771 



MECHANICAL PHILOSOPHY. PNEUMATICa [THK Tin-worn. 



water above it clones the valve 0, and the fluid is forced 

 out of the spout S, while another vi uiiin i funning be- 

 low the piiton, ami fresh water rising in the barrel. 

 Thus every time the piston is raised, the lower valve 

 open* and tli- upper one clixios ; while, on the contrary, 

 very time the piston descends, the lower valve close* 

 and the upper one opens : and in a well-made pump the 

 barrel U kept constantly full. 



It will of r :>!- be understood. in all that u nid 

 above, that the pressure of the atmosphere on the. sur- 

 face of the water in the well, or reservoir, need nut be 

 sufficiently great to cause the water in the I>UIM]> to 

 ascend as high aa the spout, when, by the. action of the 

 handle, the air is withdrawn from the tube : if the 

 length of tlie siK-t ion-pipe be 28 or 29 feet, or rather, if 

 tbe valve at the top of it be at this height nl>ove the 

 water iu the well, the exhaustion of the air will bo fol- 

 lowed by the ascent of the water, through the suction- 

 pipe, to some distance np the barrel of the pump. Tlie 

 iwbsequent raising of the water, thus introduced into the 

 barrel, to the spout, is the sole result of the mechanics, 

 force applied to the pump-handle. 



TV oF WATER DISCHARGED, AND 

 KORCK APPLIED TO RAISK IT. The quantity of 

 water discharged at each stroke of the handle, supposing 

 the barrel to be constantly full that is, supposing the 

 spout not too high is a column of water whose base is 

 the horizontal section of the piston, and of the height tc 

 which the piston is raised, called the length of the stroke 

 thus, if r be the radius of tbe section in feet, and I the 

 length of the stroke in feet, we shall have 



Quantity discharged = 3 - 1416r ! / cubic feet j 



or, since a cubic foot of water weighs 1000 ounces, or 

 about 62i pounds avoirdupois, and since an 

 gallon contains 10 Ib. of water, we have 



Quantity discharged = 3 -1416^ X 6 '25 gallons. 

 But the force necessary to raise this quantity, will be 

 that required to raise the entire column of water ex- 

 tending from the surface of the reservoir to the surface 

 of the water in the pump, the base of the column being 

 the seetion of the piston. For let a be the number of 

 liue.-r feet of water above the piston, and 6 the nunilwr 

 of feet below it, to the surface of the reservoir ; let also 

 p be the length of the column of water equal to the 

 atmospheric pressure : then since, in raising the piston, 

 the downward pressure of the atmosphere has to be 

 overcome, the height of the equivalent column of water 

 lifted is o-f-/> ; but the upward pressure against this, is 

 equal to the weight of a column of water of height j> 6: 



y, the force necessary to lift the piston is 

 that necessary to lift a column of water having the same 

 section as the piston, and the height (a + fr), of the water 

 in the pump, from the surface of that in the reservoir; 

 and, in fact, additional force must be applied to pump 

 out the water, on account of friction, and the weight of 

 tlie pinton and rod. 



In the returning stroke of the pump-handle, the piston 

 Aerr- lit, which is snllirient to over- 



com< "ii. and the slii; the water. 



K.il:' Ml 1 AM Fli:i: KNtJINE. As the 



pressure of the atmosphere varies, pumps are not con- 

 structed to raise water to a height above 23 or _'.i fed ; 

 but the water thus raised may, by an additional contri- 

 vance, be forced upwards a* much higher than this as we 

 fileane. A pump for thin purpose U called a/orcic;/ innii/i. 

 In this pump there U no valve C in the sucker or piston ; 

 so that, after the exhaustion of the pump-barrel of Mir, 

 and : lent liilin^ of it with water, no downward 



prr-n.-'- n tl. hi i.-iiiM' its descent, since the 



water in inenmpre.siMe. A pi]"', therefore, u inserted in 

 the side of tbe barrel near tlie bottom, with a valve at 

 K-rtion o| oiling outwards ; the downward pressure 

 II the . i this valve, and drives the 



water into the tut*, which may be earned upwards to 

 any height, and be made to deliver the water there, pro- 



vided only sufficient downward pressure act on the 

 piston. 



The fire-engine < is essentially the niml.in.i- 



ticm of two forcing-pumps, the j which are 



worked by a lever whose fulcrum i i l>,-, and 



Fig. IDS. 



the power is applied at each end alt mutely ; the water 

 from the fire-plug is forced into the central receptacle, 

 called the air-vessel, the air in which is thus condensed 

 the more, the more water is forced in ; the elastic force 

 of this compressed air drives the water up the leathern 

 hose, from which it issues through the delivery-pipe with 

 a velocity proportionate to the pressure applied, or the 

 condensation produced in the confined air. If the pis- 

 tons be worked with sufficient energy to supply the air- 

 vessel with water as rapidly as it is thus delivered, the 

 stream will be invariable ; if more water be forced into 

 the vessel than can escape out of the delivery-pipe, the 

 air-vessel will be in danger of bursting, by the increased 

 pressure of the condensed air. 



The intention of the air-vessel will be readily per- 

 ceived by the student : the force applied to the two ends 

 of the lever, which works the double pump, is necessarily 

 an intermitting force ; and without some contrivance to 

 render the effect continuous, the water would issue from 

 the delivery-pipe in jerks, by the separate impulsions 

 given to it ; but the continuous pressure of the con- 

 densed air in the air-vessel, causes a continuous flow of 

 water along the hose, up to the point of issue. If the 

 supply of water to the air-vessel vary, the. velocity of 

 the issuing stream will, of course, vary likewise ; but 

 there will be, nevertheless, no interruption in the coii- 

 tinuousness of the discharge. 



In fact, the air-vessel performs an office a good deal 

 like that performed by the fly-wheel in ordinary ma- 

 chinery : it gives continuity to intermittent action, tends 

 to equalise the effectsof it, and prevents that strain on the 

 stnicture which irregularities of action are apt to pro- 

 duce. 



The metal pipe, from which the stream of water issues, 

 is considerably smaller in bore than the leathern hose 

 along which the fluid flows to it. A double purpose is 

 accomplished by this disparity. In the first place, the 

 friction in the hose is diminished, as a comparatively 

 smaller portion of water comes in contact with the in- 

 terior surface j and in the next place, the contraction of 

 the stream, at the outlet, gives to it a proportional in- 

 crease of velocity, so that the water is driven out of the 

 narrow pipe with much greater force (page 764). It may 

 be observed here, that the common syringe, or boys' 

 squirt, is a miniature forcing-pump, or embryo fire- 

 engine ; the great contraction of the' tube near the out- 

 let, is the cause of the velocity with whi'-h the water 

 issues, being so much greater than that with which the 

 moves. In the syringe there is certainly no air- 

 reuol, as in the fire-engine ; but the in-H.su re on the 

 water is continuous, so long as the tube contains any. 



THK 1>IVIN<; IIKI.I. -The divn-V bell in a heavy 

 ron chest open at the bottom ; it is called a Ml, because, 

 n its original construction, a bell-form figure wasgncn 

 to it The interior U furnished with seats, sufficiently 



