358 
iy de 
Comparison with experiment. 
Figure 5 includes al] the theoretical curves for T.N.T., although it is emphasised again 
that they do not originate in precisely the same assumptions, In addition, experimental information 
on pressures very near the charge is included, taken from various published reports. It is believed 
that this covers practically all results that have been reported, at all events within 15 — 20 charge 
radii. The following is a list of instruments and methods used. 
(a) Piezo-electric gauge (Wood's Hole) (2) U.S. Report U.E. 24. 
(b) Pressure-bar (Taylor and Davies) (1) 
(c) Spray-velocity and air-shock wave (Road Research Laboratory) 
(d) Spray-velocity (Wcod's Hole) U.S. Report U.E. 27. 
(2) Optical method (effect on refractive index of water) (Wood's Hole) U.S. Report U.E. 23, 
(n addition, two empirical lines have been included, one according to the law recently 
suggested in the U.S.A. (nomogram issued by Princeton University Station and stated to be based on data 
supplied by U.E.R.L.), and the other according to Wood's formula(t), This latter formula is based cn 
measurements at conparatively large distances. As it does not scale up exactly with radius of charge, 
it has been drawn in for a 300 1b. charge. 
The following comments are made on the various experimental points:— 
(a) The line drawn through the points obtained with the piezo-electric gauge was obtained by 
Kirkwood's extrapolation method(2), of which a preliminary account is given in U.S. Report 
Uses 24 
(0) Although the one point obtained from the pressure—bar gives reasonable agreement with the 
theoretical curves, the experiment indicated a time-constant much greater than 100 
microseconds, whereas theory indicates a time-constant of the order of 50 microseconds. 
(c) The pressure was inferred by two distinct methods, by the velocity of surface spray and 
by the velocity of the shock-front rising into the air from the water surface. These 
two methods agree well except at the 2 charge radii position, where the shock-front is 
probably cbscured by the spray. The lower pcint is obtained on the assumption that the 
sheck-frent velocity in air is the same as the spray velocity. These ©xperiments were 
with C.E. and 5% has been arbitrarily deducted from the readings tc give a camparison 
with T.N.T. 
(d) The spray velocity method is applied in this case to T.N.T. 
(e) in the optical method the shock-wave was used as a lens. The explosive was Tetryl, 
which according to Kirkwood's(3) figures has a theoretical peak pressure distance 
curve almost identical with T.N.T. of density 1.59, According to experimental work 
quoted by Kirkwood (in 0.S.R.D. 2022), Tetryl has a peak pressure distinctly above the 
theoretical, at all events for points near the charge (see Figure 1 of that Report). 
This is in sharp contrast with T.N.T., for which the peak pressures are below Kirkwood's 
curve as is clear from Figure 5 of this report. (One would expect Kirkwood's theory 
to over-estimate the pressure somewhat). It is therefore not clear by what factor the 
Tetryl results should be corrected in order to give a comparison with T.N.T. 
Discussion. 
With the exception of the strange result on the time—constant revealed by the pressure—bar, 
it may be stated that the disagreements between theory and experiment are no greater than the 
discrepancies between the various theories, also that the various methods of measurement agree well 
amony themselves. We are, however, now in a position to trace at any rate some of the causes of 
these discrepancies, and to infer the approximate effect that they have. We may list them for 
Convenience under a number of headings. 
()) coco 
