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being compressed, raises the valve c, and passes above the piston (fig. 125). Once this effect is produced, when the piston ascends, the valve c closes, and the water which has passed above the piston being raised with it, ultimately flows out by a lateral tubulure in the barrel (fig. 126).

Since it is the atmospheric pressure which raises the water in the pipe, the height of the valve a, above the level in the vessel, cannot exceed a certain limit. A column of water, 34 feet in height, balances, as we have seen, the pressure of the atmosphere (117). Hence if the pipe had a greater length than this, when once water had reached this height, the column of water in the pipe would balance the pressure of the atmosphere on the water of the well, and it could not be raised any higher. Hence this would be the extreme theoretical limit which the pipe could have; but in practice the height of the tube A does not often exceed 26 to 28 feet; for, although the atmospheric pressure can support a higher column, the vacuum produced in the barrel is not perfect, owing to the fact that the piston does not fit exactly on the bottom

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Fig. 127.

Fig. 128. of the barrel. But when the water has passed the piston, it is the lifting force of the latter which raises it, and the height to which it can be brought depends on the force which moves the 'piston.

146. Force-pump. In these water is not raised by the pressure of the atmosphere, but by the pressure of the piston on the water during its descent. For this purpose the piston is solid, that is, has no valve, and there is no lifting pipe, the barrel being immersed in the liquid to be raised (figs. 127 and 128). There are two valves in the barrel ; one, a, in the bottom, opens upwards; the other, b, is placed in the orifice of a long tube in the side of the pump.

When the piston rises (fig. 127), a vacuum being produced below it, the atmospheric pressure acts on c, and closes it; while the water in which the pump is immersed being forced by its own weight and that of the atmosphere, raises the valve a, and passes into the barrel which it completely fills. The motion of the pistons is exactly reversed when the piston descends (fig. 128). By its own weight and by the pressure upon it, the valve a closes, while the valve c opens and gives exit to the water in the barrel, which then rises to a height depending on the pressure exerted by the piston. If this amounts to a pressure of one atmosphere, water rises 34 feet in the pipe H; if it is two atmospheres water rises to 68 feet, and so on ; that is, always to a height of 34 feet for a pressure of one atmosphere. The height, therefore, to which water can be raised in these pumps is not limited as it is in the friction-pump.

From what has been said, it will be seen that water only rises in the pipe H when the piston descends; there is, therefore, an intermittent flow at the end of the pipe. A more regular flow is obtained by arranging two pumps, both forcing water into the same pipe, and in such a manner that, when one piston rises, the other sinks. It is by means of such an arrangement of two pumps that air is raised to the wicks in Carcel's lamps. At the base of these lamps, and in the oil itself, are two small pumps worked by a clockwork motion, which is wound up like a clock. Such a system is also applied in fire-engines.

147. Fire-engines.-In a fire-engine water has to be forced to a great height in a continuous stream. Fig. 129 represents a section of such a pump. To the handles PQ are fixed, by means of a joint, two rods which work the pistons m and n in two brass barrels. These pumps are placed in a trough, MN, of the same metal, which is called the tank, and which is fed with water while the pump is at work. Between these two is an air-chamber R, with a lateral aperture 2, to which can be attached a long leather tube. This

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tube is provided at the end with a long conical copper tube, and which has an aperture only about three-fifths of an inch diameter.

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Fig. 129. The use of the air-chamber is as follows : Although the pistons work alternately there would necessarily be some intermittence in the jet when they are at the top or at the bottom of their course. But the water, instead of being forced by the pumps directly into the ascending pipe, first passes into the reservoir R, as shown in fig. 129. Owing to the resistance in the tube and on the jet, it flows out of the reservoir more slowly than it enters. Its level rises in the reservoir, and as the air is thereby reduced in volume, its pressure increases, so that the compressed air, reacting on the water when the pistons stop, forces out the water and thus keeps up the continuity of the jet. A good fire-engine worked by eight men will raise water to a height of 100 feet.

148. The syphon.—The syphon is a bent tube open at both ends and with unequal legs (fig. 130). It is used in transferring liquids, especially in cases in which they are to be removed without disturbing any sediment they contain. It is worked in the following manner : The syphon is filled with some liquid, and the two ends being closed, the shorter leg is dipped in the liquid, as represented

in fig. 130; or the shorter leg having been dipped in the liquid, the air is exhausted by applying the mouth at b. A vacuum is thus

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produced, the liquid in d rises and fills the tube in consequence of the atmospheric pressure. It will then run out through the syphon as long as the shorter end dips in the liquid.

A syphon of the form represented in fig. 131 is used where the presence of the liquid in the mouth would be objectionable. A tube, a, is attached to the longer branch, and it is filled by closing the end of the longer limb, and sucking at the end of a. .

To explain this flow of water from the syphon, let us suppose it filled and the short leg immersed in the liquid. The pressure then acting on d, and tending to raise the liquid in the tube, is the atmospheric pressure minus the height of the column of liquid, cd. In like manner, the pressure on the end of the tube b is the weight of the atmosphere less the pressure of the column of liquid, ab. But as this latter column is longer than cd, the force acting at b is less than the force acting at c, and consequently a flow takes place proportional to the difference between these two forces. The flow will therefore be more rapid in proportion as the difference of level between the aperture b and the surface of the liquid in d is greater.

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149. Archimedes' principle applied to gases.—The pressure exerted by gases on bodies immersed in them, is transmitted equally in all directions, as has been shown by the experiment with the Magdeburg hemispheres. It therefore follows, that all which has been said about the equilibrium of bodies in liquids, applies to bodies in air; they lose a part of their weight equal to that of the air which they displace.

This loss of weight in air is demonstrated by means of the baroscope, which consists of a scalebeam, at one of whose extremities a small leaden weight is supported, and at the other there is a hollow copper sphere (fig. 132). They are so constructed that in air they exactly balance one another, but when they are placed under the receiver of the air-pump and a vacuum is produced, the sphere sinks; thereby

Fig. 132. showing that in reality it is heavier than the small leaden weight. Before the air is exhausted each body is buoyed up by the weight of the air which it displaces. But as the sphere is much the larger of the two, its weight undergoes most apparent diminution; and thus, though in reality the heavier body, it is balanced by the small leaden weight. It may be proved by means of the same apparatus that this loss is equal to the weight of the displaced air, and we may thus generalise Archimedes' principle and say, that any body plunged in any fluid, whether it be a liquid or a gas, loses part of its weight equal to the weight of the displaced fluid. Hence bodies weighed in air usually indicate too small a weight. To have an exact weight the volume of the weights and of the displaced fluid should be exactly the same, which is seldom the case. The true weight of bodies is obtained by weighing them in a vacuum.

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