CHAPTER 2.

Accumulating Power

20. Whenever the work to be done requires more force for its
execution than can be generated in the time necessary for its
completion, recourse must be had to some mechanical method of
preserving and condensing a part of the power exerted previously
to the commencement of the process. This is most frequently
accomplished by a fly-wheel, which is in fact nothing more than a
wheel having a very heavy rim, so that the greater part of its
weight is near the circumference. It requires great power applied
for some time to put this into rapid motion; but when moving with
considerable velocity, the effects are exceedingly powerful, if
its force be concentrated upon a small object. In some of the
iron works where the power of the steam-engine is a little too
small for the rollers which it drives, it is usual to set the
engine at work a short time before the red-hot iron is ready to
be removed from the furnace to the rollers, and to allow it to
work with great rapidity until the fly has acquired a velocity
rather alarming to those unused to such establishments. On
passing the softened mass of iron through the first groove, the
engine receives a great and very perceptible check; and its speed
is diminished at the next and at each succeeding passage, until
the iron bar is reduced to such a size that the ordinary power of
the engine is sufficient to roll it.

21. The powerful effect of a large flywheel when its force
can be concentrated on a point, was curiously illustrated at one
of the largest of our manufactories. The proprietor was shewing
to a friend the method of punching holes in iron plates for the
boilers of steam-engines. He held in his hand a piece of
sheet-iron three-eighths of an inch thick, which he placed under
the punch. Observing, after several holes had been made, that the
punch made its perforations more and more slowly, he called to
the engine-man to know what made the engine work so sluggishly,
when it was found that the flywheel and punching apparatus had
been detached from the steam-engine just at the commencement of
his experiment.

22. Another mode of accumulating power arises from lifting a
weight and then allowing it to fall. A man, even with a heavy
hammer, might strike repeated blows upon the head of a pile
without producing any effect. But if he raises a much heavier
hammer to a much greater height, its fall, though far less
frequently repeated, will produce the desired effect.

When a small blow is given to a large mass of matter, as to a
pile, the imperfect elasticity of the material causes a small
loss of momentum in the transmission of the motion from each
particle to the succeeding one; and, therefore, it may happen
that the whole force communicated shall be destroyed before it
reaches the opposite extremity.

23. The power accumulated within a small space by gunpowder
is well known; and, though not strictly an illustration of the
subject discussed in this chapter, some of its effects, under
peculiar circumstances, are so singular, that an attempt to
explain them may perhaps be excused. If a gun is loaded with ball
it will not kick so much as when loaded with small shot; and
amongst different kinds of shot, that which is the smallest,
causes the greatest recoil against the shoulder. A gun loaded
with a quantity of sand, equal in weight to a charge of
snipe-shot, kicks still more. If, in loading, a space is left
between the wadding and the charge, the gun either recoils
violently, or bursts. If the muzzle of a gun has accidentally
been stuck into the ground, so as to be stopped up with clay, or
even with snow, or if it be fired with its muzzle plunged into
water, the almost certain result is that it bursts.

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
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
anvil, and strike it slightly with a hammer, it explodes; but
instead of breaking either the hammer or the anvil, it is found
that that part of the face of each in contact with the
fulminating silver is damaged. In this case the velocity
communicated by the elastic matter disengaged may be greater than
the velocity of a wave traversing steel; so that the particles at
the surface are driven by the explosion so near to those next
adjacent, that when the compelling force is removed, the
repulsion of the particles within the mass drives back those
nearer to the surface, with such force, that they pass beyond the
limits of attraction, and are separated in the shape of powder.

25. i) The success of the experiment of firing a tallow candle
through a deal board, would be explained in the same manner, by
supposing the velocity of a wave propagated through deal to be
greater than that of a wave passing through tallow.

25. ii) The boiler of a steam-engine sometimes bursts even
during the escape of steam through the safety-valve. If the water
in the boiler is thrown upon any part which happens to be red
hot, the steam formed in the immediate neighbourhood of that part
expands with greater velocity than that with which a wave can be
transmitted through the less heated steam; consequently one
particle is urged against the next, and an almost invincible
obstacle is formed, in the same manner as described in the case
of the discharge of a gun. If the safety-valve is closed, it may
retain the pressure thus created for a short time, and even when
it is open the escape may not be sufficiently rapid to remove all
impediment; there may therefore exist momentarily within the
boiler pressures of various force, varying from that which can
just lift the safety-valve up to that which is sufficient, if
exerted during an extremely small space of time, to tear open the
boiler itself.

26. This reasoning ought, however, to be admitted with
caution; and perhaps some inducement to examine it carefully may
be presented by tracing it to extreme cases. It would seem, but
this is not a necessary consequence, that a gun might be made so
long, that it would burst although no obstacle filled up its
muzzle. It should also follow that if, after the gun is charged,
the air were extracted from the barrel, though the muzzle be then
left closed, the gun ought not to burst. It would also seem to
follow from the principle of the explanation, that a body might
be projected in air, or other elastic resisting medium, with such
force that, after advancing a very short space it should return
in the same direction in which it was projected.

NOTES:

1. See Poisson's remarks, Ecole Polytec. Cahier, xxi, p. 191.