regulatable pressure can be used as well as compressed air; it is
merely a question of mechanical convenience.

In throwing small quantities of certain high explosives, powder guns
can be used satisfactorily, but when large quantities are required,
the mechanical system of guns possess numerous advantages. All the
high explosives are subject to premature detonation by shock; each of
them is supposed to have its own peculiar shock to which it is
sensitive; but what this shock may be is at present unknown. We do
know, however, that premature explosions in guns are more liable to
occur when the charge in the shell is large than when it is small.
This is due to the fact that when the gun is fired, the inertia of the
charge in the shell is overcome by a pressure proportional to the mass
and acceleration, which pressure is communicated to the shell charge
by the rear surface of the cavity, and the pressure per unit of mass
will vary inversely as this surface. If, then, the quantity of
explosive in the shell forms a large proportion of the total weight of
the shell, we approach in powder guns a condition of shock to it which
is always dangerous and frequently fatal. The pressure behind the
projectile varies from twelve to fifteen tons per square inch, but it
is liable to rise to seventeen and eighteen tons, and in the present
state of the manufacture of gunpowder we cannot in ordinary guns
regulate it nearer than that. It is not a matter of so much importance
so far as the guns are concerned, when using ordinary projectiles, as
the gun will endure a pressure of from twenty-five to thirty tons per
square inch; but with high explosives in the shell it is a vitally
serious matter. From all I can learn regarding European practice, it
appears that not only are the explosives made sluggish, but the
quantity seldom exceeds thirty per cent. of the weight of the shell,
and the velocities, notwithstanding, are kept very low. In the