516 
eel yics 
In the first series (i) 18% lbs. G.C. Charges were used. Here it was found that Prax is only 
very slightly affected by depth; when the gauge is one foot from the surface the value of eas is only 
10% lower® than at 15 feet depth, The effect on momentum and energy is, however, more serious at such 
shallow depths, for the reflected pulse arrives near the point of maximum pressure and a Considerable area 
of the pulse is therefore cancelled. At depths greater than 5 feet, however, the values of ape ff pdt 
and Jj pe dt are practically independent of depth of the gauge. 
In the second series (ii) 100 Ibs. T.N.T. charges poured filled, were used, the depth of the gauge 
in all cases being greater than 10 feet. It was found as in the previous case, that the values of P 
M and E are practically constant so long as the depth of the charge exceeds about 5 feet. The effect of 
surface reflection is most marked In relation to momentum, since after a certain interval, depending on 
the difference between the direct and reflected paths of the wave, has elapsed the pressure is suddenly 
cut off by the surface-reflected wave. Unless the depth and difference of path are small, however, 
(less than 5 or 10 feet} this effect becomes negligible. { 
As a result of these experiments therefore, we may conclude that the depths of the charge and the 
gauge are practically immaterial provided they exceed a minimum of value of about 10 feet #4. In this 
statement it is of course implied that bottom reflections are negligible, the bottom also being at a 
minimum distance of 10 feet from either gauge or charge. 
The discussion of surface and bottom reflections is reserved for a later section (see Section 5). 
3. Comparison of Explosives. Poured and Block Filling. ‘End’ and ‘Central’ 
initiation. 
T.N.T., Amato] 40/60 and gun cotton charges of the same weight give.equal values of aaa moment um 
and energy. 
A comparison of ‘poured’ and ‘block’ fillings, and of ‘end’ and ‘central’ initiation revealed no 
certain difference in these quantities. 
4. The Form of the Pressure Pulse. 
Before going on to discuss the experimental evidence relating to the form and magnitude of the 
pressure-t ime sequence in the explosion wave emanating from different sizes of charge under varying conditions, 
it may be of some assistance to consider the probable course of events when a charge is detonated under 
water. We shall assume in the first place that the charge is a sphere of radius ‘r' centrally detonated 
and enclesed in a metal case of known elasticity, density and thickness. If the rate of detonation is 
taken as 7000 metres/second % the time taken for the whole of the charge to detonate will be "/7000 seconds 
for a 100 Ibs. T.N.T. charge of radius approximately 0.2 metres this time is 2.9 x 10-5 second, It might 
reasonably be assumed that the velocity of detonation if not greater than, is at any rate equal to the 
velocity of propagation of an elastic pressure pulse through the charge. Consequently it would seem ; 
probable that the pressure on the inside of the charge case will rise instantaneously to its maximum value at \ 
the instant when all the charge is detonated and is converted into incandescent gas. The charge case and 
surrounding water now begin to move radially, starting from zero velocity and rapidly rising to a maximum. 
That is, the pressure pulse transmitted into the water will have a finite time of rise to its maximum value, 
this time of rise probably depending on a number of factors viz: (1) the maximum pressure instantaneously 
applied to the inner wall of the charge case, (2) the strength (elastic constants) of the material of the 
charge case, (3) the inertia of the charge case and surrounding water which is set in motion; it is probable 
also that the viscous resistance of the water plays an important part in the ‘degradation’ of the pressure 
pulse near the charge where the displacement amplitude is large. G. W. Walker formulated a theory of rise 
of pressure. His estimate of the time of rise of pressure in an H.2 mine cases is surprisingly near the 
observed .eoess 
This is based on a single observation. It should be notea that under similar conditions 
individual variations up to 30% have been observed. 
# This statement strictly speaking applies only to such distances apart of charge and gauge as 
are met with in the present investigation. 
* T.N.T. 6950 metres per sec. 
Ory G.C. 7998). * ae 
Wet " 5820 L] bt J 
