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tolerably accurate idea of the movements of those bodies. The syphon B may be considered as the prime mover; as the lateral apertures at D, by allowing the escape of a small portion of water from the lower extremity of the instrument, destroy the equilibrium within the tube, and give a preponderance of power to the side which is opposed to the jet, and by this simple process an equable rotatory motion is produced: the whole apparatus floating round the central wire that supports the sun.

The mode of constructing pumps must next be examined. The common sucking-pump consists of a pipe, open at both ends, in which is a moveable piston, bucket, or sucker, in which is situated the valve. The piston is leathered round, so as to move freely, but not to admit any air between it and the pump-barrel. When the pump is worked, the piston is raised, and causes a vacuum between itself and the valve below; the external pressure of the air upon the surface of the water in the well then forces it up through the lower valve to supply the vacuum, when the whole space between the valves is filled with water. At the next stroke of the pump, the piston is forced down, and the water rises through the upper valve; when the piston, on being again raised, in addition to its former operation, lifts up the water which had before Passed through the upper valve, and discharges it from the spout of the pump; and this it continues to do as long as the pump is worked. Thus, every time the piston is raised, the lower valve rises, and the upper one falls; and every time it is depressed the upper valve opens, while the lower one shuts.

As it is the pressure of the atmosphere which causes the water to rise and follow the piston or bucket as it is drawn up; and since a column of water, about thirty-two feet high, is of the same weight as a column of air of equal area, from the earth to the utmost height of the atmosphere, therefore, the perpendicular height of the piston or bucket, from the surface of the water in the well, must always be less than thirty-two feet. For, independently of the inconvenience in most cases of having so long a stroke, if it were ever so much increased, the water would never rise higher than thirty-two feet (nor indeed so high, as it would have the weight of the valve to lift), and there would be an empty space between the surface of the water in the pumpbarrel and the sucker, and consequently a considerable loss of time and labor. But, when the water has once passed through the upper valve, it may be lifted by it to any height, if the pistonrod be made long enough, and a sufficient degree of strength employed, without ever lengthening the stroke.

The construction of pumps is usually explained to the student by glass models, in which the action both of the pistons and valves may be seen. In order to understand the arrangement of the common pump, we may refer to fig. 8, plate II. A is a vessel intended to contain water, which must be deep enough to rise at least as high as from B to C. The valve a on the moveable bucket D, and the valve b on the fixed box E (which box quite fills the bore of the pipe or barrel at E), will

each lie close, by its own weight, upon the hole in the bucket and box, until the engine begins to work. The valves are made of brass, and covered underneath with leather for closing the holes more exactly; and the bucket D is raised and depressed alternately by the handle F and rod G c, the bucket being supposed at H before the working begins.

Take hold of the handle F, and thereby draw up the bucket from H to I, which will make room for the air in the pump to dilate itself, by which its spring is weakened, and then its force is not equivalent to the weight or pressure of the outward air upon the water A: and, therefore, at the first stroke, the outward air will press up the water through the notched foot B, into the lower pipe, about as far as d: this will condense the rarefied air in the pipe between d and I to the same state it was in before;.and then, as its spring within the pipe is equal to the force or pressure of the outward air, the water will rise no higher by the first stroke; and the valve b, which was raised a little by the dilatation of the air in the pipe, will fall, and stop the hole in the box E; and the surface of the water will stand at d. Then depress the piston or bucket from I to H, and, as the air in the part H cannot get back again through the valve b, it will (as the bucket descends), raise the valve a, and so make its way through the upper part of the barrel c, into the open air. But, upon raising the bucket D a second time, the air between it and the water in the lower pipe at d, will be again left at liberty to fill a larger space; and so, its spring being again weakened, the pressure of the outward air on the water A, will force more water up into the lower pipe from d to e; and when the bucket is at its greatest height I, the lower valve b, will fall, and stop the hole in the box E, as before. At the next stroke of the bucket or piston, the water will rise through the box E towards H, and then the valve b, which was raised by it, will fall when the bucket D is at its greatest height. Upon depressing the bucket again, the water cannot be pushed back through the valve b, which keeps close upon the hole whilst the piston descends. And, upon raising the piston again, the outward pressure of the air will force the water up through E, where it will raise the valve, and follow the bucket to I. Upon the next depression of the bucket D it will go down into the water in the barrel H; and, as the water cannot be driven back through the now close valve b, it will raise the valve a as the bucket descends, and will be lifted up by the bucket when it is next raised. And now, the whole space below the bucket being full, the water above it cannot sink when it is next depressed; but, upon its depression, the valve a will rise to let the bucket go down; and when it is quite down the valve a will fall by its weight, and stop the hole in the bucket. When the bucket is next raised, all the water above it will be lifted up and begin to run off by the pipe K. And thus, by raising and depressing the bucket alternately, there is still more water raised by it; which, getting above the pipe K, into the wide top L, will supply the pipe, and make it run with a continued stream.

