11 95 
point in the water, and closer consideration shows that the rule is applicable in 
the region of disintegration itself, as well as below. 
To test this theory, four 267-lb. charges of 80/20 amatol were fired.at a depth of 
344 feet, and pressure measurements were made at depths of 14 feet, 24 feet, 54 feet, 
10 feet, and 344 feet below the surface, the direct distance from the centre of charge 
to the gauges being 50 feet in every case. ‘The time-pressure curve at a depth of 
344 feet is shown in Fig. 7, which represents the average of the four shots. The 
results at other depths are shown in Figs. 8 to 11; in these figures the rectangular 
steps represent the experimental results, but the curves are simply copies of the curve 
in Fig. 7, with the pressure cut off at times corresponding to the theory ; for example, 
with the gauges at a depth of 10 feet the difference between the direct and 
reflected paths AB and ACB, Fig. 2, is 12-2 feet, corresponding to 2°5 x 10-* second, 
and the pressure is represented as ceasing at this moment. The good agreement 
between the curves drawn in this way and the rectangular steps derived from the 
experimental results is evidence of the correctness of the theory. 
Further confirmation was obtained by firing a 1,000 lb. T.N.T. charge at a depth 
of 514 feet and measuring the pressure at a point 40 feet below the surface and 
250 feet distant from the charge. The experimental results are shown by the 
rectangular steps in Fig. 23, while the curve in the same figure is derived from the 
curve shown in Fig. 20, which represents the pressure from the same charge at a 
distance of 75 feet, by reducing the ordinates in the ratio 345;. The difference 
between the direct and reflected paths is 16 feet, and the pressure is therefore 
represented as ceasing at the moment t= 3°25 x 10-*. It will be seen that in this 
case also there is good agreement between the experimental rectangles and the 
theoretical curve. 
The effect of varying the depth of the charge is covered by the same principle. 
At whatever depth a charge is fired, within practical limits, the pressure wave 
springing from it must have exactly the same character, for even at a depth of 
1,000 feet the hydrostatic pressure is less than 2 ton per square inch, and the 
density and elasticity of the water are practically the same as at the surface. The 
only effect of firing the charge deep is to remove it far from the reflecting action of 
the surface. 
A comparison of Figs. 1, 3, 4 and 5, all of which refer to 300 lb. charges of 
40/60 Amatol, shows that the pressure wave is the same whether the depth of the 
charge is 34} feet or 20 feet; with the charge at a depth of 10 feet the first part of 
the pressure wave is the same, but after 2°3 x 10-5 seconds the reflected tension 
wave arrives at the gauges and obliterates the remaining pressure; with the charge 
at a depth of 5 feet the pressure is all over in about 14 thousandtlis of a second. 
To complete this series of experiments a charge of the same sort was fired on the 
surface, slung between two barrels so as to be about three-quarters submerged. The 
result is shown in Fig. 6. ‘The maximum pressure reaches nearly its full value, but 
the pressure disappears very quickly, being practically all gone in half a thousandth 
of a second. 
In many instances the gauges gave direct evidence that the pressure in the water 
is followed by a state of tension or cavitation—see, for example, Figs. 4 to 6 and 
8 to 11. Attention may also be drawn to the figures for Shot 55 in Table VI. 
(Section 18), from which it will be seen that the pistons in the last gauge of the series 
were practically brought to rest before reaching the end of their travel, owing to the 
state of tension following the pressure wave. ‘he tensions recorded were never very 
high however, the most that was observed being °044 ton per square inch, or 6°7 
atmospheres (Fig. 11). It is probable that cavitation occurs more readily at the 
surface of the gauges than in the water itself, so that the gauges record less than the 
full tension. The zero pressure line in the diagrams represents the normal hydro- 
static pressure at the depth at which the gauges are hung; in Fig. 11 for example, 
the gauges being at a depth of 10 feet the normal pressure is 1°3 atmosphere or *(08 
ton per square inch, and a pressure indicated as “044 ton per square inch below 
normal represents only °036 ton per square inch actual tension, or 5°4 atmospheres 
negative. 
(9) Comparison of Large and Small Charges. 
The experiments on this point were based on the following theory. Suppose that 
detonation is started simultaneously at corresponding points in two charges which are 
exactly similar except that the linear dimensions of one are R times as great as the 
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