The ultimate cause of these apparently inconsistent effects
is, that every force requires time to produce its effect; and if
the time requisite for the elastic vapour within to force out the
sides of the barrel, is less than that in which the condensation
of the air near the wadding is conveyed in sufficient force to
drive the impediment from the muzzle, then the barrel must burst.
If sometimes happens that these two forces are so nearly balanced
that the barrel only swells; the obstacle giving way before the
gun is actually burst.

The correctness of this explanation will appear by tracing
step by step the circumstances which arise on discharging a gun
loaded with powder confined by a cylindrical piece of wadding,
and having its muzzle filled with clay, or some other substance
having a moderate degree of resistance. In this case the first
effect of the explosion is to produce an enormous pressure on
everything confining it, and to advance the wadding through a
very small space. Here let us consider it as at rest for a
moment, and examine its condition. The portion of air in
immediate contact with the wadding is condensed; and if the
wadding were to remain at rest, the air throughout the tube would
soon acquire a uniform density. But this would require a small
interval of time; for the condensation next the wadding would
travel with the velocity of sound to the other end, from whence,
being reflected back, a series of waves would be generated,
which, aided by the friction of the tube, would ultimately
destroy the motion.

But until the first wave reaches the impediment at the