, in Hydraulics, a machine for raising water, and other fluids.

Pumps are probably of very ancient use. Vitruvius ascribes the invention to Ctesebes of Athens, some say of Alexandria, about 120 years before Christ. They are now of various kinds. As the Sucking Pump, the Lifting Pump, the Forcing Pump, Ship Pumps, Chain Pumps, &c. By means of the lifting and forcing Pumps, water may be raised to any height, with a sufficient power, and an adequate apparatus: but by the sucking Pump the water may, by the general pressure of the atmosphere on the surface of the well, be raised only about 33 or 34 feet; though in practice it is seldom applied to the raising it much above 28; because, from the variations observed in the barometer, it appears that the air may sometimes be lighter than 33 feet of water; and whenever that happens, for want of the due counterpoise, this Pump may fail in its performance.

The Common Sucking Pump.—This consists of a pipe, of wood or metal, open at both ends, having a fixed valve in the lower part of it opening upwards, and a moveable valve or bucket by which the water is drawn or lifted up. This bucket is just the size of the bore of the Pump-pipe, in that part where it works, and leathered round so as to sit it very close, that no air may pass by the sides of it; the valve hole being in the middle of the bucket. The bucket is commonly worked in the upper part of the barrel by a short rod, and another fixed valve placed just below the descent of the bucket. Thus, (fig. 1, pl. 23), AB is the Pump-pipe, C the lower fixed valve, opening upwards, and D is the bucket, or moving valve, also opening upwards.

In working the Pump; draw up the bucket D, by means of the Pump rod, having any sort of a handle fixed to it: this draws up the water that is above it, or if not, the air; in either case the water pushes up the valve C, and enters to supply the void left between C and D, being pushed up by the pressure of the atmosphere on the surface of the water in the well below. Next, the bucket D is pushed down, which shuts the | valve C, and prevents the return of the water downwards, which opens the valve D, by which the water ascends above it. And thus, by repeating the strokes of the Pump-rod handle, the valves alternately open and shut, and the water is drawn up at every stroke, and runs out at the nozle or spout near the top.

The Lifting Pump differs from the sucking Pump only in the disposition of its valves and the form of its piston frame. This kind of Pump is represented in fig. 2, pl. 23; where the lower valve D is moveable, being worked up and down with the Pump rod, which lifts the water up, and so opens the upper valve C, which is fixed, and permits the water to issue through it, and run out at top. Then as the piston D descends, the weight of the water above C shuts that valve C, and so prevents its return, till that valve be opened again by another lift of the piston D. And so alternately.

The Forcing Pump raises the water through the sucker, or lower valve C (fig. 3, pl. 23), in the same manner as the sucking Pump; but as the piston or plunger D has no valve in it, the water cannot get above it when this is pushed down again; instead of which, a side pipe is inserted between C and D, having a fixed valve at E opening upwards, through which the water is forced out of the Pump by pushing down the plunger D.

Observations on Pumps.—The force required to work a Pump, is equal to the weight of water raised at each stroke, or equal to the weight of water filling the cavity of the pipe, and its height equal to the length of the stroke made by the piston. Hence if d denote the diameter of the pipe, and l the length of the stroke, both in inches; then is .7854d2l the content of the water raised at a stroke, in inches, or .0028d2l in ale gallons; and the weight of it is (d2l)/220 ounces or ((d2l)/3520)lb. But if the handle of the pump be a lever which gains in the power of p to 1, the force of the hand to work the Pump will be only ((d2l)/(3520p))lb, or, when p is 5 for instance, it will be ((d2l)/17600)lb. And all these over and above the friction of the moving parts of the Pump.

To the forcing Pump is sometimes adapted an air vessel, which, being compressed by the water, by its elasticity acts upon the water again, and forces it out to a great distance, and in a continued stream, instead of by jerks or jets. So, Mr. Newsham's water engine, for extinguishing fire, consists of two forcing Pumps, which alternately drive water into a close vessel of air, by which means the air in it is condensed, and compresses the water so strongly, that it rushes out with great impetuosity and force through a pipe that comes down into it, making a continued uniform stream.

By means of forcing Pumps, water may be raised to any height whatever above the level of a river or spring; and machines may be contrived to work these Pumps, either by a running stream, a fall of water, or by horses.

Ctesebes's Pump, acts both by suction and by pression. Thus, a brass cylinder ABCD (fig. 5, pl. 23) furnished with a valve at L, is placed in the water. In this is fitted the piston KM, made of green wood, which will not swell in the water, and is adjusted to the aperture of the cylinder with a covering of leather, but without any valve. Another tube NH is fitted on at H, with a valve I opening upwards.—Now the piston being raised, the water opens the valve L, and rises into the cavity of the cylinder. When the piston is depressed again, the valve I is opened, and the water is driven up through the tube HN.

This was the Pump used among the Ancients, and that from which both the others have been deduced. Sir Samuel Morland has endeavoured to increase its force by lessening the friction; which he has done to good effect, so as to make it work with very little.

There are various kinds of Pumps used in ships, for throwing the water out of the hold, and upon other occasions, as the Chain Pump, &c.

Air-Pump, in Pneumatics, is a machine, by means of which the air is emptied out of vessels, and a kind of vacuum produced in them. For the particulars of which, see Air-Pump.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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PURBACH (George)