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same proportions, however the air may be vitiated by carbonic acid, animal effluvia, and other accidental ingredients. Air has been brought from the summits of Mont Blanc and Chimborazo, and from the plains of Egypt; it has been collected and examined in crowded cities, and in fever hospitals; but in all cases the proportions of oxygen and nitrogen remain unaltered, the diffusive energy of the gases maintaining this perfect uniformity of mixture.

This mechanical compound the atmosphere forms, therefore, a fluid ocean at the bottom of which we live, and which envelopes everything, and exerts such a pressure upon everything, as is quite incredible to persons who approach this subject for the first time.

Although air is invisible, and is very much lighter than solid and liquid bodies, yet like them it is material, and possesses many of their physical properties, together with other properties peculiar to aeriform fluids. Air possesses impenetrability; that is, it will not allow the entrance of another body into the space where it is present. If a vessel be completely full of water, a solid plunged into it will displace a portion of the water equal to the bulk of the solid. So, also, if we plunge a solid into what is called an empty glass, it will displace a portion of the air contained in such glass equal to its own bulk. The impenetrability of air is well illustrated by plunging an inverted goblet into a vessel of water, keeping the edge horizontal, and it will be found that to whatever depth wc plunge it, the water will not entirely fill it. The air will be compressed into a smaller space, but will not be displaced. A diving bell at a depth of 34 feet below the surface of the water, will be half filled with water; at 100 feet it will be three-quarters filled; at. 1,000 feet it will be filled to within a thirtieth; but on drawing it up to the surface the air will expand to its original bulk, and drive out all the water.

Air being material also possesses weight; that is, it obeys the attractive influence of the earth, and gravitates towards its centre. This may be proved by suspending a copper flask, of the capacity of 100 cubic inches, to one extremity of the arm of a delicate balance, and accurately counterpoising it with weights in the opposite scale. If the air be pumped out of the flask it will be found to have lost weight: it will have lost about 31 grains; but on readmitting the air the flask will weigh as much as before. It has been found by accurate experiments, that 100 cubic inches of pure and dry air, weigh 31.0117 grains, at the temperature of 60° and under a pressure of 30 inches.

Air, then, is a ponderable substance, and, in common with all such, has inertia, that is, it cannot be set in motion without the communication of some foree, and when in motion, it cannot be retarded or brought to rest without the opposition of foree. Its inertia, like that of all other bodies, is exactly proportional to its weight; and as this is small compared with its bulk, a small foree is sufficient to impart motion to a large bulk of air. It obeys the laws of motion common to ponderable bodies, and its

momentum, or amount of foree which it is capable of exerting upon bodies opposed to it, is estimated in the same way as for solids, viz. by multiplying its weight by its velocity. The momentum of air is usefully employed as a mechanical foree in imparting motion to windmills and ships.

Another consequence of the weight of air is its pressure. We have seen that 100 cubic inches of air weigh about 31 grains. This however is only at or near the level of the sea, with the barometer standing at 30 inches; for if this 100 inches of air be taken up in a balloon, to the height of 14.282 feet, or 2.705 miles, the 100 cubic inches will expand to 200, because, at that elevation, we have ascended above half the atmosphere, and the 100 cubic inches of air has to bear only half the pressure to which it was subjected at the level of the sea. Now it is a law peculiar to gaseous matter, that its density is proportional to the pressure that confines it; that is to say, by doubling this pressure we compress any air or gas into half its former bulk; and on the contrary, by removing half the pressure from any air or gas, it expands to twice its ordinary bulk; so that there does not appear to be any limit to the space which any quantity of air, however small, would fill if relieved of all pressure. Thus, the zone or shell of air which surrounds the earth, to the height of nearly 2J miles from its surface, contains one half of the atmosphere; and the remaining half being relieved of this superincumbent pressure, expands into another zone or belt, of the thickness of 41 or 42 miles.

