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.





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