain higher pressures at equally small angles of 
incidence, it was necessary to detonate charges much closer to 
the surface, and therefore all subsequent shooting was done at 
a sheltered fresh-water pond where the surface was generally 
perfectly calm. Here it was possible to maintain accurate 
charge depths as small as 4 in Charge-to-gauge distances 
were then feasible such that w./3/R at the surface point varied 
from 1.78 (similitude pressure, 75,900 psi) at g@ = 46.4° down 
to 0,326 (pressure, 6900 psi) at M = 7.6°, 
A. Varying Gauge and Charge Depths. With a constant 
charge-to-gauge distance or 5-6 in, a series of shots as 
completed for charge and gauges at the EY, depth, D, which was 
varied from 4 to 10 in. The value of wl/3/R at the surface point 
half way between charge and gauge varied from 0.382 to 0.429 
over this range of depths corresponding to a free-water 
pressure variation from 9400 to 10,200 psi. Thus, while the 
pressure did not change more than 8%, the angle of incidence 
was varied from 10.1° to 23.9°, Table III contains a summary 
of pressure and duration results thus obtained, and Figs. 5 and 6 
represent the results graphically. 
From these results it may be seen that at a depth of 4 in, 
and for an angle of incidence of 10.1° the derived peak pressure 
at the gaugs is only 0.70 of the free-water similitude value for 
the same W 3/R. For increasing angles of incidence, the pressure 
gradually increases to about 93% of the free-water value at 10 in. 
and 23.9°. Under the same conditions the measured duration at 
10.1° is 1.59 times higher than the acoustic theory predicts 
and as the angle is increased the duration approaches the 
theoretical value. For angles larger than 17° the experimental 
results approximate the acoustical values rather well, 
Figures 14 and 15 show typical piezoelectric records for this 
series of shots. These photographs show much less rounding off of 
peaks than was seen for the low pressure shots. The difference is 
doubtless due to the smaller diameter gauges used. These figures 
show how the decay curve becomes more and more exponential as 
is increased. 
iG 
