P IT M 440 PUM

which would be the case if the main were horizontal. The waterfills it, but it is at rest. In order that a gallon, for instance, maybe delivered in a second, the whole water in the horizontal mainmust be put in motion with a certain velocity. This requiresforce. We must, therefore, always distinguish between the stateof equilibrium and the state of actual working. It is the equili-brium only that we consider at present; and no more is necessaryfor understanding the operation of the different species of pumps.The other force is of much more intricate investigation, and willbe considered by itself. Another kind of the forcing pump, is re-presented in fig. 5. It consists of a working barrel ABC1), a suc-tion-pipe, CDEF, and a main or rising pipe. This last is usuallyin three joints. The first G11KI may be considered as makingpart of the working barrel, and is commonly cast in one piecewith it. The second IKLM is joined to it by flanches, and formsthe elbow which this pipe must generally have. The thirdLNOM is properly the beginning of the main, and is continuedto the place of delivery. At the joint IK there is a hanging valveor clack S ; and there is a valve R on the top of the suction-pipe.The piston PQTV is solid, and is fastened to a stout iron-rodwhich goes through it, and is fixed bv a peg drawn through itsend. The body of the piston is a sort of double cone, wideningfrom the middle to each end, and is covered with tow bandsof very strong leather, fitted to it in the manlier already described.T. he operation of this pump is abundantly simple. 'When the pis-ton is thrust into the pump, it pushes the air before it through thevalve S, for the valve K remains shut by its own weight. Whenit has reached near the bottom, and is drawn up again, the airwhich filled the small space between the piston and the valve Snow expands into the barrel; for as soon as the air begins to ex-pand, it ceases to balance the pressure of the atmosphere, whichtherefore shuts the valve S. lly the expansion of the air in thebarrel the Equilibrium at the valve R is destroyed, and theair in the suction-pipe lifts the valve, and expands into thebarrel; consequently it ceases to be a balance for the pressureof the atmosphere, and the water is forced into the suction-pipe. Pushing the piston down again forces the air in thebarrel through the valve S, the valve R in the mean timeshutting. When the piston is again drawn up, S shuts, R opens,the air in the suction-pipe dilates anew, and the water rises higherin it. Repeating these operations, the water gets at last into theworking barrel, and is forced into the main by putting downthe piston, and is pushed along to the place of delivery. In bring-ing up great quantities of water from mines, the common suckingpump is generally employed, as really the best of them all : butit is the most expensive, because it requires the pipe to be perpen-dicular, straight, and of great dimensions, that it may contain thepiston-rods. But Ibis is impracticable when the pipe is crooked,if the forcing pump, constructed in the manner now described,be employed, we cannot use forcers with long rods. These wouldbend when pushed down by their farther extremity. In this caseit is usual lo employ only a short and stiff rod, and to hang it by achain, and load it with a weight superior to the weight of waterto be raised by it. The machinery therefore is employed, not inforcing the water along the rising pipe, but in rajsing the weightwhich is to produce this effect by its subsequent descent. In thisca-e it would be much belter to employ the lifting pump, fig. (>.For as die load on the forcer must be greater than the resistanceswhich it must surmount, the force exerted by the machine must inlike mariner lie greater than this load. This double excess wouldbe avoided by using the lifting pump. A very ingenious con-trivance has been fallen upon for obviating this inconvenience, andprocuring a stream nearly equable. At any convenient part ofthe rising pipe beyond the vaive S there is annexed a capaciousvessel VZ, (rig. 7 and 8,) close at top, and of great strength. Whenthe water is forced along this pipe, part of it gets into this vessel,keeping tiie air confined above it, and it fills it to such a height V,that the elasticity of the confined air balances a column reachingto T, we shall suppose, in the rising pipe. The next stroke of thepiston sends forward more water, which would till the rising pipeto some height above T. But the pressure of this additional co-lumn causes some more of it to go into the air-vessel, and com-press its air so much more that its elasticity now balances a longercolumn. Every succeeding stroke of the piston produces a likeeffect. The water rises higher in the main pipe, but some more

