PU M

441

P U M

discharged during its descent, some of it remains in the cistern.This added to the supply of the next stroke, makes the water risehigher in the cistern than it did by the preceding stroke. Thiswill cause the efllux to be quicker during the descent of the pis-ton, hut perhaps not yet sufficiently quick to discharge the wholesupply. It therefore rises higher next stroke; and at last it risesso high that the increased velocity of efllux makes the dischargeprecisely balance t\e supply. Now, the quantity supplied meach stroke is the same, and occupies the same room in the cis-tern at top; and the surface will sink the same number of inchesduring the descent of the piston, whether that surface has beenhigh or low at the beginning. But because the velocities of theefllux are as the square roots of the heights of the water above thespout, it is evident that a sink of two or three inches will make asmaller change in the velocity of efllux when this height and velo-city are great. It is evident that the force necessary for this ope-ration must be equal to the weight of the pillar of water dAuk), ifthe pipe he perpendicular. When the stroke of the piston isshort, in order that the water may he delivered at the spout D, letM»t,(fig. 4,) be the lowest, and N tithe highest, positions of the piston,and let HA = h be the height of a column of water equiponder-ant with the atmosphere. When the pump is filled, not with wa-ter, but with air, and the piston is in its lowest, position, and all inequilibrio, the internal air has the same density and elasticity withthe external. The space MA am, therefore, contains air of thecommon density and elasticity. These may be measured by h,Or the weight of a column of water whose height is h. Now’, letthe piston be drawn up to Nti. The air which occupied thespace MAam now occupies the space NA an, and its density is

now \ Its elasticity is now diminished, being proportion-

able to its density (see Pneumatics), and no longer balances thepressure of the atmosphere. The valve G will therefore be forcedup by the water, which will vise to the. same height SA. Now letthe piston again descend to M in. It cannot do this with its valvesshut; for when it comes down so far as to reduce the air again toits common density, it is not yet at M, because the space below ithas been diminished by the valve G. The piston-valve, there-fore, opens by the air which we thus attempt to compress, and thesuperfluous air escapes. W hen the piston has got to M, the air is■again of the common density, and occupies the space MSf/n.Now draw the piston ..up to N. This air will expand into the

space NS s:i, and its density will be reduced to

paMS.vm

and its

elasticity will no longer balance the pressure of the atmosphere,■and more water will enter, and it will rise higher. '1 his will go oncontinually. But it may happen that the water will never rise sohigh as to reach the piston, even though not 33 feet above the water inthe cistern : for the successive diminutions of density and elasticityare a series of quantities that decrease geometrically, and thereforewill have a limit; and to insure the delivery of water by the pump,the stroke must be such that the rectangle MNxAtl maybegreater than any rectangle that can be made of the parts of AN,that is, greater than the square of half AN. Or, if the length ofthe stroke be already fixed by olhercircumstances, which is a com-mon case, we must make AN so short that the square of its half,measured in feet, shall be less than thirty-three times the stroke ofthe piston. If the pump be first filled with water it will workeffectually without attending to this precaution.

Pump, Sucking, a figure or two of this kind of pump will besufficient to shew its structure, and the contrivances used in work-ing it. Fig 9 represents a simple sucking pump, in which A isthe cistern; AC, the barrel or pipe, standing in the water EF;*~1D, the piston and bucket, with iu bucket and valve D ; and C,the valve open for the ascent of the water. Fig. 10 and 11 is avery simple and useful contrivance for working two pumps bymeans of the balance AB, having large iron-balls at each end",placed in euuilibrio on the two spindles C, as represented in thefigure ; on the right and left are two boards 1, nailed to two cross-pieces fastened to the axis of the machine. On these two boards,the person who is to work the pumps, stands, and supports himselfby lour posts, E, D, erected perpendicularly, and having cross-pieces on the top. At the distance of ten inches on each side ofvot.. IV.—no. 186.

