excellent surface, is compressed considerably by the wrought-iron breech
coil holding it, which, in its turn, is compressed by the massive
exterior coil. When the gun is fired, the strain is transmitted at once,
or nearly at once, to the breech coil, and thence more slowly to the
outer one. Now, as the duration of the pressure increases, owing to the
use of larger charges of slower burning powder, it is evident that the
more complete and effective will be the transmission of the strain to
the exterior, and, consequently, the further into the body of the
gun, starting from the bore, and traveling outward, does it become
advantageous to employ the stronger material. Hence, in England, we
had reason to congratulate ourselves on the certainty and cheapness
of manufacture of wrought iron coils, as long as moderate charges
of comparatively quick burning powder were employed, and as long as
adherence to a muzzle-loading system permitted the projectiles to move
away at an early period of the combustion of the charge. Then the
pressures, though sharp, were of short duration, and were not thoroughly
transmitted through the body of the gun, so that the solidity, mass,
and compression of the surrounding coils proved usually sufficient to
support the interior lining. Now that breech-loading and slow powders
have been introduced, these conditions have been changed. The strains,
though less severe, and less tending to explosive rupture, last longer,
and are more fully transmitted through the body of the gun. Sheer
strength of material now tells more, and signs have not been wanting
that coils of wrought iron afford insufficient support to the lining.
It becomes, therefore, advantageous to thicken the inner tube, and to
support it with a steel breech piece. Carrying this principle further,
we shall be led to substitute the stronger for the weaker metal
throughout the piece. This has been done by the Germans in the first
instance, and recently by the French also. It is probable that we shall
follow the same course. When I say "probable," I intentionally guard