THE BOYAL ABTILLEBY INSTITUTION. 
199 
Irom this experiment it appears that there is no practical loss of either 
Velocity or range by reducing the cartridge (in a gun of 8*12 inches calibre) 
to 7‘0 inches diameter, and that a further reduction to 6*5 inches has a 
small effect on the velocity, but no practical effect on the range. 
It is probable that a cartridge of about 12*5 inches in length and 6*75 
inches diameter would give quite as much velocity and range to the pro¬ 
jectile, while it would, without doubt, cause a much less initial strain on 
the gun. 
Unfortunately we have no reliable means of determining the pressure of 
fired gunpowder within the bore of a cannon, and it is therefore impossible 
at present to ascertain practically the initial strain per square inch, corre¬ 
sponding to different lengths of cartridge. It is, however, self-evident that 
the greater the space occupied by the charge at the moment of its maximum 
tension the less will be the initial strain per square inch, and it is probable 
that a reduction of the 68-pr. cartridge to a diameter of 6*75 inches would 
have the effect of reducing the initial strain per square inch by at least 150 
atmospheres. 
It is an error to imagine that a reduction of the initial strain on the gun 
is necessarily attended with loss of velocity in the projectile. 
The initial velocity of a projectile is the sum of the impulses it receives 
from its first movement till it reaches the muzzle, and depends upon the 
total pressure exerted, and the space through which this pressure acts. 
The initial strain on the gun is another thing altogether, and takes place 
at the moment of maximum tension in the charge of powder, which, with 
spherical shot, is very shortly after the projectile has commenced to move, 
and with most elongated shot is probably before any movement takes place. 
It will appear from this that a gun may be very severely strained by a 
charge which will not give a corresponding velocity to the projectile. It is 
not the total strain on a gun, corresponding to the recoil, which injures it 
most, it is the blow or shock which it experiences at the moment of the 
powder reaching its maximum tension. . 
It is this which makes gun cotton so destructive. A charge of gun cotton 
may give a less velocity than a charge of gunpowder, and yet its effect on 
the gun be very much greater. 
The gas in the one case is developed more instantaneously, and the pro¬ 
jectile has not time to move, and so relieve the gun, before the tension is at 
its maximum. 
It is this which chiefly caused the bursting of cast-iron rifled guns. 
Although these guns were fired with a smaller relative charge than when 
in their smooth-bore state. 
The initial strain was, however, greaterand if a cast-iron gun is subjected 
to a greater initial pressure than the material of the gun is known to bear, 
there can be but one result*. 
Yarious methods have been proposed for strengthening cast-iron guns by 
exterior wrought-iron or steel hoops-; and guns so strengthened have 
invariably failed in the inside, where the metal was cast-iron and unable to 
bear the initial strain brought upon it. 
* See Major Palliser’s Treatise on Compound Ordnance. 
