_Mrs. B._ But as soon as I let the air again into the receiver, the apple, you see, returns to its shrivelled state. When I took away the pressure of the atmosphere, the air within the apple, expanded, and swelled it out; but the instant the atmospheric air was restored, the expansion of the internal air, was checked and repressed, and the apple shrunk to its former dimensions.

You may make a similar experiment with this little bladder, which you see is perfectly flaccid, and appears to contain no air: in this state I shall tie up the neck of the bladder, so that whatever air remains within it, may not escape, and then place it under the receiver. Now observe, as I exhaust the receiver, how the bladder distends; this proceeds from the great dilatation of the small quant.i.ty of air, which was enclosed within the bladder, when I tied it up; but as soon as I let the air into the receiver, that which the bladder contains, condenses and shrinks into its small compa.s.s, within the folds of the bladder.

_Emily._ These experiments are extremely amusing, and they afford clear proofs, both of the weight, and elasticity of the air; but I should like to know, exactly, how much the air weighs.

_Mrs. B._ A column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs about 15 lbs. therefore, every square inch of our bodies, sustains a weight of 15 lbs.: and if you wish to know the weight of the whole of the atmosphere, you must reckon how many square inches there are on the surface of the globe, and multiply them by 15.

_Emily._ But can we not ascertain the weight of a small quant.i.ty of air?

_Mrs. B._ With perfect ease. I shall exhaust the air from this little bottle, by means of the air pump: and having emptied the bottle of air, or, in other words, produced a vacuum within it, I secure it by turning this screw adapted to its neck: we may now find the exact weight of this bottle, by putting it into one of the scales of a balance. It weighs, you see, just two ounces; but when I turn the screw, so as to admit the air into the bottle, the scale which contains it, preponderates.

_Caroline._ No doubt the bottle filled with air, is heavier than the bottle void of air; and the additional weight required to bring the scales again to a balance, must be exactly that of the air which the bottle now contains.

_Mrs. B._ That weight, you see, is almost two grains. The dimensions of this bottle, are six cubic inches. Six cubic inches of air, therefore, at the temperature of this room, weighs nearly 2 grains.

_Caroline._ Why do you observe the temperature of the room, in estimating the weight of the air?

_Mrs. B._ Because heat rarefies air, and renders it lighter; therefore the warmer the air is, which you weigh, the lighter it will be.

If you should now be desirous of knowing the specific gravity of this air, we need only fill the same bottle, with water, and thus obtain the weight of an equal quant.i.ty of water--which you see is 1515 grs.; now by comparing the weight of water, to that of air, we find it to be in the proportion of about 800 to 1.

As you are acquainted with decimal arithmetic, you will understand what I mean, when I tell you, that water being called 1000, the specific gravity of air, will be 1.2.

I will show you another instance, of the weight of the atmosphere, which I think will please you: you know what a barometer is?

_Caroline._ It is an instrument which indicates the state of the weather, by means of a tube of quicksilver; but how, I cannot exactly say.

_Mrs. B._ It is by showing the weight of the atmosphere, which has great influence on the weather. The barometer, is an instrument extremely simple in its construction. In order that you may understand it, I will show you how it is made. I first fill with mercury, a gla.s.s tube A B, (fig. 3, plate 14.) about three feet in length, and open only at one end; then stopping the open end, with my finger, I immerse it in a cup C, containing a little mercury.

_Emily._ Part of the mercury which was in the tube, I observe, runs down into the cup; but why does not the whole of it subside, for it is contrary to the law of the equilibrium of fluids, that the mercury in the tube, should not descend to a level with that in the cup?

_Mrs. B._ The mercury that has fallen from the tube, into the cup, has left a vacant s.p.a.ce in the upper part of the tube, to which the air cannot gain access; this s.p.a.ce is therefore a perfect vacuum; the mercury in the tube, is relieved from the pressure of the atmosphere, whilst that in the cup, remains exposed to it.

_Caroline._ Oh, now I understand it; the pressure of the air on the mercury in the cup, forces it to rise in the tube, where there is not any air to counteract the external pressure.

_Emily._ Or rather supports the mercury in the tube, and prevents it from falling.

_Mrs. B._ That comes to the same thing; for the power that can support mercury in a vacuum, would also make it ascend, when it met with a vacuum.

Thus you see, that the equilibrium of the mercury is destroyed, only to preserve the general equilibrium of fluids.

_Caroline._ But this simple apparatus is, in appearance, very unlike a barometer.

_Mrs. B._ It is all that is essential to a barometer. The tube and the cup, or a cistern of mercury, are fixed on a board, for the convenience of suspending it; the bra.s.s plate on the upper part of the board, is graduated into inches, and tenths of inches, for the purpose of ascertaining the height at which the mercury stands in the tube; and the small moveable metal plate, serves to show that height, with greater accuracy.

_Emily._ And at what height, will the weight of the atmosphere sustain the mercury?

_Mrs. B._ About 28 or 29 inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much, in different states of the weather. The greater the pressure of the air on the mercury in the cup, the higher it will ascend in the tube. Now can you tell me whether the air is heavier, in wet, or in dry weather?

_Caroline._ Without a moment's reflection, the air must be heaviest in wet weather. It is so depressing, and makes one feel so heavy, while in fine weather, I feel as light as a feather, and as brisk as a bee.

