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thus sustains a pressure equal to the weight of the air above it. That pressure, then, balances the pressure of nearly 30 inches of mercury. In other words, the pressure of the air on a horizontal surface (which in that case is just the weight of all the air directly above it) is equal to the weight of a mass of mercury which would cover that surface to the depth of about 30 inches. And, as already explained, the pressure on a surface not horizontal is exactly the same. Suppose, now, that we wish to know the pressure of the atmosphere on a square inch of any surface. We have simply to find the weight of a column of mercury whose base is a square inch, and height 30 inches. Such a column, it is found, weighs nearly 15 lbs. avoirdupois. The astonishing result is that the atmosphere exerts a pressure of no less than about 15 lbs. on every square inch of our bodies. The surface of a man's body measures nearly 2000 square inches, so that the total pressure which he sustains from the air around him is about 30,000 lbs., or upwards of 13 tons !

Strange as it may appear, we are not in the least incommoded by this enormous load. We do not even feel it. The reason is, that the air presses upwards, downwards, and sideways with the same force, so that the different pressures are in equilibrium. Nor do our bodies stand in any danger of being crushed by this pressure on every side. For the liquids by which they are everywhere permeated are subject to the same pressure, and transmit it equally in every direction. Thus the pressure outwards is equal to and balances the pressure inwards, so that here too there is equilibrium. It is easy to show that the liquids in the body, such as the blood, do press outwards in this manner. Let the hand, for example, be placed firmly on the mouth of a vessel communicating with an air-pump. As the air is withdrawn from the vessel, and part of the skin is thus relieved from the external pressure, the blood-vessels will become distended, and may even be ruptured by the pressure from within.

The tube of quicksilver described in this lesson is nothing else than a barometer in its simplest form. The weight of the air, and the pressure depending on that weight, are different in different places, nor do they always continue the same even at the same place. To show their variations is the primary object of the barometer. Barometers are constructed in a great variety of forms, but the principle is the same in all. There is always one part of the liquid surface free from the pressure of the atmosphere, and another part exposed to it; and the difference of level gives the amount of that pressure in inches of mercury. The great weight of this liquid renders it specially suitable for barometric purposes. If water were used, it would require a tube more than 34 feet long.

Since the state of the weather depends, in a great measure, upon the atmospheric pressure, the barometer is indirectly of use as a weather-glass. The words “Fair, Rain," and so on, which we often see printed on these instruments, cannot be safely depended on, but in general, a rising of the barometer gives promise of fine weather, and a falling is indicative of wind or rain. The value of such an instrument to the farmer may easily be conceived; but it is by the mariner that it is specially prized, as it forewarns him of the coming storm, and enables him to prepare in time for the fury of the hurricane. It requires some experience, however, to interpret its prognostications with accuracy.

Another curious application of the barometer is its use in measuring the heights of mountains. It was already explained that the pressure of the atmosphere diminishes as we ascend. We leave, in fact, a part of it beneath us. Hence the mercury in the barometer stands lower and lower as we ascend. On the top of Mont Blanc, for instance, it sinks to 16 inches. Thus we are able, from the height of the barometric column at any elevated spot, to form at once a rough estimate of the elevation; and, if we have skill enough to make due allowance for temperature, and other circumstances which affect the result, a surprising degree of accuracy may in this way be attained.

THE AIR-PUMP.

We live at the bottom of a great ocean of air. Every place to which we can go filled with it; we are surrounded by it on every side; it even finds its way into our bodies, and is essential to the continuance of our very lives. Hence the effects of its pressure are so constant and universal, that our attention is little attracted by them. But, let the air be removed from any space, and we shall soon see to how great an extent the various phenomena around us are dependent on its influence.