The pressure of the air at the sea level, will be further examined when we come to notice the BaroMeter; but its amount may be shown by the Magdeburg hemispheres, fig. 16, which consist of 2 hollow hemispheres of brass, which fit together with smooth edges. The lower hemisphere is furnished with a short tube opening into it, which can be opened or closed by means of a stopcock. On screwing this tube into the table of an air-pump and placing the two hemispheres together, the air can be withdrawn from the hollow sphere thus formed, and on turning the stop-cock and removing the apparatus the air cannot re-enter. A handle may be screwed to the short tube, and if two persons pull in opposite directions they will be unable to separate the hemispheres. If the sphere be 6 inches in diameter, its section through the centre will be about 29 square inches, and supposing the vacunm to be perfect, a weight of 420 lbs. will be required to separate the hemispheres.1

(1) This experiment derives its name from Magdeburg, the place where it was first invented by Otto Guricke, a wealthy magistrate, who, in the year 1654, had the honour of exhibiting it on a large scale, before the princes of the empire and the foreign

Fig. 16. Now *gf = about 14£ lbs. which is the amount of atmospheric pressure upon 1 square inch of surface.

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To this pressure all bodies animate and manimate, situated at or near the level of the sea, are subject. By caleulating the number of square inches on the surface of the body of a man of ordinary stature, and multiplying this number by 14£, we get the Tiumber of pounds pressure to which his body is subject from the atmosphere: this will be found to amount to no less than 33,600 lbs., or about 15 tons. Why then is he not crushed beneath so enormous a load? is a very natural inquiry. To this it may be answered, that at the bottom of the ocean of water, many frail and delicate animals live and enjoy life, and move about with perfect freedom, just as we do at the bottom of our ocean of air; and yet these creatures are subject to a pressure of from 60 to 90 times greater than we sustain. They arc not crushed because the hydrostatic pressure is equal on all sides; the bodies of these animals are equally pressed above, below, and around, and the fluids within the animal are also either of similar density or they are nearly incompressible. So, also, our atmosphere presses equally in all directions, and our bodies are filled with liquids capable of sustaining pressure, or with air of the same density as the external air, so that the external pressure is met and counteracted by the internal resistance. Fishes which live at great depths in the sea, are as effectually destroyed when drawn up to the surface and the hydrostatic pressure removed, as we should be if taken to a great height in the atmosphere, where the pneumatic pressure would be removed or nearly so. We become painfully sensible of atmospheric pressure in the operation of cupping, and also in the experiment with the hand-glass, fig. 17,

Fig. 17. which is a stout glass open at both ends: if the broader end be placed on the table of the air-pump and the upper end be closed with the palm of the hand, the 'removal of a portion of air from the interior of the glass will cause an intolerable load to be felt on the back of the hand: this is the atmospheric pressure now unbalanced, because it does not press below as well as above the hand.

It has been stated that when a given volume of air is released from pressure it will expand to an indefinite extent, and that when the same amount of pressure is restored it will regain its former bulk. This property of elasticity is one of the most striking features of aeriform matter, and is a consequence of its peculiar structure. The atoms or particles of solids are held together by an attractive force called cohesion, which differs in different solids, as is evident from the various degrees of force required to overcome it, as in breaking them, or crushing and grinding them to powder. In liquids, the cohesive attraction is so weak that the particles glide over each other and instantly mould themselves to the form of the vessel or channel necessary to contain them. In air,

ministers, assembled at the diet of Katisbon. The force of two teams, each consisting of a dozen horses made to pull in opposite directions, was found insufficient to xpaiate the hemispheres.

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Fig. 18.

gases, and vapours, conesive attraction is altogether absent; the particles not only do not cohere, but repel each other with so powerful a force, that they constantly tend to separate themselves from each other, so as to occupy a larger space. It is this repulsive force which constitutes the elasticity of gaseous bodies.

This property of air, and the law by which it is governed, can be illustrated by means of a long bent glass tube, open at its longer extremity, and furnished with a stop-cock at the shorter. The stop-cock being open, a quantity of mercury is poured into the open end, and the surfaces of the mercury A a will of course stand at the same level in both limbs of the tube. The two columns of air, A C and a D, sustain a pressure equal to the weight of a column of air continued from A and a to the top of the atmosphere. If we now close the stop-cock 1), the effect of the whole weight of the atmosphere above that point is cut off, so that the surface a can sustain no pressure from the weight of the atmosphere. The level of the mercury, however, remains the same, because the elasticity of the column of air a D, is equal to the weight of the whole column before this small length was cut off. Under this condition of the experiment, it may be said that the surface A is pressed by the weight of the whole atmospheric column, and the surface a, by the elasticity of a portion of air which has been subjected to the weight of the whole atmospheric column; and that these two different properties of the atmosphere, its weight and its elasticity, exactly counterbalance each other; because the pressure of the atmosphere on A is transmitted to the surface a, and the elasticity of the confined portion of air on a is transmitted to the surface A, thus producing equilibrium.

