ment, we should find, that, after certain portions had gone into intimate union, according to the laws by which they combine, the surplus portions of each would assume relative positions according to their respective weights the heaviest to the bottom, and the lightest to the top. Such an experiment would resemble that previously noticed, of the mixture of mercury, oil, water, and spirits. Air and all kinds of gases are rendered lighter by the application of heat, for then the particles in the mass are repelled from each other, and occupy a greater space; this process of lightening or thinning is called rarefaction. Rarefied air, being specifically lightest, mounts above that of a common density. The warmest air is always at the top of a room, and the coldest at the bottom. Air is distinguished from water not only by its extreme comparative lightness, but by the property of elasticity; it is a compressible and elastic fluid. When any quantity of air is compressed into a smaller space than it naturally occupies, it will return to its natural bulk on the pressure being withdrawn. A small bladder of air may be squeezed between the hands so as to be considerably reduced in size; and on opening the hands again, and withdrawing the pressure, it will instantly resume its former bulk. If a metallic tube or barrel be fitted with a moveable plug or piston, which is made to work in it perfectly air-tight, the air which occupies the space between the top and the bottom of this barrel when the piston enters, can be compressed to a hundredth part, or even less, of its usual bulk. If the force, however, by which the piston is pushed down, be withdrawn, the air, by its elasticity, will force it up again with a power equal to that by which its descent was resisted. In proportion as any given volume of air is diminished by pressure, its elastic force is increased; in other words, the elastic force or elasticity of air is proportional to its density. THE ATMOSPHERE. The air, as formerly expressed, is a great ocean wrapped round the earth to a depth of from forty-five to fifty miles above the highest mountains, and forms a menstruum which is essential to the existence of all animals and plants. This ocean of air penetrates into all unoccupied places, in the same manner as water flows into all crevices and holes beneath the level of its surface; and it also finds a place in the bodies of animals, plants, and liquid substances; hardly any thing, indeed, that we see in nature or art, is free from air, unless force has been employed to extract it. The height of the atmosphere, though usually estimated at forty-five or fifty miles, is in reality unknown. The highest point above the level of the sea, which has ever been reached by any human being, is 21,000 feet, which has been attained in a balloon. It is only conjectured, from the refraction of the sun's rays and other circumstances, that the height of the atmosphere is about fifty miles. At and near the level of the ocean it is most dense, in the same manner as water at the bottom of the sea is more dense than it is at the surface, on account of the incumbent pressure. As we ascend mountains, or in any other way penetrate upwards into the atmosphere, the air becomes gradually less dense, and so thin is it at the height of three miles on the summit of Mont Blanc, that breathing is there performed with some difficulty. Beyond this limited height, the density of the air continues to diminish, and at the elevation of about fifty miles, it is believed to terminate. The extreme height of the atmosphere is not observable from the situation in which we are placed on the earth. Our eye, on being cast upwards, perceives only a vast expanded vault, tinted with a deep but delicate blue colour; and this in common language is called the sky. The blueness so apparent to our sense of sight is the action of the rays of light upon the thin fluid of the upper atmosphere, and the brightness is in proportion to the absence of clouds and other watery vapours. In proportion as the spectator rises above the surface of the earth, and has less air above him, and that very rare, the blue tint gradually disappears; and if he could attain a height at which there is no air, say at above fifty miles in height, the sky would appear perfectly dark or black. Travellers who have ascended to great heights on lofty mountains, describe the appearance of the sky from these elevated stations as dark or of a blackish hue. The atmosphere possesses the capacity of absorbing and sustaining moisture, but only to a limited extent. When saturated to a certain degree, it is relieved by the falling of the moisture in the form of rain. It is calculated that the whole atmosphere round the globe could not retain at one time more moisture than would produce about six or seven inches of rain. By an elevation of temperature, the capacity of the atmosphere to absorb and sustain moisture is increased, and by a lowering of temperature, decreased. Cold breezes, by lowering the temperature of the air, cause the aëriform moisture to assume the appearance of clouds, and then to fall as rain. LAWS OF AIR. First-The pressure of the air is equal in all direc tions: Second-Its degree of pressure depends on the vertical height or depth, and at any place is proportional to its density: Third-Its surface is level in all parts of its volume: Fourth-It affords support according to its density and to the weight of the fluid displaced. That air presses equally in all directions may be rendered evident by filling a bladder with that fluid, and then pressing upon it so as almost to make it burst. The pressure is freely communicated through the mass, as in the case of the bag of water, and it will be observed that the confined air will rush out with equal impetuosity at whatever part you make a hole in the surface. The level of surface of air is less perfect than the uniform level of water, on account of the greater elasticity of the