The air contained in a vessel may be almost entirely withdrawn from it by means of an air-pump, one of the most interesting and instructive of scientific instruments. It consists, as do also water-pumps, of a suitable combination of cylinders and air-tight pistons with certain contrivances called valves. The object of a valve is to prevent any fluid from passing through a tube or aperture in one direction, without interrupting its progress when flowing in the opposite direction. It is like a door which opens inwards, but not outwards; or, which opens outwards, but not inwards. Its simplest form is the clack-valve, which resembles in its motion the lid of a box. It plays upon FIG. 44. a hinge, as at A, and being a little larger than the aperture which it covers, it effectually stops any fluid flowing downwards. For it is evident that the greater the pressure of the fluid, the more firmly will the valve be shut. On the other hand, a fluid pressing upwards opens the valve by its pressure, and thus makes a passage for itself.

Suppose now that we have a hollow cylinder A B with an air-tight piston working in it, as at C. Into an aperture in the bottom of the cylinder is inserted a bent tube D, the mouth of which is closed by a valve v opening upwards, to prevent air from passing out of the cylinder into the tube. The other end of the tube is fitted into an opening in the centre of a flat plate of metal E, on which is placed, mouth downwards, a glass vessel R, called the receiver. It is from

А.

That it may

F

D

this vessel that the air is to be extracted, and the name of Fic. 45.

receiver is given to it, be-
cause it receives, or holds the
objects on which experiments
are to be made.
lie air-tight on the plate, its
edge is ground very smooth,

and it must also, from time R to time, be rubbed over with

grease. The only essential part of the machine, which has not been already mentioned, is an aperture at F

in the piston itself, also closed by a valve opening upwards, to exclude the surrounding atmosphere.

We shall suppose that the piston is at the bottom of the cylinder. When it is drawn up, by means of its rod G, a vacuum is left in the cylinder, which cannot be filled by air from without, for the valve at F is shut. The pressure of the air in the tube, being no longer balanced by a corresponding pressure in the cylinder, will open the lower valve, and the whole of the air in the tube and receiver will diffuse itself equally, in virtue of its elasticity, throughout the receiver, the tube, and the part of the cylinder below the piston. Its volume being thus increased, its elasticity will be proportionably diminished, and therefore it will not be able to open the upper valve, which is kept shut by the atmospheric pressure without. But as soon as the piston begins to descend again, the air in the cylinder is compressed, and by its pressure towards the receiver the lower valve is immediately shut. The air in the cylinder, thus cut off from communication with the receiver, gains elasticity as the piston descends, until at last its pressure exceeds that of the atmosphere. It then forces up the upper valve, and, as the piston continues to descend, gradually escapes through the opening at F.

The piston, having reached the bottom of the cylinder, is again drawn up, and the whole process is

repeated. At every upward stroke, part of the air in the receiver passes into the cylinder, and the valve placed between them prevents its return. At the downward stroke, it is discharged into the atmosphere. The receiver is thus gradually emptied, till the air that remains in it has too little elasticity to lift the valves. It is then said to be exhausted.

It must not be understood that this is the only form of the air-pump. It is not even the usual form. For, generally speaking, it is found convenient to have two cylinders, with a branch of the tube D opening into each. They are so connected that, while the one piston ascends, the other descends. The piston rods are provided with teeth, into which is fitted a toothed wheel worked by a winch. The exhaustion is thus completed in one-half the time that would be required with one cylinder; for each cylinder acts separately, as if there were but one. Sometimes, too, the receiver is of a different form. It may, for example, be a vessel with a small neck, which could not conveniently stand inverted on the plate. In this case, the mouth of the receiver must be screwed into the mouth of the tube at E.

Many interesting experiments, illustrating the elasticity and pressure of air, may be made by means of an air-pump. The following is one of the most astonishing. Two hollow hemispheres of equal size, furnished with handles, are placed with their edges in contact, the junction Fig. 46. being made air-tight by rubbing one of the edges, as A B, with grease. One of the handles can be taken off, being screwed upon a narrow neck provided with a stop

A cock. Through this neck the air inside the hemispheres is extracted, and, the stopcock being shut, the apparatus is then detached from the air-pump, and the handle again screwed on. If the hemispheres be a few inches in diameter, two boys will be unable to pull them asunder. sooner is the air re-admitted, than they will fall asunder by their own weight. This experiment was first tried at

B

Yet no

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