THE ROYAL ARTILLERY INSTITUTION. 
419 
also small, the ignition will not be so rapid. Large grains will burn slowly, 
but will admit of rapid ignition. The shape of the grain is another element 
to be considered, but one of still more importance is the density; for, if the 
grain be porous, the flame will not only pass between the grains, but also 
penetrate into their mass. We shall obtain the most rapid combustion, 
therefore, by combining a certain size and shape of grain with a certain 
density of grain; and, if we wish to moderate the rate of combustion, we 
must not only increase the size of the grain, but also increase its density, 
especially in large guns, where the pressure is very great. 
The objects to be attained in regulating the size and density of the grains 
are, the greatest possible velocity of projectile combined with the least strain 
on the gun. These cannot be obtained by one set of conditions for all 
natures of ordnance. A small projectile moves quickly, and relieves the 
strain in a still greater ratio. A heavy projectile not only moves slowly, 
but even a considerable motion does not relieve the strain in a proportionate 
manner, because the column of powder is longer in a large than a small 
gun. With small-arms consequently we must use fine grain powder, but 
large grain powder with heavy guns. Owing to the effect heat and pressure 
have in accelerating the combustion, we cannot determine a priori what 
size or density of grain will suit any particular gun. This, and as a conse¬ 
quence the actual pressure in the gun itself, can only be determined 
2. Pradically . 
Eumford was the first who attempted to obtain the pressure of gunpowder 
by actual experiment. Not being able to find any material strong enough 
to confine it when exploded in its own space, he measured the pressure when 
exploded in closed spaces bearing various relations to the bulk of the charge, 
and had it been possible to combine this method witli a knowledge of the 
rate of combustion, he would have completely solved the question. The 
plan he adopted was to make a small, short, strong gun, the muzzle of 
which he confined with a heavy weight, varying the charges and so regulating 
the weight that the force of explosion was just sufficient to lift it. 
In this way he obtained the various pressures, and made a calculation of 
what the pressure would be if the charge were exploded in its own space. 
No vent or escape of any kind was allowed. The charge was fired by means 
of a red-hot ball, which communicated the heat through the walls which 
contained a long narrow chamber at the base of the charge. Tig. 1 shows the 
method used; the enclosing weight was placed on the top, and is not shown. 
The bore was J inch diameter at the top, and the actual capacity of the 
whole gun was 25J grains. 
The following table gives the pressures obtained with a varying quantity 
of powder in grains. The first column gives the actual pressures, the second 
the pressures calculated according to what appeared to be a law. Some¬ 
times the weight was lifted with an explosion like that of a gun, at others 
it was not raised at all. The result w r as taken when it was just lifted with 
a very small escape of gas.* 
Philosophical Transactions of the Royal Society of London, 1797. 
