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Page 19
Probably one reason why we have so many conflicting yet positive
accounts of great successes in Europe with torpedo shells is because
each nation wishes its neighbors to think that it is prepared for all
eventualities, and they are obliged to keep on hand large quantities
of some explosive, whether they have confidence in it or not.
Fortunately we are not so situated, but singularly enough what we have
done in the field of high explosive projection has been accomplished
by private enterprise, and we have attacked the problem at exactly the
opposite point from which European nations have undertaken it. While
they have assumed that the powder gun with its powerful and relatively
irregular pressures was a necessity and have endeavored to modify the
explosive to suit it, we have taken the explosive as we have found it,
and have adapted the gun to the explosive. At present the prominent
weapon in this new field is the pneumatic gun, but it is obvious that
steam, carbonic acid gas, ammonia or any other moderate and
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
pneumatic gun the velocity is low also, but so is the pressure in the
gun. The pressure in the firing reservoir is kept at the relatively
low figure of 1,000 pounds per square inch or less, and the air is
admitted to the chamber of the gun by a balance valve which cuts off
just the quantity of air (within a very few pounds) that is required
to make the shot. The gun is long, and advantage is taken of the
expansion of the air. In no case can the pressure rise in the gun
beyond that in the reservoir.
Up to the present time there have been no accidents in using the most
powerful explosives in their natural state, and in quantities over
fifty per cent. of the weight of the projectile. I have seen
projectiles weighing 950 pounds, and containing 500 pounds of
explosives (300 pounds of the blasting gelatine and 200 pounds of No.
1 dynamite) thrown nearly a mile and exploded after disappearing under
water. According to Gen. Abbot's formula such a projectile would have
sunk any armorclad floating within forty-seven feet of where it
struck. Apparently there is no limit to the percentage of explosive
that can be placed in the shell except the mechanical one of having
the walls thick enough to prevent being crushed by the shock of
discharge. In the large projectiles a transverse diaphragm is
introduced to strengthen the walls and to subdivide the charge.
The development of the pneumatic gun has been attended with some other
important discoveries, which may be of interest. It is well known that
mortar fire is very inaccurate, except at fixed long distances, in
consequence of the high angle, the slowness of flight of the
projectile, the variability of the powder pressure, and the inability
to change the elevation and the charge of powder rapidly. In the
pneumatic gun, the complete control of the pressure remedies the most
important of the mortar's defects and makes the fire accurate from
long ranges down to within a few yards of the gun. It is obvious that
the pressure can be usefully controlled in two ways: (1) by keeping
the elevation of the gun fixed and using a valve that can be set to
cut off any quantity of air, according to the range desired; (2) by
keeping the pressure in the reservoir constant, and using a valve
which will cut off the same quantity of air every time, changing the
elevation of the gun according to the distance. Another important
discovery consists in the application of subcalibered projectiles for
obtaining increased range.
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