98 14 
from a hemispherical charge resting on the bottom would obviously be the same as 
that from a spherical charge of the same diameter in mid-water, that is to say, the 
charge on the bottom would give the same effect as a charge twice as heavy with 
water all round it. A hard rock bottom should approximate to this extreme, while 
sand or mud should give a similar but smaller effect. 
Experiments were made by firing three 1,000-lb. T.N.T. charges on a sand 
bottom in 10 fathoms, about 14 miles off Irvine; the pressure wave was measured at 
a distance of 75 feet from the centre of the charge, in a direction making an angle of 
about 60° with the vertical. In the case of shot 78 (Fig. 22) the result approximates 
to the calculated effect of a 2,000-Ib. T.N.T. charge in mid-water ; in shot 75 (Fig. 21) 
the pressure was considerably less, but stil) greatly in excess of the effect of the same 
charge in mid-water (Fig. 20); the third shot gave results intermediate between the 
other two. 
The differences between the results of these three shots fired on the bottom were 
much greater than was ever observed in the case of shots fired in mid-water; this is 
not altogether surprising, seeing that the local configuration of the bottom and 
the depth of sand covering the underlying rock were quite likely different in all three 
cases. 
(11) Effect of surrounding the charge with an Air Chamber. 
Fig. 28 shows the time-pressure curve at a distance of 50 feet from the centre of 
an H 2 mine (a spherical mild steel shell 38 inches in diameter and } inch thick, with 
a charge of 320 lbs. of 40/60 amatol in a central container, as shown in Fig. 46). 
Comparing Fig. 283 with Fig. 1 it will be seen that both the maximum pressure and the 
time-integral of pressure of the air-surrounded charge are slightly less than for the 
naked charge, but the difference in both respects is not much more than 5 per cent. 
This is rather surprising, seeing that the volume of air is nearly four times the volume 
of explosive. 
Some H 2 mines filled with an additional 500 lbs. of 40/60 amatol (as at X 7, 
Fig. 46) gave the result shown in Fig. 29, which is very nearly the same as the 
calculated effect of the same charge in naked form. 
(12) Comparison of different Explosives. 
Figs. 12, 1, and 7 give a comparison between T.N.T. (tri-nitro-toluene), 40/60 
amatol (ammonium nitrate 40, T.N:T. 60) and 80/20 amatol (ammonium nitrate 80, 
T.N.T. 20). Jt will be seen that T.N.T. gives practically the same effect as 40/60 
amatol, except that the maximum pressure is about 5 per cent. lower. Allowing for 
the slight difference in the weight of explosive, 80/20 amatol and T.N.T. give 
practically identical results. A comparison between 300-lb. charges of 40/60 amatol 
and 50/50 amatol showed no difference. 
Figs. 30 and 31 show results obtained with guncotton and ammonium perchlorate 
charges. These results are not directly comparable with Figs. 1, 7 and 12, as the 
charges were made up in very thick walled mines (spherical mild steel shells, 
33 inches in diameter and 3%; to } inch thick). The maximum pressure in both cases, 
but especially for the guncotton charge, is a good deal lower than for amatol ; this 
may be partly due to the thickness of the mine shell. On the other hand it is 
noticeable that the pressure of the ammonium perchlorate charge is very well 
sustained ; the whole time-integral of pressure is decidedly higher than for amatol ; 
with charges of equal weight and similar make-up the difference would be even more 
marked. 
(13) Gunpowder Charges. 
Fig. 32 shows the results obtained with some 500-lbs. charges of E.X.E. powder. 
This is a very slow-burning powder, density 1°8, pressed into hexagonal prisms 
14 inches across the flats and with a central hole about ‘4 inch in diameter. The 
charge was built up of these prisms in layers, and was fired by a central igniter, 
