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divers are generally let down from a ship, and taking a rope with them, to which is fixed a bell in the vessel, they have only to pull the string, and the people in the ship draw them up; but if business requires it, they will stay several hours at the bottom of the sea without the smallest difficulty. By means of a strong globular cap with circular glasses in front to give light, it has been found practicable for a diver to go out of the engine to the distance of eighty or a hundred yards, the air being conveyed to him in a continued stream by small flexible pipes. Accidents, which through carelessness have sometimes occurred, may be readily prevented, by a proper degree of attention, and people may descend to very great depths without danger. The diving bell has often been used in bringing up the goods from a vessel which has sunk in deep water, and in blowing rocks which impeded navigation.

The Steam Engine is one of the most useful, curious, and important machines that have ever been invented. It consists of a large cylinder or barrel, in which is fitted a solid piston like that of the forcing pump. Steam is supplied from a large boiler, which in forcing up the piston, instantly opens a valve, through which cold water rushes, on the principle of the common pump. Other steam is then introduced above the piston, which forces it down, and drives the water out of the pipe. Steam raises the piston agam, and agair makes it fall, and thus produces an alternate motion, which is communicated, by an upright iron rod, to a large beam or lever, that is lifted up and pulled down with wonderful precision and force. This regular and powerful motion is easily applied by the mechanic to all kinds of machinery. The apparatus has been varied by different persons, and ior different objects; but the principle remains the same.

By the admirable contrivances of Watt and Fulton, the steam-engine has become a thing stupendous alike for its force and flexibility,--for the prodigious power which it can exert, and the ease, and precision, and ductility with which it can be varied, distributed, and applied. The trunk of an elephant, that can pick up a pin or rend an oak, is nothing to it. It can engrave a seal, and crush masses of obdurate metal beiore it,--draw out, without breaking, a thread as fine as gossamer, and lift up a ship of war like a bauble in the air. It can embroider muslin and forge anchors-cut

56

NATURE AND PROPERTIES OF AIR.

steel into ribands, and impel loaded vessels against the fury of the winds and waves. It has armed the feeble hand of man, in short, with a power to which no limits can be assigned; completed the dominion of mind over the most refractory qualities of matter; and laid a sure foundation for all those future miracles of mechanic power which are to aid and reward the labour of after generations.

Questions.-1. What is the principle of the diving bell? 2. What were the dimensions of Dr. Halley's diving bell? 3. How was light let in? 4. Fresh air? 5. How do divers make known their wish to be drawn up ? 6. Of what use is this invention? 7. Describe the steam-engine.

LESSON 27.

Nature and Properties of Air.
Den'sity, the degree of closeness and compactness of the parti-

cles of a body, the property directly opposite to rarity. Ab'solutely, completely, without restriction, positively. Hem'isphere, half a globe, or sphere. The science which treats of the inechanical properties of elastic or aëriform fluids, such as their weight, density, compressibility, and elasticity, is called Pneumatics. The air in which we live surrounds the earth to a considerable height, revolves with it in its diurnal and annual motion, and, together with the clouds and vapours that float in it, is called the atmosphere. The height to which the atmosphere extenu. has never been ascertained; but at a greater height than forty-five miles it ceases to reflect the rays of light from the sun. The air is invisible because it is perfectly transparent; but it may be felt on moving the hand in it, or when it moves and produces what we call wind. It is nearly nine hundred times lighter than water, but the whole atmosphere presses on all sides like other fluids, upon whatever is immersed in it, and in proportion to the depths. Its pressure upon a mountain is known to be less than in the plain or valley beneath. If a glass tumbler be completely filled with water, and covered with a piece of writing paper, so as to hold it tight, and accurately'even, the water will not run out although the glass be inverted and the band removed. The

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weight of the water is sustained by the upward pressure of the air

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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.

rofied 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 o: her is d'scharging 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 communi cate 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

If two brass hemispheres of three or four inches in diameter be put together, and the internal air exliausted, the pressure from without will require one hundred and fifty pounds to separate them; but if the external air se 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. Ву thrusting down the piston the air is forced through the valve, which is afterwards held close by the elasticity of the con: densed air. When the piston is raised up a vacuum is pro

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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 ihe 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 er 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 small 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

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