168 
III. PRESSURE nave 8 
1. OBSERVATIONS 
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Pm = 13,000 — pounds per square inch 
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(c) Velocity. The velocity of the wave is nearly that of sound, within the 
range specified. 
(d) Regularity and Symmetry. The pressure wave does not vary much from one 
charge to another of the same kind and size, and it is spherically symmetrical, pro- 
vided the charge is approximately symmetrical. When the charge is decidedly elon- 
gated or flattened, the pressure wave is not quite the same in different directions 
from the charge. The observed differences due to this cause, or to initiation of the 
detonation on one side, can be explained qualitatively by imagining the pressure wave 
to be made up of component waves emitted by the various parts of the charge, and then 
allowing for the differences in the time of travel of the component waves. Thus the 
wave from a long rod detonated at one end would be strongest but of shortest duration 
at points lying on the prolongation of the axis and in the direction of travel of the 
detonation wave, and weakest but of greatest duration in the opposite direction. If 
a maximum effect were desired in a particular direction, the best shape would probably 
be a curved disk, concave toward the side of the given direction, detonated at the 
center. 
Surrounding a 320-pound charge of amatol with four times its own volume of 
air at atmospheric pressure produced little effect on the pressure wave. 
(e) Reflections. When the charges were fired rather close to the surface of 
the water, the pressure wave was observed to be cut off at a time corresponding to 
the arrival of the wave reflected from the surface of the water. According to acous- 
tic theory, the reflected wave should consist of a rarefaction which is the mirror 
image in the pressure axis of the incident pressure wave, atmospheric pressure being 
taken as zero. 
Actually, although the positive pressure instantly disappeared at the calcu- 
lated moment of arrival of the reflected wave, the maximum negative pressure observed 
did not exceed 90 pounds per square inch below the hydrostatic pressure at the level 
of the gauge. There is known to be a rather low limit to the negative pressure that 
water can stand, without the occurrence of cavitation, when it is in contact with 
solid objects. Very likely cavitation occurred around the gauges used in these ob- 
servations and the true negative pressure occurring in the water was not indicated. 
Reflection from the bottom was also observed. The pressure reflected from 
a mud bottom was only 0.4 times that in the incident wave. On the other hand, when 
a charge of 1000 pounds of TNT was laid directly on a sand bottom at 10 fathoms, the 
pressure wave was not much less than that to be expected from a 2000-pound charge 
surrounded by water. This is understandable, for the situation in question could be 
imitated roughly by passing a rigid diaphragm through the center of a 2000-pound 
