Otto Guericke's instrument was imperfect in construction and difficult of management. The apparatus required to be kept under water. More con

venient machines have therefore been devised. The following is a description of one of the most simple:-Upon a metalic basis, h h, Fig. 13, are fastened two exhausting syringes, a a, which are worked by means of a handle, b, the two screw columns, ee, aided by the cross piece, ff, tightly compressing them into their places. A jar, c, called a receiver, the mouth of which is carefully ground true, is placed on the plate of the pump, hh, which is formed of a piece of metal or glass ground quite flat. This pump-plate is perforated in its centre, from which air-tight passages lead to the bottom of each syringe, and when the handle, b, is moved, the syringes withdraw the air from the interior of the jar. From the same central perforation there is a third passage, which can be opened or closed by the screw at g, so that when the experiments are over, by opening it, the air can be re-admitted into the interior of the receiver.

Fig. 13.

So far as its exterior parts are concerned, this air-pump consists of a pair

P

Fig. 14.

of syringes worked by a handle, and producing exhaustion of the interior of a jar, with a vent which can be closed or opened for the re-admission of air.

The syringes are constructed exactly alike. The glass model, represented in Fig. 14, exhibits their interior ; each consists of a cylinder, a a, the interior of which is made perfectly true, so that a piston, p, introduced at the top may be pushed to the bottom, and, indeed, work up and down without any leakage. There is a hole made through the piston, p, and over it a valve is laid. This consists of a flexible piece of membrane,

-as leather, silk, &c.,--which being placed on the aperture, opens in one Such a valve is in the piston, and there is another one, c, resting on an aperture in the bottom of the cylinder. To understand the action of this instrument, let us suppose a glass globe, full of atmospheric air, to be fastened air-tight to the bottom of such a syringe, and the piston then lifted to the top of the cylinder. As it moves without leakage, it would evidently leave a vacuum below it, were it not that the air in the globe, exerting its elastic force, pushes up the valve, c, and expands into the cylinder. In this way, therefore, by the upward

movement of the piston, a certain quantity of air comes out of the globe and fills the cylinder. The piston is now depressed: the moment it begins to descend

the valve, c, which leads into the globe, shuts; and now, as the piston comes down, it condenses the air below it, and as this air is condensed, it resists exerting its elastic force. The piston-valve, p, under these circumstances, is pushed open, and the compressed air gets away into the atmosphere. As soon as the piston has reached the bottom of the cylinder all the air has escaped, and the process is repeated precisely as before. The action in the syringe is, therefore, to draw out from the globe a certain quantity of air at each upward movement, and expel this quantity into the air at each downward

movement.

For reasons connected with the great pressure of the air, and also for expediting the process of exhaustion, two syringes are commonly used. To their pistons are attached rods which terminate in racks, bb ; between these is placed a toothed wheel, which is turned on its axis by the handle, its teeth taking into the teeth of the racks. When the handle is set in motion, and the wheel made to revolve, it raises one of the pistons, and at the same time depresses the other. The ends of these racks are seen in Fig. 14. The wheel is included in the transverse wooden bar, ff, Fig. 13. By the aid of this invaluable machine numerous striking and important experiments may be made. The form described here is one of the most simple, and by no means the most perfect. For the higher purposes of science, more complicated instruments have been contrived, in which,

I

Fig. 15.

Fig. 16.

d

a

C

with the utmost perfection of workmanship, the valves are made to open by the movements of the pump itself, and do not require to be lifted by the elastic force of the air. In such pumps, a far higher degree of rarefaction can be obtained.

No air-pump, no matter how perfect it may be, can ever make a perfect vacuum, or withdrew all the air from its receiver. The removal of the air depends on the expansion of what is left behind, and there must always be that residue remaining which has forced out the portion last removed by the action of the syringes.

The fundamental fact in the science of Pneumatics is, that atmospheric air is a heavy body, and this may be proved in a very satisfactory manner by the

aid. of the pump. Let there be a glass flask, a, Fig. 15, the mouth of which is closed with a stop-cock, through which the air can be removed. If from this flask we exhaust all the air, and then equipoise it with weights at a balance, as soon as the stop-cock is opened and the air allowed to rush in, the flask preponderates. By adding weights in the opposite scale, we can determine how much it requires to bring the balance back to equilibrio, and therefore what is the weight of a volume of air equal to the capacity of the flask.

Upon the same principles we can prove that all gasses, as well as atmospheric air, have weight. It is only requisite to take the exhausted flask, and having counterpoised it as before, screw it on the top of a jar, a, Fig. 16, containing the gas to be tried. On opening the stop-cocks, bc, the gas flows out of the jar and fills the flask, which, being removed, may be again counterpoised at the balance, and the weight of the gas filling it determined. There are very great differences among gases in this respect.

