weight of the water is sustained by the upward pressure of the air upon the paper.

The most essential point in which air differs from other fluids, is by its spring or elasticity, that is to say, its power of increasing or diminishing in bulk, according as it is more or less compressed. The elasticity of air differs from that of bodies in general; for when solid bodies are compressed they have an elastic power, which causes them to resume the same figure they possessed before compression: but on removing the pressure on air, it will not only resume its first bulk, but expand to an indefinite extent. With regard to animal and vegetable bodies, the gravity of the air is destroyed by its elasticity. It is true, that the atmosphere presses with a weight of fifteen pounds upon every square inch of the earth's surface, when the air is heaviest, and that consequently a man's body, which contains nearly fifteen square feet, will sustain a weight equal to about fourteen tons and a half; but this pressure is so great that it would be absolutely insupportable, and even fatal to us, were it not equal in every part, and counterbalanced by the spring of that air which fills all the vesicles of the body, and reacts with an outward force equal to that with which the atmosphere presses inward.

By means of an air-pump, the air may be drawn out of a large glass vessel, or receiver, and a vacuum produced, in which a great number of curious experiments may be performed, showing at once the properties and usefulness of the air. We shall give a brief description of the air-pump, though a view of the machine itself will convey a much better idea of the important purposes to which it is applied, than any description can afford. Two brass cylinders are closely and firmly fastened down to the table or base of the machine, by means of what are called the head and the columns. The receiver is made to fit very accurately on a brass circular plate, which has a hole in the middle, through which the air passes from the receiver into a tube made of brass, that communicates with the cylinders. Near the bottom of each cylinder is a valve opening upwards, and above these valves are two others in pistons which are moved up and down by toothed rods that fall into a toothed wheel, to the axis of which a handle is fixed. On turning the handle one of the pistons is raised and the other depressed, consequently a ra

58

CONDENSING SYRINGE.

refied space is formed between the upper and lower valve in one cylinder; then the air which is contained in the receiver rushes through the brass tube and by its elasticity forces up the lower valve and enters the cylinder; then the valve closes and prevents the air from returning into the receiver. When the motion is reversed, the other piston ascends, and the first is depressed; in its depression, the elasticity of the air contained between the two valves, forces open the uppermost valve, and it escapes into the upper part of the cylinder; then the valve closes and prevents its return. Whilst one piston, therefore, exhausts the air from the receiver, the other is discharging it from the top of the cylinder. Thus by continued exhaustion, the density of the air keeps decreasing in the receiver, till its elasticity is no longer able to force up the lower valves, which terminates the effect of the machine. The air is admitted into the receiver again by unscrewing a small nut which is so situated as to communicate with the air channel.

If the air be exhausted from a receiver, it will be held fast by the pressure of the external air. If a small receiver be placed under a larger, and both exhausted, the larger will be held fast, while the smaller will be easily moved. If a guinea and a feather be dropped from the top of the receiver, they will reach the bottom at the same instant, because there is then no resisting medium. Animals cannot live in an exhausted receiver, and the continuance of life varies according to the strength or size of the animal. A man requires a gallon of fresh air every minute. If a lighted candle be covered with a receiver containing a gallon of air, the candle will burn a minute; and then the flame, after having gradually decayed, will go out. A constant supply of fresh air, therefore, is as necessary to feed flame as to support life. If two brass hemispheres of three or four inches in diameter be put together, and the internal air exhausted, the pressure from without will require one hundred and fifty pounds to separate them; but if the external air be taken away, they will separate of themselves.

The Condensing Syringe has a solid piston, and a valve in the lower part of its barrel which opens downwards. By thrusting down the piston the air is forced through the valve, which is afterwards held close by the elasticity of the condensed air. When the piston is raised up a vacuum is pro

duced, till it is raised above a small hole in the barrel, when the air rushes in, and is again discharged through the valve. An instrument of this kind is used to produce what is called the artificial fountain.

QUESTIONS.-1. What is Pneumatics? 2. What is the atmosphere? 3. What is said of its height? 4. What is wind? 5. What is said of the weight and pressure of the atmosphere? 6. What experiment illustrates the upward pressure of the atmosphere? 7. How does the elasticity of air differ from the elasticity of bodies in general? 8. What is the weight of the atmosphere upon a square inch? 9. Upon the surface of a man's body? 10. How is the pressure of the air upon the body counterbalanced? 11. Describe the air-pump. 12. Show the method by which the air is drawn from the receiver. 13. What are some of the experiments that may be performed by an air-pump ? 14. Describe the condensing syringe, and its action. 15. Look at fig. 16. and describe the air-pump, and show its action. 16. Look at fig. 26. and describe the artificial fountain.

