THE ROYAL ARTILLERY INSTITUTION. 
213 
that opinions of any value can be formed respecting the relative merits of 
different materials and constructions. 
When a charge of gunpowder is ignited in the bore of a gun the gas 
exerts equal pressure in every direction, and therefore, neglecting windage, 
the pressure on the bottom of the bore is equal to that on the base of the 
shot, and the pressures on the top and bottom as well as those on the sides 
of the bore balance each other. As the shot moves towards the muzzle, so 
will the space in which the gas is confined be increased, and the pressure be 
decreased. It is evident then that the thickness of metal may (as it always 
is) be safely reduced from breech to muzzle. A curve representing the 
pressure of the gas along the bore might be constructed on the assumption 
that the whole charge is converted into gas before the shot moves, but this 
is not the case in practice; in fact, as we know, the amount of gas generated, 
previously to motion being communicated to the projectile, varies according 
to circumstances in different guns. 
The force exerted by the gas produces a double strain inside the bore— 
a tangential strain , tending to rend the metal lengthwise (through AB, 
Eig. 1); and a longitudinal strain , which tends to fracture the gun across 
(through CD), or to drive out the breech. 
_D_ 
a --mu z z 3 Kg.l. 
c 
The most important point to consider is the initial strain, or that to 
which the metal is subjected before the shot moves; for the gas then exerts 
its maximum pressure per square inch, and it is this first severe strain that 
usually causes the destruction of the gun. Now, in ordinary cases, the 
initial pressure exerted at the bottom of the bore by a given charge 
depends upon the amount of force necessary to set the shot in motion, for 
the greater this force, the longer is the time allowed for the ignition of the 
powder before the gas can expand. 
In comparing then the strain in different guns, it is necessary to consider 
the resistance the projectile opposes in each case to the force of the gas, or 
in other words, the work the gas has to perform in putting the shot in 
motion. Eor instance, in a smooth-bore gun the spherical shot has merely to 
be rolled along the surface of the bore, and is therefore probably set in motion 
when only a small portion of the powder is converted into gas. Should a 
cylindrical shot be fired from the same piece, there will be a considerable 
amount of friction between the shot and the bore, which must be overcome 
and cause delay. In a rifled gun additional work must be expended in 
giving a rotatory motion to the shot; besides which, the area of projectile 
presented to the force of the gas is much smaller for a given weight of shot 
in a rifled gun, than in a smooth bore piece.* It is thus easy to account for 
the great proportional strain caused by a charge of powder in a rifled gun. 
* In all these cases it must be remembered that although the actual differences in time elapsing 
between the ignition of the powder and the moving of the shot under various circumstances may be 
very small indeed, the ratios of the times may be high; 
