29 113 
which differed from type GF only in having plates 3 inch thick instead of 4 inch, and 
with a time-constant therefore twice as great, T= 2 x 10°*. 
These gauges gave remarkably consistent results; the difference between the 
pressures recorded by the two tripods in each gauge averaged only 2 per cent., and in 
only one or two cases exceeded 5 per cent. 
The GF and GH gauges are suitable for pressures from *6 ton per square inch 
(A= 10 x 10-8inch) to 1°8 ton per square inch (A= 61 ¥ 107° inch). In the few 
instances in which higher pressures were measured a modified form of gauge (type GJ) 
was used, with a plate 3 inch thick operating on a ring of 9 coppers. This gauge, 
which has a time-constant T = 10~* second, is suitable for pressures from 1°8 ton per 
square inch (A = 10 x 107° inch) to 5°4 ton per square inch (A = 61 x 10~* inch). 
The results from the GF and GH gauges proved that a time-constant of 
10-* second is small enough to give the true maximum pressure, provided the charge 
is not too small. (This was very fortunate, as it would have been impossible to 
design a gauge of the same type, that is to say, using the same coppers, with a time- 
constant substantially smaller). The proof rests on a comparison of the results 
obtained with very big charges. Taking the average of 3 shots of 1,600 lbs. 80/20 
amatol the ratio of the pressures recorded by the GF and GH gauges was 100 : 99°1, 
and taking the average of 3 shots of 1,900 Ibs. 50/50 amatol the ratio was 100: 100°1, 
that is to say, the pressure recorded by a gauge with a time-constant of 10~* second 
was the same, within the limits of error of the gauge, as the pressure recorded hy a 
gauge with a time-constant of 2 x 10~* second. The pressure therefore cannot have 
fallen more than a very small fraction during the first two ten-thousandths of a 
second. Taking the average of 11 shots of 300 lbs. 40/60 amatol the ratio of the 
pressures recorded by the GF and GH gauges was 100 : 94°6 ; and taking the average 
of 8 shots of 40 lbs. 40/60 amatol the ratio was 100:91°1. These figures illustrate 
the more rapid decline of the pressure from a small charge. Since the pressure from 
a 1,600-lb. charge does not fall appreciably in 2 x 10~* second it may be concluded, 
from the considerations set out in Section 9, that the pressure from a 2U0-lb. charge 
falls equally little in half the time, z.e., in 10-* second ; consequently a gauge with a 
time-constant of 10-* second may be assumed to be rapid enough for the correct 
determination of the maximum pressure from a charge of 200 Ibs. or over. On the 
other hand, since a gauge with a time-constant of 2 x 10~* second underestimates the 
maximum pressure from a 300-lb. charge by 5 or 6 per cent., it may be concluded 
that a gauge with a time-constant of 10~* second will underestimate the pressure from 
a 40-lb. charge by about the same amount. 
The gauges with free pistons described in the previous section differ from those 
dealt with in the present section in having no definite time-constant ; the time from 
the first onset of pressure to the striking of the copper depends un the intensity of 
the pressure. It is possible, however, in certain cases to compare the results given by 
the two types of gauge, and it is always found that the pressure registered by the 
free-piston gauge is from 10 to 15 per cent. lower than the pressure registered by the 
other type of gauge. For example, at a distance of 924 feet from a 1,900-lb. charge 
of 50/50 amatol a GX gauge with a $-inch piston registers a pressure of *82 ton per 
square inch, the time of operation of the gauge being 1°95 xX 10‘ second (Table VI.). 
Under the same conditions a GH gauge, with a time-constant of 2 x 10-‘*second, 
registers *95 ton per square inch. Similarly, at 50 feet from a 300-lb. charge of 
40/60 amatol a GX gauge with a }-inch piston registers ‘75 ton per square inch, the 
time of operation being 2°2 xX 10~* second, while a GH gauge registers ‘34 ton per 
square inch. It is hardly surprising that there should be a small systematic 
difference between the results of the two kinds of gauge, but the discrepancy gives 
rise to a difficulty in drawing the time-pressure curve. If this curve is started from 
a maximum pressure corresponding to the indications of the GF gauge it is impossible 
to make the curve fall evenly through the rectangular steps calculated from the GX, 
GY, &c., gauges except by representing the initial decline of the pressure to be much 
more rapid than can be reconciled with the results described in the preceding para- 
graph. Some sort of accommodation had to be made, and it was decided to reduce 
the indications of the GF gauges in every case by 10 percent. It should be under- 
stood therefore that wherever the ‘“‘ maximum pressure”’ is quoted in Part I. of this 
report. or in the diagrams illustrating it, the expression denotes the pressure recorded 
by a GF (or GJ) gauge minus 10 per cent. 
The methods that have been described are not capable of determining the form 
of the rising part of the pressure curve; it can only be inferred that the pressure 
D3 