The common sucking-pump may, by a small addition, be converted into a lifting-pump, fitted for propelling the water to any distance, and with any velocity. Fig. 9 is a sucking-pump whose working-barrel A B has a lateral pipe, C, connected with it close to the top. This terminates in a main, or rising-pipe, furnished, or not, with a valve. The top of the working-barrel A B is shut by a strong plate, having a hollow neck terminating in a small flanch. The pistonrod passes through this neck, and is accurately turned and polished. A number of rings of leather are put over the rod, and strongly compressed round it by another flanch and several screwed bolts. By this contrivance, the rod is closely grasped by the leathers, but may be easily drawn up and down, while all passage of air and water is effectually prevented. The piston is perforated, and furnished with a valve open ing upwards. There is also a valve T on the top of the suction-pipe; and it will be of advantage, though not absolutely necessary, to put a valve L at the bottom of the rising-pipe. Now, suppose the piston at the bottom of the workingbarrel; when it is drawn up, it tends to compress the air above it, because the valve in the piston remains shut by its own weight. The air, therefore, is drawn through the valve L into the rising-pipe, and escapes. In the mean time, the air, which occupied the small space between the piston and the valve T, expands into the upper part of the working-barrel; and its elasticity is so much diminished thereby, that the atmosphere presses the water of the cistern into the suction-pipe, where it rises until an equilibrium is again produced. The next stroke of the piston, downwards, allows the air which had come from the suction-pipe into the barrel during the ascent of the piston, to get through its valve. Upon drawing up the piston, the air is also drawn off through the rising pipe. Repeating this process brings the water at last into the working-barrel, and it is then driven along the rising-pipe by the piston.

This is one of the best forms of a pump. The rarefaction may be very perfect, because the piston can be brought so near to the bottom of the working barrel; and, for forcing water in opposition to great pressures, it appears preferable to the common forcing-pump; because in that, the piston-rod is compressed and exposed to bending, which greatly hurts the pump, by wearing the piston and barrel on one side. This soon renders it less tight; and much water squirts out by the sides of the piston. But in this pump the piston-rod is always drawn, or pulled, which keeps it straight, and rods exert a much greater force in opposition to a pull than to compression. The collar of leather round the pistonrod is found by experience to be very impervious to water; and, though it needs but little repair, yet the whole is very accessible; and, in this respect much preferable to the common pump in deep mines, where every fault of the piston obliges us to draw up some hundred feet of piston-rods. By this addition, too, any common pump, for the service of a house, may be converted into an engine for extinguishing fire; or may be made to convey the water to every

part of the house; and this without hurting or obstructing its common uses. All that is necessary is, to have a large cock on the upper part of the working-barrel, opposite to the lateral pipe in this figure. This cock serves for a spout, when the pump is used for common purposes; and the merely shutting this cock converts the whole into an engine for extinguishing fire, or for supplying distant places with water. It is scarcely necessary to add, that, for these services, it will be requisite to connect an air-vessel with some convenient part of the rising-pipe, in order. that the current of water may be continual.

It is of considerable importance that as equable motion as possible be produced in the main pipe, which diminishes those strains to which it is otherwise liable. The application of an air vessel, at the beginning of the pipe, answers this purpose. In great works it is usual to effect this by the alternate action of two pumps. It will be rendered still more uniform if four pumps be employed, succeeding each other at the interval of one quarter of the time of a complete stroke.

But ingenious men have attempted the same thing with a single pump; and many different constructions for this purpose have been proposed and executed. Fig. 10 represents one of the best. It consists of a working-barrel a b, closed at both ends; the piston c is solid, and the pistonrod passes through a collar of leathers at the top of the barrel. This barrel communicates laterally with two pipes n and k, the communications being as near to the top and bottom of the barrel as possible. At each of the communications are two valves, opening upwards. The two pipes unite in a larger rising-pipe at b, which bends a little back, to give room for the pistonrod. Suppose the piston down close to the entry of the lateral pipe h; when it is drawn up, it compresses the air above it, and drives it through the valve in the pipe k, whence it es capes through the rising-pipe; at the same time it rarefies the air below it. Therefore the weight of the atmosphere shuts the valve m, and causes the water in the cistern to rise through the valve n, and fill the lower part of the pump. When the piston is pushed down again this water is fresh driven through the valve m, because n immediately shuts; and then most of the air which was in this part of the pump at the beginning goes up through it, soine of the water coming back in its stead. In the mean time the air which remained in the upper part of the pump after the ascent of the piston, is rarefied by its descent; because the valve o shuts as soon as the piston begins to descend, the valve p opens, air in the suction-pipe h, expands into the barrel, and the water rises into the pipes by the pressure of the atmosphere. The next rise of the piston must bring more water into the lower part of the barrel, and must drive a little more air through the valve o, namely, part of that which had come out of the suction-pipe h; and the next descent of the piston must drive more water into the rising-pipe k, and along with it most, if not all, of the air which remained below the piston, and must rarefy still more the air remaining above the piston; and more water

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w come in through the pipe h, and get into the barrel. It is evident, that a few repetitions wili at last fill the barrel on both sides of the piston with water. When this is accomplished, there is no difficulty in perceiving how, at every ise of the piston, the water of the cistern will come in by the valve n, and the water in the upper part of the barrel will be driven through the valve o; and, in every descent of the piston, the water of the cistern will come into the barrel by the valve p, and the water below the piston will be driven through the valve m; and thus there will be a continual influx into the barrel through the valves n and p, and a continual discharge along the rising-pipe I, through the valves m and o.

