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tudinal, and the latter a transverse section. In all of the figures, the same letters indicate corresponding parts.

The air-pump consists of two glass cylinders, called barrels, in which are pistons, P and Q, made of leather, thoroughly soaked in oil. The pistons are attached to rods, and are elevated and depressed by a lever, NM, Fig. 89, which imparts an oscillating motion to a pinion, K. The teeth of this pinion engage with corresponding ones

Give a complete description of the air-pump. Barrels. Pistons

Rode.

on the inner sides of the piston rods, A and B. The machine is so arranged that one rod ascends whilst the other descends. The cylinders rest upon and are firmly attached to a platform, H, Fig. 88. On the same platform, H, is a column, I, which supports a plate, G. Resting upon the plate G, is a bell glass, R, called a receiver. The receiver communicates with both cylinders by a pipe, shown in Fig. 88.

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This pipe branches near the cylinders, one branch leading to each cylinder, as shown in Fig. 89. The pipe communicates with the cylinders by openings, which may be closed by conical valves, a and b. The valves a and b are attached to rods which pass through the pistons, and fitted to slide with gentle friction as the pistons move up and down. In the pistons are valves, s and t, which are gently

Receiver. Pipe. Valves. Valve rods.

pressed by spiral springs so as to permit the condensed air to escape and then to close the orifices in the valves. All of the valves, a, b, *, and t, open upwards.

In explaining the action of the air-pump, it will be sufficient to consider a single barrel, as shown in Fig. 88. The piston, P, being at the bottom of the barrel, the valves a and t are closed. If the piston be raised, the valve a ic opened, whilst the valve t is kept closed by the spiral spring and the pressure of the atmosphere. The valve a is soon arrested by its rod coming in contact with the top of the barrel, and it then remains open during the ascent of P. The air in the barrel above the piston is driven out at the opening, r, and that in the receiver and pipe expands so as to fill the receiver, pipe, and barrel.

If the piston, P, be depressed, it at once closes the valve a, and compresses the air in the barrel till its elastic force becomes great enough to force open the valve t, when it escapes into the atmosphere.

B

Fig. 89.

M

By this double stroke of the piston, P, a portion of the air is exhausted from the receiver, and if a second double stroke be made, a portion of what remains may in like manner be exhausted, and so on until nearly a perfect vacuum is formed in the receiver,

Describe the action of the air-pump in detail.

R, or in any other closed vessel attached to the pipe of the machine.

What has been said of one barrel, is equally true of the other; in fact, the instrument, as figured, is a double pump.

Measure of the Rarefaction produced.

126. In order to measure the degree of rarefaction produced, a glass cylinder, E, Fig. 87, is connected with the pipe by means of an opening through the column I. In this cylinder, is a glass tube bent into the form of the letter U, one branch being closed at the top, and the other open. The tube has its closed branch filled with mercury, and is called a siphon gauge.

The mercury, under ordinary circumstances, is kept in the closed branch by the atmospheric pressure, but as the air becomes rarefied in the receiver, the tension of the air becomes less and less, and finally the mercury falls in the closed branch and rises in the open one. The difference of level between the mercury in the two branches, is due to th tension of the rarefied air, and if this difference be aeter mined by means of a proper scale attached to the gauge. the tension can be found. Thus, if the difference of level is reduced to one inch, the tension of the air in the receiver will be only one thirtieth part of the tension of the external atmosphere.

Experiments with the Air-pump.

127. We have already described several experiments requiring the employment of the air-pump, such as the shower of mercury, Fig. 1: the fall of bodies in a vacuum, Fig. 2; the bladder in a vacuum, Fig. 70; the bursting membrane, Fig. 72; and finally, the hemispheres of Magdebourg, Fig. 73.

(126.) How may the degree of rarefaction be measured? What is the siphon gauge? Explain its action and use.

The machine may be used to show that the air is necessary to the

support of combustion and ani

mal life. If a lighted taper be placed under the receiver, and the air exhausted, the light will grow dim, and finally will go out entirely. If an animal or bird be placed under the receiver, and the air exhausted, it will struggle and soon die. This experiment is shown in Fig. 90.

Animals and birds die as soon as they are placed in a vacuum; reptiles support life longer when deprived of air. as to certain insects, they live for many days under an exhausted receiver. They are enabled to live on the small supply of air which remains in the receiver, after as much of it as possible is extracted.

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Preservation of Food in a Vacuum.

128. It has been discovered that articles of food which would soon perish if exposed to the air, may be preserved fresh for a long time if kept in a vacuum.

If fruits, vegetables, and the like, be placed in a bottle with water, and then heated gradually till ebullition takes place, all of the air will be driven out, being replaced by steam. If the bottle is corked and sealed in this condition, the fruit will remain fresh for years. On this principle, vast quantities of meat, fruit, vegetables, and the like, are prepared for naval and other purposes. Instead of bottles, tin canisters may be employed, which, after expelling the air, are hermetically sealed by soldering.

(127.) How is it shown that air is necessary to combustion and animal life! What animals support life longest in a vacuum? (128.) How are articles of food preserved in vacuo? What applications are made of this principle?

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