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longer balanced by the elasticity of the air in the tube, forces the liquid up into the mouth. If the straw were gradually increased in length, without increasing its whole capacity, we should find a certain length at which (however small the tube) we should not be able to raise the water into the mouth, for it would not rise more than a certain number of feet above its former level; and no sucking, even by the most powerful machinery instead of the mouth, could ever raise it more than 34 feet above that level, because the atmospheric pressure cannot counterbalance a column of water of a greater height than between 33 and 34 feet. And if we try to suck up mercury through a long tube, we shall be unable to raise it more than a few inches by the mouth, nor by the best air-pump more than about 30 inches (viz., the height at which the mercury of the barometer stands at the time), because the atmosphere cannot balance a column higher than this. Our reputed powers of suction, therefore, have nothing to do with the fluid. to be raised, for that work is done by the atmospheric pressure only: we assist it or bring it into play by using the mouth as an air-pump in withdrawing the air from the tube by enlarging the cavity of the mouth or lungs. In a boy's squirt, Fig. 7, the same principle is in operation. This simple but ingenious little instrument consists of a metal cylinder drawn at one end into a point, and furnished with a plug or piston. On introducing the point into water and drawing up the plug, a vacuum would be formed below it did not the water rise and fill it by atmospheric pressure.

Fig. 7.

13. A similar but somewhat more complicated process is carried on in the common household pump. long pipe A B, Fig. 8, dips at one end into the water of

A

THE HOUSEHOLD PUMP.

Fig. 8.

a well w, the other end is furnished with
a valve v. Above this pipe is another
and wider pipe c called the barrel, con-
taining a plug or piston p, furnished
with a valve v' in the centre; both these
valves open upwards. The piston is
worked up and down by means of a
lever called a brake. There are, how-
ever, various ways of working the piston,
which of course do not affect the prin-
ciple of the pump. At the commence-
ment of the operation of pumping, the
water in the well and in the pipe A B
stands at the same level. On raising
the piston a vacuum would be formed
below it, but the air in A B by its elas-
ticity raises the valve v and fills the
barrel. This increased expansion of
the air in A B diminishes its elasticity,
so that water is forced up into A B to a
certain height by the atmospheric pres-
sure on the exposed surface of the water
in the well. On depressing the piston
the valve v is closed and the valve B
forced open (as in Fig. 8, 2), through

27

[graphic]

which the air between v and the bottom of the piston escapes. On raising the piston a second time, more air rushes from A B, and the column of water in the pipe rises higher. In this way, by alternately raising and depressing the piston, all the air is drawn out of the pipe, and the column of water rises up to the valve v. On again raising the piston, water instead of air now opens the valve v, and rushes into the barrel, and, on lowering the piston, the water closes this valve v, thereby preventing it from again flowing back into the well. At the same time the water forces open the

valve v, and streams through it, so that water is now both above and below the piston. On continuing the action, the water rises higher and higher above the piston, until it reaches the spout s, where it is discharged. Now, it is quite evident that the length of the pipe AB must have a limit; since the atmosphere by its pressure is capable of supporting a column of mercury 30 inches high, and as the specific gravity of water is about 13 times less than that of mercury, it follows that a force which can sustain 30 inches of the heavier fluid will sustain a column of the lighter fluid 13 times greater in height, or about 405 inches, or 34 feet instead of 30 inches. But, as the barometric column in this country oscillates between 28 and 31 inches in height, it follows that a column of water supported by the atmosphere must also be subject to a proportional variation. Besides, as the mechanism of a common pump is by no means exact, some allowance must be made for its imperfections. Hence the length of the pipe A B ought never to exceed 30 feet above the level of the water in the well. It was the lucky accident of erecting a pump over a deep well at Florence that led to the discovery of the barometer and the pressure of the atmosphere, as already narrated in a companion treatise.

14. When it is desired to raise water to a great height, advantage is taken of the elasticity of a confined portion of air condensed into a smaller space than it usually occupies under atmospheric pressure. Such is the fire engine, the principle of which will be understood from the section shown in Fig. 9. H is the pipe, or hose, which is prolonged to the plug, or water pipe, whence the supply of water is obtained. This pipe H communicates with two valves v v, which open into the pump barrels of two forcing pumps * AA, containing

* I.e., pumps like the exhausting syringe, first described (10) instead of the air-pump (11) and common water-pump just mentioned.

THE FIRE-ENGINE.

29

solid pistons p p. The piston rods of these are connected with a working beam so arranged as to be worked by a number of per

sons on each side.

From

the sides of the pump barrels above the valves v v proceed force pipes which communicate with an air chamber c by means of valves v' v' opening upwards into it. Through the top of the air chamber descends nearly to its bottom a pipe e, to the upper part of which is attached the hose or jet used for directing a stream of water on the fire. By the alternate action of the pis

Fig. 9.

[graphic]

tons pp, water is drawn through the valves v v and propelled through the forcing valves 'v' precisely as took place with the air in Fig. 8, and, when the surface of the water rises above the lower mouth of the pipe e, the air in the air chamber c is confined above the water; and, as the water accumulates in the air chamber, the inclosed air is compressed and acts with increased elastic force on the surface of the water, thereby forcing a column of water into the pipe e and out through the hose and jet attached to it with a force depending partly on the degree of condensation and partly on the elevation of the extremity of the hose above the level of the engine. The pressure of the condensed air has first to support a column of water, whose height is equal to the level of the end of the tube above the level of the water in the air chamber; and until the pressure of the condensed air exceeds what is necessary for this pur

pose no water can spout from the end of the hose; and, secondly, the force of the jet will be proportional to the excess of the pressure of the condensed air above the weight of the column of water, whose height is equal to the elevation of the end of the hose above the level of the water in the air vessel. When the air in the air chamber is condensed into half its original bulk, it will act upon the surface of the water with double the atmospheric pressure; while, the water in the force pipe, being subject to only one atmospheric pressure, there will be an unresisted upward force equal to one atmosphere which sustains the column of water in the tube; consequently, a column will be sustained or projected to the height of 34 feet. When the air is reduced to 3rd of its original bulk, the height of the jet will be 68 feet, and so on.

Fig. 10.

15. The siphon is not a pump, but a bent tube so contrived as to convey liquids from one vessel to another at a lower level, by raising them first above their natural level in the first vessel. In Fig. 10, the siphon, or bent tube A B C, has its shorter leg A B immersed in the liquid which is to be transferred from the vessel D to E. At the commencement of the operation the shorter leg is immersed in the vessel D, and the air removed from the tube by applying the mouth to the extremity c of the longer limb, or the end c may be closed and the air sucked out through an opening at the top of the bend,

[graphic]

F

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which is afterwards closed; or, lastly, E the siphon may be held with its ends

upwards, filled with liquid, the ends

closed, turned downwards, and the shorter immersed in D. But in any case, however the siphon may be

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