We have seen that the pressure of the air is equal to about 14£lbs. on the square inch. This pressure will sustain a column of mercury 30 inches high and an inch square; or in other words, the weight of such a column of mercury is equal to about 14£ lbs. Now if we pour an additional quantity of mercury into the long limb, so as to compress the air in a D into half its former limits, that is, until the surface of the mercury rise from a to b, and then draw a horizontal line from h to the opposite point V in the longer limb, it will be found that the column of mercury V B required to compress the air in a D to half its former limits, measures exactly 30 inches, the weight of which is equal to the atmospheric pressure. The force with which the surface b is pressed upwards towards D is therefore equal to two atmospheres, or double the force with which a was pressed upwards towards D. Consequently the elasticity of the confined column b D, is double its former elasticity or that which it had when filling the space a I), so that when air is compressed into half its volume its elasticity is doubled. On again pouring mercury into the tube at C until the air in the shorter limb is reduced to a third of its bulk as at c D, the compressing force will be equal to 3 atmospheres, and the height of the column of mercury would extend to C or 60 inches above the level c. If we wish to compress the confined air into one-fourth part of its onginal volume, mercury must be poured in to the height of 90 inches, and then the elastic force of the confined air would be 4 times greater than at first.

It appears from these experiments that the elastic force of air varies in exactly the same proportion as its density. This important law, named, after its discoverer, the law of Marioite, applies not only to air, but to all gaseous bodies when subject to such variations of pressure as can be readily obtained. Air has been allowed to expand into more than 2,000 times its usual bulk, and compressed into less than a thousandth of its usual bulk; but at these extreme degrees of rarefaction or condensation, it is difficult to determine its elasticity with rigour, so that it is uncertain whether the law of Mariotte applies so extensively. It is probable that when gases are subjected to very great compression, their density increases in a greater ratio than their elasticity; but Mariotte's law has been found to apply to air when condensed as much as 50 times, and also when allowed to expand to several times its usual volume.

The elasticity of the air is taken advantage of in the construction of those useful instruments, the exhausting syringe and the air-pump. The exhausting syringe consists of a brass cylinder, fig. 19, with an accurately Fig. 1g. titling piston. The lower part of the cylinder contains two valves, one a opening upwards into the cylinder, and the other b at the side opening out into the air. The vessel V to be exhausted is screwed into a short tube projecting from the cylinder. On closing the stop-cock of this vessel and drawing up the piston, a vacuum or empty space must evidently be left between the bottom of the cylinder and the piston. Then on opening the stop-cock, the air in the vessel no longer being counterbalanced by the atmospheric pressnre, expands by its elasticity, forces open the valve a and fills the empty space below the piston. On driving the piston forcibly down so as to condense the air the valve a is closed and b is forced open, and through this valve b the contents of the cylinder arc expelled. When the piston has been thus forced to the bottom of the cylinder it is again drawn up, and the stop-cock being left open, the air from the vessel expands and follows it all the way; but the external air cannot enter through b, because this valve opens outwards, and the external atmospheric pressure only serves to close

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it more securely. On again depressing the piston the valve a is closed and b is forced open; and in this way the action is carried on until the air left in the vessel is so greatly expanded that its elasticity is insufficient to open the valve at a. The exhaustion of the vessel is then said to be complete. It must, however, be evident that a perfect vacuum cannot in this way be formed in the vessel. A small portion of air must always be left in it, and this portion can easily be caleulated. If the cylinder be of the same capacity as the vessel, and the weight and friction of the valve be regarded as nothing, one half of the air will pass out of the vessel by the first stroke of the piston; the remaining half will still completely fill the vessel, but its atoms or particles will be further apart; its density will be diminished one half, and its elasticity will be diminished in the same proportion. The second stroke of the piston will again diminish the air in the vessel by one half, and the air left after the second stroke will have one-fourth of its former density and elasticity. The following table will show the progress of the exhaustion during nine strokes of the piston; the quantity of air in the vessel before the first stroke being taken as unity.

