gas, tending to raise the pressure, and the growing space behind the
advancing shot, tending to relieve it. As artillery science progresses,
so does the duration of this contest extend further along the bore
of the gun toward the great desideratum, a low maximum pressure long
sustained.

When quick burning powder was used for ordnance, the pressures were
short and sharp; the metal in immediate proximity to the charge was
called upon to undergo severe strains, which had scarcely time to reach
the more distant portions of the gun at all; the exterior was not nearly
so much strained as the interior. In order to obviate this defect, and
to bring the exterior of the gun into play, the system of building up
guns of successive tubes was introduced. These tubes were put one over
the other in a state of tension produced by "shrinkage." This term is
applied to the process of expanding a tube by the application of heat,
and in that condition fitting it over a tube larger than the inner
diameter of the outer tube when cold. When the outer tube cools it
contracts on the inner tube and clutches it fast. The wrought-iron guns
of England have all been put together in this manner.

Prussia at first relied on the superior strength of solid castings
of steel to withstand the explosive strain, but at length found the
necessity for re-enforcing them with hoops of the same material, shrunk
on the body of the piece.

The grand principle of shrinkage enables the gunmaker to bring into play
the strength of the exterior of the gun, even with quick powders, and to
a still greater extent as the duration of the strain increases with the
progress of powder manufacture. Thus, taking our largest muzzle-loaders
designed a few years ago, the thin steel lining tube, which forms an