LESSON 28.

The Barometer.

Hermetically, a term applied to the closing of the orifice of a glass tube by fusion, so as to render it air-tight.

Respira'tion, the act of alternately inspiring air into the lungs, and expiring it from them.

THE Barometer is a very useful instrument for determining the variations of the weather. If a glass tube of about thirty-two or thirty-three inches long, hermetically sealed at one end, be filled with mercury, and then inverted in a basin or cup of the same fluid, the mercury in the tube will stand at an altitude above the surface of that in the basin between twenty-eight and thirty-one inches. The tube and the basin 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 a smail moveable metallic plate, called a vernier, an inch of which is divided into a hundred equal parts, serves to show that height with greater accuracy. The height at which the mercury will stand depends upon the weight of the atmosphere, which varies much according to the state of the weather. The air is 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. It is true that in damp weather the air feels heaviest, but it is on account of its being less salubrious. The lungs under these circumstances do not play so freely, nor does the blood circulate so well; and thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, and fevers. The thinness of the air in elevated situations 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 distension, and sometimes causes the bursting of smaller blood-vessels.

The barometer has been used for the purpose of measuring the heights of mountains and towers, and of estimating the elevation of balloons. The weight of one hundred and three feet of air is equal to that of one tenth of an inch of mercury. If a barometer, therefore, be carried to any great eminence, the mercury will descend one tenth of an inch for every one hundred and three feet that the barometer ascends. When the surface of the mercury is convex, or stands higher in the middle than at the sides, it is a sign the mercury is then in a rising state; but if the surface be concave, or hollow in the middle, it is then sinking. In very hot weather, the falling of the mercury indicates thunder. In winter, the rising indicates frost, and in frosty weather if the mercury falls three or four divisions, there will be a thaw. But in a continued frost, if the mercury rises, it will snow. In wet weather, when the mercury rises much and high, and so continues for two or three days before the bad weather is entirely over, then a continuance of fair weather may be expected. In fair weather, when the mercury falls low, and thus continues for two or three days before the rain comes, then much wet weather may be expected and probably high winds. The unsettled motion of the mercury denotes unsettled weather. The words engraved on the scale are not so much to be attended to, as the rising and falling of the mercury. It always sinks lowest of all for great winds, though not accompanied with rain; but it falls more for wind and rain together than for either of them alone. Barometers are frequently made of a tube with a curved neck and bulb, being more commodious than the basin and tube. To make these tolerably exact, however, the circular area

of the bulb should be at least thirty or forty times larger than that of the tube; so that the mass of mercury may be as little affected as possible whilst it rises and falls; for the height of the column is taken from the surface of the mercury in the bulb to its height in the tube.

QUESTIONS.-1. What is the construction of the barometer? 2. Upon what does the height of the mercury depend? 3. Why is the air heaviest in dry weather? 4. Why does it feel heaviest in damp weather? 5. How may the height of a mountain be ascertained by the barometer? 6. What is indicated by the convexity and concavity of the mercury? 7. Upon what other construction are barometers made than that first described?

LESSON 29.

Sound.

Humid'ity, moisture. The degrees of moisture in the air are measured by an instrument called a Hygrometer, of which there are various kinds; whatever contracts or expands by the moisture or dryness of the atmosphere is capable of being formed into one.

SOUND arises from a tremulous or vibrating motion in elastic bodies, which is caused by a stroke or collision, and is carried to the ear through the medium of the air. The production of sound therefore depends upon three circumstances, a sonorous body to give the impression, a medium to convey it, and the ear to receive it. Sonorous bodies, however, are merely the instruments by which a peculiar species of motion is communicated to the air. It is true that when you ring a bell, both the bell and the air are concerned in the duction of sound: but sound, strictly speaking, is a perception excited in the mind by the motion of the air on the nerves of the ear; the air, therefore, as well as the sonorous bodies which put it in motion, is only the cause of sound,—the immediate effect is produced by the sense of hearing: for without this sense, there would be no sound. The vibrating air strikes the ear, and causes in the mind the perception of sound.

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If you endeavour to ring a small bell, after you have suspended it under the receiver in an air-pump, from which the air has been exhausted, no sound will be produced. By