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Hence it appears, that after the ninth stroke, the air left in the vessel will be only 6-j^th of its original quantity, and as it still occupies the same space, it has only j^th the original density and clastie force, which is equal to a pressure of only 0.028lb. to the square inch, which would scarcely be sufficient to raise the valve. If however the valve a be fastened to the piston by a loose string, so long that it may become tightened just before the piston reaches the top of the cylinder, the string will open the valve, and the exhaustion may be carried much further.

The air pump, Fig. 20, is a double exhausting syringe, bur, the valve through which the air is foreed out of the cylinder, instead of being placed in the side of the cylinder, as at b, Fig. 19, is contained in the piston or ping itself. The cylinders, or barrels, as they are called, are placed side by side, and motion is given to their pistons by means of a toothed wheel and racked piston rods, so contrived, that while one piston is ascending and drawing out the air, the other is descending and expelling the air already withdrawn from the vessel under exhaustion. At the bottom of each barrel is a valve opening upwards, so that during the ascent of cither piston, the air below the valve

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the piston itself opening upwards, allows the escape of the air between the bottom of the barrel and the piston. The vessel to be exhausted is called a receiver, R; it is made of stout glass, with a thick rim at the bottom, which is ground perfectly smooth, and is smeared over with pomatum before being placed on the metal table t, thus ensuring air-tight contact therewith. The receiver thus forms a transparent chamber, in which any substance or arrangement of apparatus adapted to the purpose may be observed under any amount of rarefaction that may be given to the inclosed air. The table is perforated in the centre with a hole which communicates by a bent metal tube c with the barrels a b. This tube has a stop-cock, which, when closed, prevents any air from leaking into the receiver from the barrels, and when open, allows them to communicate with the inclosed air. When the experiment is over, air can be readmitted into the receiver by a hole in the bent metal tube at h; this hole is closed by a thumb-screw, and is made air-tight by a washer of leather. In order to ascertain the amount of exhaustion in the receiver, one extremity of a bent glass tube, d, opens into the metal tube c, and the other extremity dips into a cistern of mercury. This tube, which is more than 30 inches long, acts as a gauge, and indicates by the ascent of the mercury within it the amount of rarefaction in the receiver. As the pump is worked, the air in the receiver diminishes in density and consequently in pressure, so that the external atmospheric pressure on the surface of the mercury in the cistern forces some of the metal up the tube. The weight of the column of mercury thus raised, combined with the clastic pressure of the air remaining in the receiver, is equal to the atmospheric pressure, and the elastic force of the air in the receiver is equal to the excess of the atmospheric pressure above the weight of the column of mercury in the tube. If a common barometer stand at 30 inches, and the mercury in the

gauge at 20 inches, the pressure of the air in the receiver is equal to 10 inches of mercury or one-third of that of the external atmosphere. The density of the air in the receiver is also one-third of that of the external air, showing that two-thirds of the air have been removed. In the best air pumps, the valves at the bottom of the barrels are not opened by the elastic force of the air, but by a mechanical contrivance working in the piston rods.

AIR-BEDS AND CUSHIONS. The elasticity of the air is taken advantage of in applying it as a stuffing material for cushions, pillows and beds. A textile fabric is rendered air-tight by the application of a solution of India rubber, and when made up into the required form, the seams are rendered tight by means of the same substance. At one corner is a short tube fitted with a screw, by loosening which the bag can be distended, and by tightening it the air is prevented from escaping. If too much air be introduced, the pillow or cushion becomes too hard, but when moderately distended, it is a tolerably soft surface. When not in use, the air can be let out and the cover folded up into a small space. The principal objection to its use arises from its great heat: air being a bad conductor of heat, the enclosed air, when made warm by contact with the body, retains its warmth and produces an unpleasant sensation of dry heat to the part which rests upon it. The water-bed or water-cushion is a much softer and cooler article, and has in great measure superseded air-beds and cushions.

