535. The air gun is an instrument of no practical importance, but which may be noticed as its action depends on the condensation of air. A strong chamber is constructed into which air is condensed until the elastic force of the whole is very great. The chamber is connected with a tube in which a bullet is placed; by opening a valve the condensed air rushes out and sweeps the bullet along the tube, from which it issues with great velocity: the force which drives the bullet along the tube is the excess of the pressure of the condensed air behind, above the ordinary pressure of the atmosphere in front.

XLIX. PUMPS.

536. There are various machines for raising water from one level to another which is higher; and we will now describe some of them.

D

537. The common pump sometimes called the suction pump. AB is a cylinder having at the bottom a valve C opening upwards. A piston works up and down in the cylinder, having a valve D opening upwards. A pipe BE passes from the bottom of the cylinder, and the end of it is below the surface of the water in a well; let E denote the level of the water in the well. Suppose the piston to be at C, and the pipe to be full of air. Let the piston be raised to A; then the pressure of the atmosphere keeps the valve D closed, and the pressure on the valve C being lessened the air in the pipe opens this valve and fills the cylinder below the piston. The pressure of the air in the pipe is now

C

B

M

E

less than that of the atmosphere, and accordingly the pressure of the atmosphere on the surface of the water in the well forces water up the pipe EB to such a height as to make the pressure at E equal to that of the atmo

sphere. When the piston descends the valve C closes, and the air between C and the piston escapes through D. The water will rise in EB each time this operation is repeated until at last it passes through C; and now when the piston descends to C the water passes through D and is then carried up by the piston as it ascends and discharged through the spout at A.

538. It will be observed that the ascent of the water consists in general of two distinct processes. The water is received from E to B by the pressure of the atmosphere, and in consequence of this EB must not be higher than the column of water which this pressure would support, that is about 34 feet. But the length AB may be as great as we please, provided that we have a cylinder and a piston rod of sufficient strength, and force enough to do the requisite work. For when the water reaches to a point in the cylinder the height of which above E is greater than the standard 34 feet, the pressure of the atmosphere will take it no further, and it must be lifted by the ascending piston from this point up to the spout. If the height of the spout above E is not greater than the standard 34 feet, then we have not the two processes but only the first of those just noticed.

539. An error is frequently made with respect to the amount of force which must be used to work the piston; it seems to be imagined that the pressure of the amosphere renders, or ought to render, any application of force to the piston unnecessary. Let us suppose that the piston is at some point between A and B, and that the water in the pipe has risen to the level M; so that between M and the piston there is air. Then above the piston we have the pressure of the atmosphere; and below the piston we have this pressure diminished by so much as corresponds to the height of the column EM. Thus on the whole the piston is urged down by a pressure which is measured by the height of the column EM of water; and so force must be applied sufficient to overcome this. But if the height of the piston above E is greater than the standard 34 feet, then below the piston there is a vacuum, and the pressure above it is the pressure of the atmosphere increased by the weight of the water which is to be lifted.

540. The forcing pump. AB is a cylinder having at the bottom a valve Copening upwards. A piston D works up and down in the cylinder. A pipe BE passes from the bottom of the cylinder, and the end

D

C

B

E

A

of it is below the surface of the water in a well; let E denote the level of the water in the well. Just above Ca tube BF passes from the cylinder and has a valve at F opening upwards. Suppose the piston to be at C, and the pipe to be full of air. Let the piston be raised; then the pressure on the valve C being lessened the air in the pipe opens this valve and fills the cylinder below the piston. The pressure of the air in the pipe is now less than that of the atmosphere, and accordingly the pressure of the atmosphere on the surface of the water in the well forces water up the pipe EB to such a height as to make the pressure at E equal to that of the atmosphere. When the piston descends the valve C closes, and part of the air between the piston and C is forced through the valve F. The water will rise in EB each time this operation is repeated until at last it passes through C; and now when the piston descends some of the water is forced through the valve F. As in the common pump EB must not be greater than the standard 34 feet; but the ascending tube may be as long as we please, and if the pump be of sufficient strength and the force enough for the work, we may raise water to any height we please.

K

Sometimes instead of a piston D there is a solid cylinder working through a water-tight collar at A. 541. The stream of issuing water may be made continuous by connecting the tube BF with a large vessel having a pipe HK which reaches nearly to the bottom. Suppose the water to be forced into this vessel and to reach the level G. Then above G there is condensed air which formerly occupied all the vessel above the level H, and the pressure of this condensed air on the

H
F

water in the lower part of the vessel forces out the water through HK in a continuous stream.

542. The fire engine is a forcing pump with the appendage just described; there are usually two cylinders worked simultaneously, so that one ascends while the other descends in the manner mentioned in Art. 525 with respect to the air pump.

543. There are various other contrivances for raising water, but we need not delay long upon them, as they do not involve any new principle. We may just notice a pump called the lifting pump, which differs from the suction pump in having the fixed valve above the piston instead of below it. The piston is moved up and down by a frame-work of which PQR represents part. When the piston descends the valve D in it opens, and water rises above it; and when the piston ascends it lifts this water through the fixed valve C to any height that may be desired. This contrivance is said to avoid the inconvenience arising "from the length of the barrel through which the piston rod of a sucking pump would have to descend in order that the piston might remain within the limits of atmospheric pressure."

D

P

544. It will be observed that in all the pumps which we have described the pressure of the atmosphere discharges a very important function. There are however processes for raising water in which this pressure is not concerned. A simple example is that of drawing water from a well by the aid of a bucket. The chain pump is of the same kind; through a vertical cylinder moveable bottoms or pistons are drawn one after another lifting the water above them. In the plunging pump a long hollow cylinder having at its lowest part a valve opening upwards is inserted in water; the cylinder is so long as to reach considerably above the surface of the water. The water enters through the valve, and rises to the same level inside

the cylinder as outside. A solid cylinder somewhat less in diameter than the hollow cylinder is plunged into the hollow cylinder; the water having no other escape is driven up in the hollow cylinder, and may be conducted through a spout provided at the highest part. Such a pump has been constructed with two cylinders, and two plungers working simultaneously, one ascending as the other descends; a plank moveable about its middle point has its ends connected with the plungers, and a man walking backwards and forwards on the plank continually by his weight supplies the force necessary to raise one plunger and depress the other.

545. The Screw of Archimedes is a machine for raising water, which is said to have been invented by that ancient philosopher for the purpose of enabling the inhabitants of the low grounds of Egypt to clear away the stagnant water left by the Nile after its inundations. This machine may be presented in slightly different forms, and we will confine ourselves to the simplest. A hollow tube is bent into the form of a corkscrew and placed inclined to the horizon. The screw can be turned round in the manner of a corkscrew and is so fixed that its lower end alternately dips below the surface of the water and rises above it as the screw is turned round. Then during each turn of the screw water enters at the lower end; and in successive turns of the screw the water thus entering passes on up the screw until at last it issues from the top. The fact that the water will thus pass along the screw is not very easy to establish by the aid of diagrams so as to be intelligible to the early student; but it becomes clear on examining a model of the machine. The screw must obviously not be inclined at too great an angle to the horizon for instance, if it is placed vertically no water will be raised. Let the screw be at rest, and suppose that as we pass along it we find points such that the screw rises on each side of them, then the proper inclination is not exceeded. Moreover, of the two directions in which the screw can be turned round only one is suitable, namely, that in which a corkscrew would be turned round in order to penetrate a cork occupying the place of the water.