515 
-19- 
the distance D and preSsure p belng measured in feet and 1bs/sq.in. respectively. 
Converting to C.G.S. units v2 f™ p2 dt = 5.85 x 1026 per 1b. wt. of explosive. whence we see 
0 
that the total energy in the pressure wave is +Z x 5.85 x 104 = 4.72 x 1042 ergs. per 1b. weight of 
pC 
charge. 
Comparing this with Sir R. Robertson's statement (Journal Chemical Society Vol. 119 Part | 1921) 
that the chemical energy per gramof T.N.T. is 9g4 Calories, Corresponding to 17,5 x 10 12 ergs. per 1b., 
we at once obtain the interesting result that 27% of the chemical energy of the TNT. appears in the 
pressure wave. 
It is also interesting to observe that the relative chemical energies per gram of T.N.T, amatol 
(40/60) and gun cotton, are given as 924, 920 and 892 respectively. These values only differ by about 3%. 
It will be Seen from the graph in Figure 18 that the energies in the pressure waves from T.N.T. amato) and 
gun cotton are almost the same - a result in good agreement with the chemical data. 
Summarising the principal experimental results to which reference has been made in this section, 
we find that the piezo electric-records indicate 
(1) Pmax = 12,000 w*387p1-00 
(2) hee pdt = 14.90 we 8575 
° 
ape 1.00,.2 
(3) f° p@ at = 13200 wh 0 
fe] 
where W is weight of charge in 1bs., distances D are measured in feet and pressures | 
in 1bs/sq.in. 
o 2 (oo) 
(Note — Momentum = f pdt Energy = 270° ff p2 dt) 
° C ) 
(4) About one quarter of the chemical energy of the explosive appears in the pressure wave. 
(5) Relations (1) (2) (3) and (4) are equally applicable for T.N.T. Amato] (40/60), or 
gun cotton. 
Effect of Depth of Charge and Gauge. 
In the foregoing experiments on the variation of spill and — with weight of charge w, particular 
Care was taken to ensure that the charges and gauges were suspended at a known depth below the surface. 
For the purpose of comparison of charges of different weights it was considered desirable, on general 
grounds, that the depth of submergence of the Charge and gauges should be approximately proportional to the 
cube root of the weight of the charge taking 23 feet for 100 1b. charge as standard. Previous to this, 
however, it had been ascertained that, except at very small depths, the influence of depth on the pressure 
was almost if not entirely negligible. It will be seen in what follows that this was confirmed by 
subsequent experiments. It is to be expected, of course, that as the depth of the charge or the gauge 
becomes small in comparison with their distance apart, the ‘negative’ wave reflected from the surface will 
interfere with the direct positive wave received by the gauge. As the depth diminishes this effect will 
become more and more serious but it will only be at very shallow depth that the pressure maximum will be 
affected, as we shalj see, The experimental observations may be divided into two sections. 
(i) In which the depth of the charge is kept constant whilst the depth of gauge 
is varied. 
and (ii) In which the depth of gauge is kept constant and the depth of charge is varied. 
IM sacese 