_Mrs. B._ Would it not have been better to have answered with a moment's reflection, Caroline? It would have convinced you, that the air must be heaviest in dry weather; for it is then, that the mercury is found to rise in the tube, and consequently, the mercury in the cup, must be most pressed by the air.

_Caroline._ Why then does the air feel so heavy, in bad weather?

_Mrs. B._ Because it is less salubrious, when impregnated with damp. The lungs, under these circ.u.mstances, do not play so freely, nor does the blood circulate so well; thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, agues, fevers, &c.

_Emily._ Since the atmosphere diminishes in density, in the upper regions, is not the air more rare, upon a hill, than in a plain; and does the barometer indicate this difference?

_Mrs. B._ Certainly. This instrument, is so exact in its indications, that it is used for the purpose of measuring the height of mountains, and of estimating the elevation of balloons; the mercury descending in the tube, as you ascend to a greater height.

_Emily._ And is no inconvenience experienced, from the thinness of the air, in such elevated situations?

_Mrs. B._ Oh, yes; frequently. It is sometimes oppressive, from being insufficient for respiration; and the expansion which takes place, in the more dense air contained within the body, is often painful: it occasions distention, and sometimes causes the bursting of the smaller blood-vessels, in the nose, and ears. Besides in such situations, you are more exposed, both to heat, and cold; for though the atmosphere is itself transparent, its lower regions, abound with vapours, and exhalations, from the earth, which float in it, and act in some degree as a covering, which preserves us equally from the intensity of the sun's rays, and from the severity of the cold.

_Caroline._ Pray, Mrs. B., is not the thermometer constructed on the same principles as the barometer?

_Mrs. B._ Not at all. The rise and fall of the fluid in the thermometer, is occasioned by the expansive power of heat, and the condensation produced by cold: the air has no access to it. An explanation of it would, therefore, be irrelevant to our present subject.

_Emily._ I have been reflecting, that since it is the weight of the atmosphere, which supports the mercury, in the tube of a barometer, it would support a column of any other fluid, in the same manner.

_Mrs. B._ Certainly; but as mercury, is heavier than all other fluids, it will support a higher column, of any other fluid; for two fluids are in equilibrium, when their height varies, inversely as their densities.

We find the weight of the atmosphere, is equal to sustaining a column of water, for instance, of no less than 32 feet above its level.

_Caroline._ The weight of the atmosphere, is then, as great as that of a body of water of 32 feet in height.

_Mrs. B._ Precisely; for a column of air, of the height of the atmosphere, is equal to a column of water of about 32 feet, or one of mercury, of from 28 to 29 inches.

The common pump, is dependent on this principle. By the act of pumping, the pressure of the atmosphere is taken off the water, which, in consequence, rises.

The body of a pump, consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it, by means of a metallic rod, fastened to the centre of the piston.

_Emily._ Is it not similar to the syringe, or squirt, with which you first draw in, and then force out water?

_Mrs. B._ It is; but you know that we do not wish to force the water out of the pump, at the same end of the pipe, at which we draw it in. The intention of a pump, is to raise water from a spring, or well; the pipe is, therefore, placed perpendicularly over the water, which enters it at the lower extremity, and it issues at a horizontal spout, towards the upper part of the pump; to effect this, there are, besides the piston, two contrivances called valves. The pump, therefore, is rather a more complicated piece of machinery, than the syringe.

_Caroline._ Pray, Mrs. B., is not the leather, which covers the opening, in the lower board of a pair of bellows, a kind of valve?

_Mrs. B._ It is, valves are made in various forms; any contrivance, which allows a fluid to pa.s.s in one direction, and prevents its return, is called a valve; that of the bellows, and of the common pump, resemble each other, exactly. You can now, I think, understand the structure of the pump.

Its various parts, are delineated in this figure: (fig. 4. plate 14.) A B is the pipe, or body of the pump, P the piston, V a valve, or little door in the piston, which, opening upwards, admits the water to rise through it, but prevents its returning, and Y, is a similar valve, placed lower down in the body of the pump; H is the handle, which in this model, serves to work the piston.

When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by working the handle of the pump, the piston ascends; it raises a column of air which rested upon it, and produces a vacuum, between the piston, and the lower valve Y; the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through it; the water, then, relieved from the pressure of the air, ascends into the pump. A few strokes of the handle, totally excludes the air from the body of the pump, and fills it with water, which, having pa.s.sed through both the valves, runs out at the spout.

_Caroline._ I understand this perfectly. When the piston is elevated, the air, and the water, successively rise in the pump, for the same reason as the mercury, rises in the barometer.

_Emily._ I thought that water was drawn up into a pump, by suction, in the same manner as water may be sucked through a straw.

_Mrs. B._ It is so, into the body of the pump; for the power of suction, is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced, by the pressure of the atmosphere, on another part. The action of sucking through a straw, consists in drawing in, and confining the breath, so as to produce a vacuum in the mouth; in consequence of which, the air within the straw, rushes into the mouth, and is followed by the liquid, into which, the lower end of the straw, is immersed. The principle, you see, is the same, and the only difference consists in the mode of producing a vacuum. In suction, the muscular powers answer the purpose of the piston and valve.