90 6 
(4) Effect of a 300-lb. Charge of 40/60 Amatol at a discance 
of 50 feet. 
Towards the end of .1917 Mining School carried out a series of experiments in 
which H.M. Submarine 1D). 1 was attacked at different distances with depth charges 
containing 300 lbs. of 40/60 amatol. Roughly speaking, the result of these experi- 
ments was to show that at distances above S() feet the damage was not very heavy, 
while at smaller distances the damage was vital. The gauges for the present experi- 
ments were accordingly designed to be suitable for the region between 45 and 100 feet 
from a charge of this type, and the pressure at a distance of 5!) feet was made the first 
object of investigation. 
The result is shown in Fig. 1, which represents the average of five shots. The 
pressure rises to its maximum intensity (‘80 ton per square inch) almost instan- 
taneously, at most in a few hundred-thousandths of a second ; it-falls in a thousandth 
of a second to a quarter of its maximum intensity, and afterwards continues to fall 
more and more slowly ; after five thousandths of a second there still remains a very 
small pressure. 
The time integral of the pressure 
(= { pat = the area of the time-pressure curve) 
up to five thousandths of a second is I (t = 5 x 10-*) = ‘68, a pressure of one ton 
per square inch for a thousandth of a second being taken as the unit. The whole- 
time integral of pressure, up to the moment when the pressure ceases or becomes 
negative, can only be very slightly greater than this. More than half the time 
integral of pressure occurs in the first thousandth of a second, and more than four 
fifths in three thousandths of a second, I (t = 10-*) = -40, 1 (t = 3 x 107%) = 60. 
A time-pressure curve such as that shown in Fig. 1 gives complete information 
as to what happens in the water when the pressure wave passes. For example, the 
velocity of a particle of the water at any moment is 
De 
ap 
a being the velocity of the pressure wave (Section 6) and p the density of sea-water, 
or 33 p, if the velocity is expressed in feet per second and p in tons per square inch. 
The particle-velocity is greatest in the front of the wave, where the pressure is greatest, 
and there amounts, in the present case, to 26°4 feet per second. Again, the displace- 
ment of a particle from its original position is 
= | pdi, or 0° 40_I, 
if the displacement is expressed in inches and the time integral of pressure I in the 
dimensions defined above; thus in the present case the total displacement of a particle 
by the pressure wave is ‘27 inch. 
The flux ef energy, that is to say, the energy which crosses each unit surface of a 
sphere with the charge at its centre, Is 
F = + | pidt, or F = 774 x 10*| pat 
ap- 
if I"is expressed in foot-pounds per square inchand p in tons per square inch. In the 
present case F (t = 10~*) = 14, F (¢ = 3 x 10) = 15°7, F (t= 5 x 107%) = 16°80. 
Since nearly nine-tenths of the energy passes in the frst thousandth of a second, 
which corresponds to 4 radial distance of about 5 feet, it is clear that nearly all the 
energy of the pressure wave is concentrated in the front of the wave in a layer only a 
few feet thick. The whole energy of the pressure wave, assuming that it springs from 
the charge with equal strength in all directions, is 4 * D°F = 72 x 10° foot-pounds, or 
-24 x 10® foot-pounds per pound of explosive. Since the total energy liberated by the 