This machine is certainly equivalent to two forcing-pnmps, although it has but one barrel and one piston; but it has no sort of superiority. It is not even more economical, in most cases; because, probably, the expense of the additional workmanship will equal that of the barrel and piston, which is saved. There is, indeed, a saving in the rest of the machinery, because one lever produces both motions. It therefore cannot be called inferior to two pumps; and there is undoubtedly some ingenuity in the contriv-,

ance.

The forcing-pump represented at fig. 1, plate III., consists of a barrel A B, and a piston or forcer C. It is also provided with an airvessel v.

When the forcer is first moved upwards in the barrel, the air between that and the water below, having room to dilate, by its natural spring, will of course be rarefied; the pressure of the atmosphere being intercepted by the force of the barrel A Bon one hand, and by the upper valve at S in the branching-pipe, on the other, the water will rise from the spring into A B, for the reason already given; and repeated strokes of the piston will fetch up the fluid to the forcer, and fill the cavity of the pipes between the fixed valves D and S. The water in this manner raised, being hindered from going down again by the lower valve, will be pressed by the forcer every time it descends, and be thereby obliged to make its way where there is least resistance, viz. through the upper valve at S. And whenever, on the rising of the forcer, this pressure intermits, the valve at S will immediately close under the weight of the upper water, and prevent its return that way, while the piston is rising with a fresh supply; and this is repeated at every stroke of the forcer.

It is evident that the operation of a pump is by starts, and that the water in the main remains at rest, pressing on the valve during the time that the piston is withdrawn from the bottom of the working-barrel. It is in most cases desirable to have this motion equable, and in some cases it is absolutely necessary. Thus, in the engine for extinguishing fires, the spout of water, going by jerks, could never be directed with a certain aim, and half of the water would be lost by the way; because a body at rest cannot in an instant be put in rapid motion; and the first portion of every jerk of water would have put a small velocity. A very ingenious contrivance has been fallen upon for obviating this in VOL. XI.

stream nearly

convenience, and procuring a equable. At any convenient part of the rising pipe, beyond the valve S, there is annexed a strong and capacious vessel V, closed at top by a small pipe T fixed into it, which reaches nearly to the bottom of the vessel. When the water is forced along the rising-pipe, S, it gets into this vessel, and rises above the lower part of the pipe T. The air, which is above the water in the vessel, being now confined, and being condensed into a smaller space by the admission of more water at each action of the piston, presses by its elasticity upon the surface of the water, which cannot return by the valve S, and forces it up the pipe T in a continued stream. This air-vessel must be so large, that the change of bulk of the compressed air, during the inaction of the piston, may be inconsiderable, otherwise the stream will not continue until the next stroke. We must not imagine, that because the stream produced by the assistance of an air-vessel is almost perfectly equable, and because as much water runs out during the returning of the piston as during its active stroke, that it therefore doubles the quantity of water. No more water can run out than what is sent forward by the piston during its effective stroke. The continued stream is produced only by preventing the whole of this water from being discharged during this time, and by providing a propelling force to act during the piston's return. It is, however, a matter of fact, that a pump, furnished with an air-vessel, delivers a little more water than it would do without it.

In forcing-pumps it is of the utmost consequence to avoid all contractions in the pipes. The main, which leads from the forcing-pumps, should be equal to the working barrel. If it be only half the diameter, it has but onefourth of the area; the velocity in the main is four times greater than that of the piston; and the force necessary for discharging the same quantity of water is sixteen times greater. It is not, however, possible to avoid these contractions altogether, without making the main wider than the barrel; for if only so wide, with an entry of the same size, the valve makes a considerable obstruction. Unskilful engineers endeavour to obviate this, by making an enlargement in that part of the main which contains the valve. If this be not done with great judgment, it will increase the obstruction; for, if this enlargement be full of water, the water must move in the direction of its axis with a diminished velocity; and, when it comes to the main, it must again be accelerated. In short, any abrupt enlargement, which is to be afterwards contracted, does as much harm as a contraction, unless it be so short that the water in the axis keeps its velocity until it reach the contraction. All angular enlargements, all boxes into which the pipes, from different working-barrels, unite their water before it goes into a main, must therefore be avoided by an artist who would execute a good machine; and the different contractions, which are unavoidable at the seats of valves, and the perforations of pistons, &c., should be diminished, by giving the parts a trumpet shape. In the airvessel, for producing a constant stream, this is of 2 L

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