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CHAPTER III.

APPARATUS WHICH DEPEND ON THE PROPERTIES OF AIR.

141. Air-pump.—The air-pump is an instrument by which a vacuum can be produced in a given space, or rather by which air can be greatly rarefied, for an absolute vacuum cannot be produced by its means. It was invented by Otto von Guericke in 1650, a few years after the invention of the barometer.

Fig. 121 gives a perspective view of the pump, fig. 122 gives a detailed longitudinal section, and fig. 123 gives a cross section.

The pump consists of two stout glass barrels in which two pistons, P and Q, made of leather well soaked with oil, move up and down, and close the barrels air-tight. The pistons are fixed to two racks, A and B, working with a pinion K (fig. 123), which is moved by a handle MN, so that when one piston rises the other descends.

The two barrels are firmly cemented on the base, H, which is of brass; on this plate is a column, I, terminated by a plate, G. On this plate is a glass bell jar which is called the receiver. In the interior of the column is a conduit, which is prolonged below the base between the two barrels. It there branches in the shape of a T, terminating in two apertures, a and b, in the bottom of the cylinders. These apertures are conical, and are closed by two small conical valves; these latter are fixed to metal rods which work air-tight, but with gentle friction in the pistons. In the pistons is a cylindrical cavity communicating with the lower part of the pump by two apertures, s and t (fig. 123). These apertures are closed by small clack valves, kept in position by springs which surround each of the rods themselves. The four valves, a, b, s, t, open upwards.

These details being known, the working of the machine is readily understood. It is sufficient to consider what takes place in a single piston, fig. 122. The piston P being first at the bottom of its stroke, on rising it raises the rod which traverses it, and therewith the valve a, which remains open during the ascent. The

valve which is in the piston, remains closed by the action of the spring and by the pressure of the atmosphere, which acts in the barrel through an aperture, r, in the cover. From this position of the two valves, it will be seen that, as the piston rises, the external

pressure of the atmosphere cannot act in the bottom of the barrel, but the air of the receiver, in virtue of its elasticity, expands and passes by the conduit, I and H, into the barrel. The receiver is still full of air, but it is more rarefied; it is less dense.

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When the piston descends, the rod which bears the valve, a, descending with it, communication between the receiver and the barrel is cut off. The air in the barrel becomes more and more compressed, its elastic force increases, and finally overcomes the atmospheric pressure; so that the valve t, being pressed upwards by the elastic force of the air in the interior more strongly than it is pressed downwards by the atmosphere, is raised, and allows the air of the barrel to escape into the upper part of the barrel, and

thence into the atmosphere. Thus a certain quantity of air has been removed. A fresh quantity is removed at a second stroke of the piston, another at the third, and so on. The air in the receiver is thus gradually more and more rarefied; yet all the air cannot be entirely extracted, for it ultimately becomes so rarefied, both in the receiver and in the barrel, that when the piston P is at the bottom of its stroke, the compressed gas below the piston has no longer

valve t which is in the piston, remains closed by the action of the spring and by the pressure of the atmosphere, which acts in the barrel through an aperture, r, in the cover. From this position of the two valves, it will be seen that, as the piston rises, the external

pressure of the atmosphere cannot act in the bottom of the barrel, but the air of the receiver, in virtue of its elasticity, expands and passes by the conduit, I and H, into the barrel. The receiver is still full of air, but it is more rarefied; it is less dense.

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When the piston descends, the rod which bears the valve, a, descending with it, communication between the receiver and the barrel is cut off. The air in the barrel becomes more and more compressed, its elastic force increases, and finally overcomes the atmospheric pressure; so that the valve t, being pressed upwards by the elastic force of the air in the interior more strongly than it is pressed downwards by the atmosphere, is raised, and allows the air of the barrel to escape into the upper part of the barrel, and

thence into the atmosphere. Thus a certain quantity of air has been removed. A fresh quantity is removed at a second stroke of the piston, another at the third, and so on. The air in the receiver is thus gradually more and more rarefied; yet all the air cannot be entirely extracted, for it ultimately becomes so rarefied, both in the receiver and in the barrel, that when the piston P is at the bottom of its stroke, the compressed gas below the piston has no longer