AIR-GUN. The exhausting syringe, Fig. 19, may also be used as a condensing syringe. If the vessel be removed from the end of the syringe and screwed into the short tube b at the side, it will be evident that on drawing the piston to the top of the cylinder, air will rush through the valve a and fill it. On depressing the piston, the valve a will close and the valve b will open, so that the air contained in the cylinder will be forced into the vessel V through the valve b, and if the vessel be strong enough it will accommodate this increased quantity of air without bursting. On again raising the piston to the top of the cylinder, a fresh supply of air will fill it, and on again depressing the piston an additional quantity will be forced into the vessel. Each succeeding descent of the piston will, however, become more difficnlt, for the air contained in the cylinder will not force open the valve b until it is more compressed than the air within the vessel, which presses up against the valve b. On closing the stop-cock and removing the vessel from the syringe, we have a volume of condensed air which will rush out with great force the moment the stop-cock is opened, and this force has been used for projecting balls or other missiles.

In the air-gun, the vessel for containing the condensed air is a strong metal ball furnished with a small hole and a valve opening inwards. This ball is screwed to a barrel containing a bnllet, when upon turning a cock and opening a communication between the condensed air and the bullet, the latter will be projected forward with a greater or less velocity according to the state of condensation and the weight of the bullet. In air-guns, the reservoir of condensed air is usually very large in proportion to the tube which contains the ball, so that its elastic force is not greatly diminished by expanding through it, and the ball is urged all the way by nearly the same uniform force as at the first instant. The elastic fluid arising from inflamed gunpowder, on the contrary, is very small in proportion to the barrel of the gun, and occupies only a narrow space next the butt-end; so that by dilating into a comparatively large space as it urges the ball along the barrel, its elastic force is proportionately weakened, and it acts always less and less on the ball in the barrel. "Whence it happens, that air, condensed into a pretty large machine only 10 times, will project its ball with a velocity but little inferior to that given by gunpowder; and if the valve of communication be snddenly shut again by a spring after opening it tc let some air escape, then the same charge may serve to impel several balls in succession. In all cases where a considerable force is required and consequently a great condensation of air, it will be requisite to have the condensing syringe of a small bore, perhaps not more than halfan-inch in diameter, otherwise the force requisite to produce the compression will become so great that the operator cannot work the machine; for as the pressure against every square inch is about 15lbs., and against every circular area of an inch diameter 12 lbs., if the syringe be an inch in diameter, it will require a force of as many times 12 lbs. as the density of the air in the receiver exceeds that of the common atmosphere; so that when the condensation is 10 times, the force required will be 120 lbs., whereas with a half-inch bore it will only amount to 30lbs.'"

There are various forms of air-gun, but perhaps the best is Martin's, Fig. 21. It consists of a lock, stock, barrel, ramrod, &c. of about the size and weight of a common fowling-piece. Under the lock at b is screwed on a hollow copper ball (c) perfectly air-tight. This ball is charged with condensed air by means of the syringe, Fig. 22. When the ball is charged and screwed on, and a bullet is rammed down in the barrel, if the trigger a be pulled, the pin in b will by the spring-work within the lock strike into the copper ball, and thereby snddenly pushing in the valve within it, let out a portion of the condensed air, which rushing up through the aperture of the lock, and forcibly striking on the bullet, will propel it to the distance of 60 or 70 yards, or further if the air be strongly condensed. The gun may in this way be discharged many times before the condensed air has lost its propelling power.

The air is condensed in the copper ball by screwing it to the top of the syringe. At the lower end of the rod a is a stout ring, through which passes the rod k, upon which the feet are firmly placed; the hands are then applied to the two handles i i fixed on the side of the barrel of the syringe, when by moving the barrel B steadily up and down on the rod a, the ball c will

(1) Encyclopaedia Metropolitans. Article, Pneumatic!.

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a key to fix the ball fast to the screw b of the gun and syringe. When the barrel is drawn up the air will rush in at the hole h, and when it is pushed down the air contained in it will have no other way to pass from the pressure of the air-tight piston but into the ball c at the top. The barrel being drawn up the operation is repeated until the condensation is so strong as to resist the action of the piston.

In other forms of air-gun there are two barrels, one of small bore from which the bullets are shot, and a larger barrel on the outside of it which forms the reservoir of condensed air: the stock of the gun contains the syringe for condensing the air. The ball is inserted by means of the rammer, and by pulling a trigger a valve is opened which allows the air to come behind the ball so as to drive it out with great force. If this valve be opened and shut snddenly, one charge of condensed air may make several discharges of bullets; but if the whole of the air bo discharged on a single bullet, it will impel it more forcibly.

The air-gun is sometimes made in the form of a cane or walking-stick. Fig. 23. It is then called

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