Part 2 (1/2)

23 The power accuunpowder is well known; and, though not strictly an illustration of the subject discussed in this chapter, soular, that an atteun is loaded with ball it will not kick so st different kinds of shot, that which is the sun loaded with a quantity of sand, equal in weight to a charge of snipe-shot, kicks stilland the charge, the gun either recoils violently, or bursts If the round, so as to be stopped up with clay, or even with snow, or if it be fired with its ed 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 circuun loaded with powder confined by a cylindrical piece of wadding, and having itsa ree of resistance In this case the first effect of the explosion is to produce an enor 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 i is condensed; and if the wadding were to rehout the tube would soon acquire a uniform density But this would require a s would travel with the velocity of sound to the other end, froenerated, which, aided by the friction of the tube, would ultimately destroy the motion

But until the first wave reaches the iainst it Now if the velocity coreater than that of sound, the condensation of the air ireat 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 ireater than that hich a wave can be transh them It also affords a reason for the success of athe upper part of the hole above the poith sand, instead of clay raun barrel does not arise from the property possessed by fluids, and in so 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 plu pieces ater, instead of loading the explains a curious pheno a still more powerfully explosive substance If we put a s silver upon the face of an anvil, and strike it slightly with a ha either the hammer or the anvil, it is found that that part of the face of each in contact with the fuled In this case the velocity coreater 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 co 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 experih a deal board, would be explained in the saated through deal to be greater than that of a wave passing through tallow

25 ii) The boiler of a stea the escape of steah the safety-valve If the water in the boiler is thrown upon any part which happens to be red hot, the steahbourhood of that part expands with greater velocity than that hich a wave can be transh the less heated steaainst the next, and an almost invincible obstacle is formed, in the saun 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 ti ought, however, to be admitted with caution; and perhaps some induce it to extreme cases It would seeht be h no obstacle filled up its ed, the air were extracted froun ought not to burst It would also seem to follow froht be projected in air, or other elastic resistinga 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

Chapter 3

Regulating Power

27 Uniformity and steadiness in the rate at which machinery works, are essential both for its effect and its duration The first illustration which presents itself is that beautiful contrivance, the governor of the steaine, which must immediately occur to all who are faine Wherever the increased speed of the engine would lead to injurious or dangerous consequences, this is applied; and it is equally the regulator of the water-wheel which drives a spinning-jenny, or of the windmills which drain our fens In the dockyard at Chathae platforovernor; but as the weight is very considerable, the velocity of this governor is still further checked by causing its motion to take place in water

28 Another very beautiful contrivance for regulating the nuine, is used in Cornwall: it is called the cataract, and depends on the ti of the valve through which the fluid is adine-ularity of the supply of fuel to the fire under the boilers of stea to the uniformity of their rate, and also economizes the consumption of coal Several patents have been taken out forto ine supply the fire with sular intervals by means of a hopper, and to ine works too quickly One of the incidental advantages of this plan is, that by throwing on a very small quantity of coal at a time, the smoke is almost entirely consumed The dampers of ashpits and chimneys are also, in soulate their speed

30 Another contrivance for regulating the effect of ht, but presenting a large surface This revolves rapidly, and soon acquires a uniforreatly exceed, because any addition to its velocity produces a reater addition to the resistance it meets with from the air The interval between the strokes on the bell of a clock is regulated in this way, and the fly is so contrived, that the intervalthe arms of it more or less obliquely to the direction in which they enerally used in the smaller kinds of mechanism, and, unlike the heavy fly, it is a destroyer instead of a preserver of force It is the regulator used in musical boxes, and in almost all ests the principle of an instru the altitude of mountains, which perhaps deserves a trial, since, if it succeed only tolerably, it will form a much more portable instrument than the baroht of a column of the atmosphere above it, whose base is equal to the bore of the tube It is also known that the density of the air adjacent to the instruht of air above it, and on the heat of the air at that place If, therefore, we can ht of a column of mercury which it would support in the baroives infor the teht be ascertained by means of a watch and a small instrument, in which the number of turns istered The less dense the air in which the vane revolves, the greater will be the nuiven time: and tables could be formed from experiments in partially exhausted vessels, aided by calculation, from which, if the temperature of the air, and the nu height of the baroht be found(1)

NOTES:

1 To persons who may be inclined to experiest the perusal of the section 'On the art of Observing', Observations on the Decline of Science in England, p 170, Fellowes, 1828

Chapter 4