unable to sustain the violence of the pressure, burst with the explosion you have just heard.*

I shall now show you an experiment, which proves the expansion of the air, contained within a body when it is relieved from the pressure of the external air. You

would not imagine that there was any air contained within this shrivelled apple, by its appearance; but take notice of it when placed within a receiver, from which I shall exhaust the air.

Caroline. How strange! it grows quite plump, and looks like a fresh-gathered apple.

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 atmospherical 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 quantity 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

* The weight of the atmosphere can also be ascertained from the following experiments.-The air being exhausted, by an airpump, from a glass receiver, the receiver will be held fast by the pressure of the external air. If a small receiver be placed under a larger one, and the air be exhausted from both, the larger one will be held fast by the pressure of external air, while the smaller one will be easily moved. Or, if the hand be placed upon a small open vessel in such a manner as to close its upper orifice, it will be heid down with great force.

663. What experiments named in the note prove that air has weight?-664. How may the elasticity or expansive power of the air be shown?

and shrinks into its small compass 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 15lbs. when the air is heaviest therefore every square inch of our bodies sustains a weight of 15lbs. 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 are there no means of ascertaining the weight of a small quantity of air?

Mrs. B. Nothing more easy. 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.

* If a tube, closed at one end, be inserted at its open end, in a vessel of water, the fluid in the tube will not rise to the level of the water in the vessel, being resisted by the elastick force of the air within the tube. It is on this principle that the diving bell is formed.

It has been computed that the pressure of the atmosphere on the whole surface of the earth is equivalent to that of a globe of lead sixty miles in diameter.

665. How much does a column of air, reaching to the top of the atmosphere, of an inch in diameter, weigh ?-666. How great has been estimated the whole pressure of the atmosphere upon the earth ?- -667. How can the weight of a small quantity of air be ascertained?

Mrs. B. That weight, you see, is almost two grains. The dimensions of this bottle are six cubick inches. Six cubick inches of air, therefore, at the temperature of this room, weigh nearly two 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 specifick gravity of this air, (we need only fill the same bottle with water, and thus obtain the weight of an equal quantity of water which you see is 1515 grains; now by comparing the weight of water to that of air we find it to be in the proportion of about 800 to 1.

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. 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 a glass tube A B, (fig.3, plate XIV.) about three feet in length, and open only at one end, with mercury; 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 in the cup, 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 space in the upper part of the tube, to which the air cannot gain access; this space is therefore a perfect vacuum; and consequently the

668. Why is it necessary in this experiment to observe the temperature of the room in which it is made?-669. How much heavier is water than air?-670. How is the specifick gravity of air determined?-671. What is a barometer ?- -672. Which figure represents a barometer?-673. How is the weight of the atmosphere determined by a barometer ?

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 it sustains no 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 vase are fixed on a board, for the convenience of suspending it; the board is graduated 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 at`mosphere sustain the mercury?

Mrs. B. About 28 inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much according to the state 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 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

674. At what height will the weight of the atmosphere sustain the mercury? -675. According to what does the weight of the atmosphere vary?676. When is the air the heaviest, in wet or dry weather?

must be most pressed by the air and you know, that we estimate the dryness and fairness of the weather, by the height of the mercury in the barometer.

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 circumstances 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. The hills in this country are not sufficiently elevated to produce any very considerable effect on the barometer; but 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.

Emily. And is no inconvenience experienced from

the thinness of the air in such elevated situations?

Mrs. B. Oh, yes; frequently. It is sometimes op. pressive, 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 distension, 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

677. Why then do our feelings indicate that the air is heaviest in wet weather, if that is not the fact?678. Is the atmosphere of the same density on a hill or mountain as in a valley?

-679. Does a person in elevated situations feel any inconvenience from the thinness of the atmosphere ?-680. What causes the rise and fall of the fluid in the thermometer ?