muzzle, the air can exert no pressure against it. Now if the
velocity communicated to the wadding is very much greater than
that of sound, the condensation of the air immediately in advance
of it may be very great before the resistance transmitted to the
muzzle is at all considerable; in which case the mutual repulsion
of the particles of air so compressed, will offer an absolute
barrier to the advance of the wadding.(1*)

If this explanation be correct, the additional recoil, when a
gun is loaded with small shot or sand, may arise in some measure
from the condensation of the air contained between their
particles; but chiefly from the velocity communicated by the
explosion to those particles of the substances in immediate
contact with the powder being greater than that with which a wave
can be transmitted through them. It also affords a reason for the
success of a method of blasting rocks by filling the upper part
of the hole above the powder with sand, instead of clay rammed
hard. That the destruction of the gun barrel does not arise from
the property possessed by fluids, and in some measure also by
sand and small shot, of pressing equally in all directions, and
thus exerting a force against a large portion of the interior
surface, seems to be proved by a circumstance mentioned by Le
Vaillant and other travellers, that, for the purpose of taking
birds without injuring their plumage, they filled the barrel of
their fowling pieces with water, instead of loading them with a
charge of shot.

24. The same reasoning explains a curious phenomenon which
occurs in firing a still more powerfully explosive substance. If
we put a small quantity of fulminating silver upon the face of an