1b 99 
consisting of a tin containing fine grain powder with a fulminate of mercury 
detonator. The time-pressure curve o: this charge is an interesting contrast to that 
of a high explosive such as amatol. The pressure builds up slowly to a maximum 
which is not much more than 3';th of the maximum pressure given by an equal weight 
of'amatol at the same distance ; on the other hand the pressure lasts much longer ; it 
does not reach its maximum until a time when the pressure from the amatol charge 
would practically have disappeared. 
The surface above the charge showed no sign of breaking at the moment of 
explosion; the usual white dome was entirely absent ; the tension in fact was too 
weak to break the water and the pressure wave was completely reflected. It was 
possible therefore to observe the first stages of the arrival of the burnt gases at the 
surface, which is masked, in the case of high explosives, by the dome of broken water 
thrown up by the pressure wave. ‘The first effect, which appeared at a distinct 
interval after the moment of explosion, was the heaping up of a small mound of green 
water (Fig. 56); this was probably a sort of water-piston pushed up ahead of the 
ascending gas bubble; a moment later the gases burst through, forming plumes not 
unlike those given by a high-explosive charge (Fig. 57). 
(14) Composite Charges. 
Some composite charges were made by lashing together three 30U-lb. charges of 
40/60 amatol. Each 300-lb. charge was a cylinder 28 inches long and 18 inches in 
diameter; the three charges were lashed with their axes horizontal and parallel, so 
as to give a figure of three touching circles in cross-section ; only the top charge was 
primed, the two others being fired by the explosion of the first. The result, shown in 
Fig. 24, approximates very closely to the calculated effect of a single 900-lb. charge. 
(15) Influence of the Shape of the Charge. 
Experiments on this point were made by lashing together three 300-lb. charges 
of 40/60 amatol end-to-end. Hach charge was a cylinder 28 inches long and 
18 inches diameter, so that the composite charge had a length of nearly five diameters. 
Owing to the dished shape of the ends of the charges there was an average distance 
of about 3 inches between the amatol in one charge and the amatol in the next. 
One of the end charges was primed, the other two being fired by the explosion of the 
first. The composite charge was hung horizontally, and pressure measurements were 
taken in three directions, (1) in line with the axis of the charge from the primed end 
towards the unprimed end, (2) in line with the axis of the charge from the unprimed 
end towards the primed end, (3) at right angles to the axis of the charge. All three 
sets of gauges were at the same distance from the centre of the charge. The results 
are shown in Figs. 25, 26, and 27. It will be seen that the pressure is strongest but 
least sustained in the broadside direction, and weakest but most sustained in the 
direction opposite to that in which detonation proceeds. 
These results admit of a simple explanation; in the broadside direction the effects 
of the three charges arrive simultaneously, or nearly so, while in the endwise 
directions they arrive more or less in turn; but there is a difference between the two 
endwise directions, because in the direction in which detonation proceeds the effects 
of the three charges arrive more nearly simultaneously than in the opposite direction, 
the time taken by detonation in travelling from one end of the charge to the other 
being in the first case subtracted from and in the second case added to the time taken 
by the pressure wave in travelling the same distance in water. 
Comparing Figs. 25, 26, 27 with Fig. 24, it appears that the three charges lashed 
end-to-end give on the whole a less powerful effect than when bunched together. It 
is probable that, for general purposes, the best disposition of a given weight of 
explosive is in spherical form, which most nearly enables the effects of all its parts to 
arrive at any given point simultaneously. On the other hand, if a maximum effect is 
desired in a single direction the best shape would probably be a flat disc, the effect of 
which should be greatest in the direction of its axis. 
