466 
THE 80-TON GUN. 
presses against it, the action of the charge on ignition is to evolve its 
gas from the surface of the grains in the space behind the stationary 
shot till a pressure is established sufficient to push it away. It moves 
tardily at first, and as the powder continues to burn up rapidly in a 
slowly increasing space, the pressure rises and the shot moves faster. 
As soon as the increase of room behind overtakes the increase of evo¬ 
lution of gas, a momentary equilibrium is established, and then the 
pressure begins to fall again. With the large-grained cubical powders 
rammed up, the maximum pressure is usually reached when the shot 
has travelled about 6 ins.; and probably, except under abnormal cir¬ 
cumstances, all pressures of importance arise after the motion of the 
shot has commenced, but before it has had time to travel far enough 
to relieve the tension of the gas. 
When rammed up hard, the space occupied by the cubical powders 
is about 24*6 cub. ins. per pound, against 26'0 cub. ins. for pebble 
powder. If, now, we lengthen the cartridge by making it thinner, 
leaving an air space over the top of such an extent that the content of 
the space behind the projectile is in increased proportion to the weight 
of the charge, each pound of powder will have more room at first in 
which to burn up and evolve its gas, which will therefore never reach 
the same density as when the cartridge has been rammed home. The 
pressures and velocities will thus be reduced. This much has long 
been known, and many proposals have been made involving the use of 
air spaces. The experiments tried have, however, always failed; the 
start being rendered unpromising by the difficulty of obtaining a satis¬ 
factory velocity. Still, the principle of reducing the maximum pressure 
by an air space, and keeping up this low maximum as long as possible 
by increasing the amount of charge, appeared to be right; and the last 
experiments with the 80-ton gun indicate that the reason that success 
had not attended previous attempts in this direction was simply that 
the question had not been pushed far enough. 
I believe that the researches made by Captain A. Noble, of Elswick, 
into the phenomena of the explosion of charges at different densities in 
closed vessels gave a stimulus to farther trials of air-spaced cartridges ; 
and certain remarkable results obtained by the Germans (see Table B) 
caused the Committee on Explosives to make experiments of this 
character. 
The Germans, as my readers are probably aware, use prismatic 
powder for their heavy guns—which are breech-loaders. The latest 
grain of prismatic powder is a cake with one hole through it; so that 
in burning the gas is evolved from both the inside and outside, and 
thus, while the outer surface diminishes, the inner increases. The 
evolution of gas is therefore more rapid than with our solid grains in 
the later stages of combustion. The projectiles for breech-loading 
guns have bands on them, larger than the lands, and have to be forced 
through the bore—requiring much greater pressure to start them than 
is the case, cat eris paribus, with muzzle-loaders, whose projectiles slip 
easily away, especially when the increasing twist is employed. Erom 
these two causes it appears probable that if air spaces are found 
beneficial, a larger proportionate air space would be necessary with 
