THE BOYAL ARTILLERY INSTITUTION. 4$1 
the pressure varies inversely as the amount of extra space which is added to 
that of the charge consumed :—■ 
If P = pressure in tons, 
l — length of charge, 
y — proportion of charge consumed, 
x = distance moved by the shot, 
j _ 2Q .y . 
X 
j + l ~y 
te What was very remarkable in all the experiments in which the generated 
elastic vapour was completely confined, was the small amount of expansive force 
which the vapour appeared to possess after it had been suffered to remain a few 
minutes, or even only a few seconds, confined in the barrel; for, in raising the 
weight by means of its lever, and suffering this vapour to escape, instead of 
escaping with a loud report it rushed out with a hissing noise hardly so loud or 
so sharp as the report of a common air-gun, and its effects against the leathern 
stopper by which it assisted in raising the weight were so very feeble as not to be 
sensible. On examining the barrel, however, this diminution of force in the 
generated elastic fluid was easily explained; for what was undoubtedly in the 
moment of explosion in the form of an elastic fluid, was now found transformed 
into a solid body, as hard as a stone. 
“ That this hard substance, found in the barrel after an explosion in which the 
generated elastic fluid had been completely confined, was actually in a fluid or 
elastic state in the moment of explosion, is evident from hence, that in all those 
cases in which the weight was raised and the stopper blown out of the bore, 
nothing was found remaining in the barrel. It was very remarkable that this 
hard substance was not found distributed about in all parts of the barrel indif¬ 
ferently, but more of it was always found near the middle of the length of the 
bore than at either of its extremities; and the upper part of the vent-tube in 
particular .was always found quite filled with it. It should seem from hence that 
it attached itself to those parts of the barrel which were soonest cooled; and hence 
the reason, most probably, why none of it was ever found in the lower part of the 
vent-tube, where it was kept hot by the red-hot ball by which the powder was set 
on fire,”* 
A better plan than that adopted by Eumford for measuring the actual 
pressure cannot be conceived. We have considered the nature of a suddenly 
applied pressure in doubling the strain on yielding material, but a suddenly 
applied pressure in this case would make no difference, for the absolute 
weight keeping the powder enclosed would not yield to any pressure less 
than itself, and so would show the same result whether the pressure were 
suddenly or slowly applied. 
There is, however, another kind of pressure to be considered. It was 
investigated by Eobins in the following manner. By firing a musket charged 
with a light wad against a pendulum, he considered the velocity of forced 
gunpowder to be 7000 f.s. He says :— 
“ From these determinations may be deduced the force of petards, since the 
action depends entirely on the impulse of the flame, and it appears that a quantity 
* Philosophical Transactions of the Royal Society of London, 1797; 