of it goes into the air-vessel. At last the water appears at the placeof delivery; and the air in the air-vessel is now so much com-pressed, that its elasticity balances the pressure of the whole co-lumn. The next stroke of the piston sends forward some morewater. If the diameter of the orifice of the main be sufficient tolet the water How out with a velocity equal to that of the piston, itwill so flow out, rising no higher, and producing no sensible addi-tion to the compression in the air-vessel. But if the orifice of themain be contracted to half its dimensions, the water sent forwardby tiie piston cannot flow out in the time of the stroke without agreater velocity, and therefore a greater force. Part of it, there-fore, goes into the air-vessel, and increases the compression. Whenthe piston lias ended its stroke, and no more water comes forward,the compression of the air in the air-vessel being greater than whatwas sufficient to balance the pressure of the water in the mainpipe, now forces out some of the water which is lying below it.This cannot return towards the pump, because the valve S is newshut. It therefore goes forward along the main, and produces auefflux during the time of the piston’s rising in order to make ano-ther stroke, In order that tills efflux mav he very equable, theair-vessel must be very large. If it be small, the quantity of wa-ter that is discharged by it during the return of the piston makesso great a portion of its capacity, that the elasticity of the confinedair is too much diminished by this enlargement of its bulk, and therate of efilux must diminish accordingly. The capacity of theair-vesseL should be so great that the change of bulk of the com-pressed air during the inaction of the piston may be inconsiderable.It must therefore be very strong. It is indifferent in what waythis air-vessel is connected with the rising pipe. It may join itlaterally, as in fig. 7, ami the main pipe go on without interruption.;or it maybe made to surround an interruption of the main pipe,as in fig. 8. It may also be in any part of the main pipe. If tbqsole effect intended by it is to produce an equable jet, as in theornamental water-works, it may be near the end of the main.This will require much less strength, because there remains but ashort column of water to compress the air in it. JBut it is, on thewhole, more advantageous to place it as near the pump as possible,that it may produce an equable motion in the whole main pipe.

Pump, Lifting. The simplest form and situation of the lift-ing pump, is represented by the sketch, fig. 3. The pump is i im-mersed in the cistern till both the valve G and piston F are underthe surface All of the surrounding water. The water thus entersthe pump, opening both valves, and finally stands on a level with-in and without. Flow draw up the piston to the surface A. Itmust lift up the water which is above it (because the valve in thepiston remains shut by its own weight) ; so that its surface willnow. be at a, Aa being made equal to AK. In the mean time, thepressure of the surrounding water forces it into the working bar-rel through the valve G; and the barrel is now filled with water.Now let the piston be pushed down again; the valve G imme-diately shuts by its own weight, and in opposition to the endea-vour which the water in the barrel makes to escape this way.This attempt to compress the water in the barrel causes it to openthe valve F in the piston ; or rather this valve yields to our en-deavour to push the piston down through the water in the work-ing barrel. Tims we get the piston to (lie bottom of the barrel;and it has now above it the whole pillar of water reaching to theheight a., Drawing up the piston to the surface A, a secondtime, must lift this double column along with it, and its surfacenow will be at />. The piston may again be thrust down throughthe water in the barrel, and again drawn up to the surface, whichwill raise the water to c. Another repetition will raise it to d, andit will now shew itself at the intended place of delivery. Anotherrepetition will raise it to c ; and while the piston is now descend-ing to make another stroke, the water in cd will be running offthrough the spout D; and thus a slrcainwill.be produced, insome degree continual, but very unequal. This is inconvenientin many cases : thus, in a pump for domestic uses, such a hob-bling stream would make it verjTroublesome to fill a bucket, (tis, therefore, usual to terminate the main by a cistern l.MNO,and to make the spout small. By these means the water broughtup by the successive strokes of the piston rises to such a height inthis cistern as to produce an efflux by (he spout nearly equable.The smaller we make the spout D, the more equable will be thestream; lor when the piston brings up more water th.ui can be

discharged