the axis, are fastened the pistons M, N, which go to the suckers.The man, by leaning alternately on his right ami left foot, puts thebalance in motion, by which means the pumps O, P, are worked,and the water thrown into the pipe II, and carried to a height pro-portional to the diameter of the 1 valves, and the action of the ba-lance. It will be necessary to place on each side an iron-spring,as F, G, in order to return the balance, and prevent, its motionfrom being too great. The American pump-engine, lately con-trived by Mr. Benjamin Dearborn, which is so constructed, that itmay be conveniently added to a common pump, in order to an-swer the purposes of a fire-engine, deserves a description in thisplace. In fig. 12, A, B, C, D, represents a pump, the form ofwhich is similar to that of the pumps commonly employed onship-beard. E, the spout. F, a stopper. I), (I, a plank-cap,that is filled to the pump, and provided with leather on its lowersurface ; being secured by the screws a, b: in the centre is a hole,through which the spear of the pump passes, and roundwhich aleather-collar is made, as represented at the letter c. g, a nut forthe screw b. f, a square piece of wood, that is nailed across oneend of the plank-cap, through both which the screw a is intro-duted: a hole is made through such piece and the cap, that com-municates with the bore of the pump. G, G, a wooden tube,which may be of any requisite length, and consist of any numberof joints: it is made square at the lower extremity, and perforatedfor the reception of the cock; the upper end being made with anice shoulder, e, a wooden cock, that opens or shuts the com-munication between the pump and the tube; being furnished onthe opposite side with a handle and with a lock, in case it should 'be found necessary, h, h, are two ferules, the object of which isto prevent the tube from splitting. II, II,- braces, -each of whichought to he crossed over another, as nearly at right angles as pos-sible. i. i, are irons in the form of a staple, which surround thetube, and pass through the braces; (heir ends being perforatedwith holes for fore-locks. K, L, M, N, is a head, made of livepieces of wood; h, l, m, n, a square piece, in the lower part ofwhich is a hole for the reception of the extremity of the tube, andwhich piece rests on the shoulder o, p; to the lower end of thishead is nailed a piece of leather, with a hole in its centre, similarto that made in the wood. Another piece of leather of the sameform is placed on the top of the tube, and between both is a circleof thin plate-brass; the two pieces of leather and the brass beingpressed between the lower end of the head, and the shoulder olthe tube. Their edges are delineated at o, p. K, N, and L, M,are the edges of two pieces of plank, of a similar width with thehead, to which they' are closely nailed; each being provided witha tennon, that passes through a mortice in the end ol the piece O,P ; both tennons have holes for a forelock at q. O, P, a piece ofplank, of the same width as the sides; the centre of which is per-forated, in order that the tube may pass through ; and in each endof which is a mortice for the reception of the tennons. N, M, acap. r, r, are two pieces nailed to the side of the tube ; the lowerextremity of each is provided with a truck, with a view to .lessenthe friction of the head in its horizontal revolution, q, q, reprersent forelocks, the design of which is to fasten down the head, andprevent the water from escaping at the joint o, p. Q, R, is awooden conductor; the extremity marked with the letterQ, beingsolid, while the opposite end, K, is bored with a small auger. .?, abolt, that passes through the conductor and head,. being securedon the back with a fore-lock, or nut: this bolt is rounded near thehead, and square in the middle, t, u, tv, x, represents a piece ofiron or brass, designed to prevent the head ot the bolt from wear-ing into the wood. S, S, are ropes for the direction of the con-ductor. Fig. 13 represents the head without such conductor.a, b, c, d, is a thick brass-plate, the centre of which is perforated,so as to admit a passage to impurities, that might otherwise ob-struct the conductor: for which purpose a piece of leather is nailedunder it to the head. The square hole in the centre is adapted tothe size of the bolt, which it prevents from turning. The con-ductor has a hollow cut round the holt on the inside, of the samesize as the circle of holes in the brass: round such cavity is nailed,on the face of the conductor, a piece ot leather, that plays on themargin of the brass-plate, when the conductor is in motion. Themost common kind of valve consists of a piece of stiff leather, suchas is applied for soles in shoes, and is generally known by thename of pump-leather. On its upper side a piece of milled-leart5 U is