The following table will show the respective weights of equal quantities by measure of several elastic fluids, including those which are of the greatest importance on account of their frequent occurrence, and the valuable purposes to which they have been applied :

:

The specific gravity of the atmosphere being the standard with which the density of all gaseous substances is compared, it is considered as unity. [At 30 Bar. and 32 deg. it is 769-4 times lighter than water, and 10,462 than mercury or at 62 deg. 815 times lighter than water, and 11,065 times lighter than mercury. The knowledge of its exact weight is an essential element in many physical and chemical researches, and has been determined with very great care by Prout, who finds that 100 cubic inches of pure and dry atmospheric air at 60 deg. and 30 Bar. weigh 31·0117 grains.-Turner's "Elements of Chemistry."]

From the fact that the air has weight, it necessarily follows that it exerts pressure on all those portions that are in the lower regions, having to sustain the weight of the masses above. And not only does this hold good as respects the aërial strata themselves; it also holds for all objects immersed in the air. In most cases, the resulting pressure is not detected, because it takes effect equally in all directions, and pressures that are equal and opposite mutually

neutralize each other.

But when by the air-pump we remove the pressure from one side of a body, and still allow it to be exerted on the other, we see

at once abundant evidence of the intensity of this force. Thus, if we take a jar, Fig 17, open at both ends, and having placed it on the pump-plate, lay the palm of the hand on the mouth of it on exhausting the air, the hand is pressed in firm contact with the jar, so that it cannot be lifted without the exertion of a very considerable force.

Fig. 17.

Fig. 18.

In the same way, if we tie over a jar a piece of bladder, and allow it to dry, it assumes, of course, a perfect horizontal position; but on exhausting the air within very slightly, it becomes deeply depressed, and is soon burst inward with a loud explosion. This simple instance illustrates, in a very satisfactory way, the mode in which the pressure of the air is thus rendered obvious; for so long as the jar was not exhausted, and had air in its interior, the downward pressure of the atmosphere could not force the bladder inward, nor disturb its position in any manner: for any such disturbance to take place, the pressure must overcome the elastic force of the air within, which resists it, pressing equally in the opposite way. But on the removal of the air from the interior, the pressure above is no longer antagonized, and it takes effect at once by crushing the bladder.

CHAPTER V.

THE PRESSURE OF THE AIR.

The Magdeburgh Hemisphere-Water supported by Air-The Pneumatic Trough-Description of the Barometer-Cause of its Action-Different kinds of Barometers-Measurement of Accessible Heights.

a

MANY beautiful experiments establish the fact that the atmosphere presses, not only in the downward direction, but also in every other way. Thus, if we take a pair of hollow brass hemispheres, a b, Fig. 19, which fit together without leakage, by means of a flange, and exhaust the air from their interior through a stop-cock affixed to one of them, it will be found that they cannot be pulled apart, except by the exertion of a very great force. [In order to make the contact more perfect, the hedges of the hemispheres are rubbed with grease previous to the air being exhausted.] Now it does not matter whether the handles of these hemispheres are held in the position represented in the figure, or turned a quarter way round, or set at any angle to the horizon, they adhere with equal force together; and the same power which is required to pull them asunder in

Fig. 19.

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aid of the pump. Let there be a glass flask, a, Fig. 15, the mouth of which is closed with a stop-cock, through which the air can be removed. If from this flask we exhaust all the air, and then equipoise it with weights at a balance, as soon as the stop-cock is opened and the air allowed to rush in, the flask preponderates. By adding weights in the opposite scale, we can determine how much it requires to bring the balance back to equilibrio, and therefore what is the weight of a volume of air equal to the capacity of the flask.

Upon the same principles we can prove that all gasses, as well as atmospheric air, have weight. It is only requisite to take the exhausted flask, and having counterpoised it as before, screw it on the top of a jar, a, Fig. 16, containing the gas to be tried. On opening the stop-cocks, b c, the flows out of the jar and fills the flask, which, being removed, may be again counterpoised at the balance, and the weight of the gas filling it determined. There are very great differences among gases in this respect.

gas

The following table will show the respective weights of equal quantities by measure of several elastic fluids, including those which are of the greatest importance on account of their frequent occurrence, and the valuable purposes to which they have been applied :

The specific gravity of the atmosphere being the standard with which the density of all gaseous substances is compared, it is considered as unity. [At 30 Bar. and 32 deg. it is 769-4 times lighter than water, and 10,462 than mercury or at 62 deg. 815 times lighter than water, and 11,065 times lighter than mercury. The knowledge of its exact weight is an essential element in many physical and chemical researches, and has been determined with very great care by Prout, who finds that 100 cubic inches of pure and dry atmospheric air at 60 deg. and 30 Bar. weigh 31.0117 grains.-Turner's "Elements of Chemistry."]

From the fact that the air has weight, it necessarily follows that it exerts pressure on all those portions that are in the lower regions, having to sustain the weight of the masses above. And not only does this hold good as respects the aërial strata themselves; it also holds for all objects immersed in the air. In most cases, the resulting pressure is not detected, because it takes effect equally in all directions, and pressures that are equal and opposite mutually neutralize each other.