substance. In a series of strata of air of different densities, one above the other, a small portion of each mingles with those which immediately adjoin it the particles of one commingle to a certain extent with those of another. There is thus, as respects aërial bodies, a modification of the law of uniform levelness of surface in all parts of the volume of fluid. PRESSURE OF AIR. The pressure depending on the vertical height or depth of air, is an important property in the atmosphere, and on it depends the explanation of numerous phenomena. Air being a substance possessing gravity, it must of necessity press downwards in the direction of the centre of the earth, and therefore the degree of pressure on any given point will be equal to the weight of the column of air above the point, and proportional to the density of the air at that point. The idea of the atmosphere possessing the property of gravity or pressure, is of comparatively modern date. No such notion was entertained by the ancients, in consequence of living animals being observed to move with perfect ease in all directions, and because there was no other appearance in nature calculated to suggest it to their minds. It was however remarked, that, when the air was sucked out of a small glass tube, the lower end of which was immersed in water, the water rushed up into the tube and occupied the situation of the displaced air. In consequence of this and similar phenomena, it was alleged as a doctrine in physics, that "nature abhors a vacuum." A vacuum is a space destitute of air or any other kind of matter; and the notion was, that whenever by any chance such an empty space was found, nature in terposed with all imaginable haste to fill it. With this very rude idea, pumps were formed to raise water, the rising of the water in these instruments being ascribed simply to nature's abhorrence of a vacuum. At length it was discovered that water could not be drawn up by a pump above a height of about thirty-two feet, and that a vacuum above that elevation remained unfilled; whereupon the terms of the doctrine were changed, and it was said that nature abhorred a vacuum only to a height of thirty-two feet, but no farther. This explanation was seemingly unphilosophical, and men's minds being carefully turned to the subject, various experiments were performed, and the important truth became manifest, that the atmosphere possessed gravity or pressure; also, that that pressure was the sole cause of the rushing of liquids into tubes exhausted of air-the height of the ascending liquids being in every case limited by the degree of pressure of the incumbent atmosphere. Thus, the discovery of a simple truth in science at once abolished the fantastic doctrine of nature's abhorrence of a vacuum, and all the laboured sophistry with which it was supported.* Nature has no dislike to a vacuum; a vacuum will occur in all situations from which solids or fluids are accidentally or artificially excluded. The degree of pressure imposed by the atmosphere on any given spot on the earth's surface, as already noticed, is equal to the weight of the column of air above that spot, and is also proportional to the density of the air at the place. The atmosphere is deepest or of greatest vertical height at the level of the ocean, and there it exerts the greatest pressure. The pressure of the air at the level of the sea is usually reckoned to be about 15 pounds on every square inch.† The pressure of 15 lbs. to the square inch refers to every shape of surface at or near the sea's level. The pressure is sideways, upward, oblique, and in every other direction, as well as downward, because fluids press equally in all directions. Thus, in every crevice, nook, or vessel, in which air happens to be, the pressure is equally intense. The human being, for example, sustains the pressure of 15 lbs. to the square inch all over his person, and this is a load under which he could not possibly move, unless the pressure was also exerted in the interior of his body, or through his whole system of muscles, viscera, and bones, by which means the external pressure is counteracted, and he feels no pressure whatever. If, however, the air by any means be withdrawn from the interior of any object, that object becomes immediately susceptible of the external atmospheric pressure. There are many familiar examples of this pressure around us. One of the most common consists in causing a thimble to adhere to the hand by sucking the air from beneath it: the adhesion is the result of the pressure of the atmosphere on the exhausted space on the hand. Another consists in lifting a stone by means of a sucker, formed of a string and a wetted piece of leather, as in the accompanying figure. The wetted leather is in this case pressed down upon the stone, and the string is then pulled: if air were admitted under the end of the string, the sucker Fig. 18. would come off; but none being admitted, the atmosphere presses on the sucker, a rigid adhesion of the * This great discovery in physical science was made by Torricelli, an eminent Italian mathematician, about the year 1644. It was suggested by an ineffectual attempt to raise water from a deep well near Florence, by means of a pump of a greater height than thirty-two feet. sucker to the stone is produced, and the stone, if not too heavy, is lifted. The surgical process of cupping is upon the same principle. A small glass cup is held with its mouth near the part to be operated on, and the air being consumed within it by a lighted taper, it is instantly applied, and adheres with great force. The part having been previously lanced, the blood, rushing to fill the vacuum, enters the cup in copious small streams. The feeling endured in cupping is that of considerable weight. The feet of flies and some other insects are formed on the principle of the sucker, by which means they are enabled to walk and run with security on the ceiling of an apartment, back downwards, or on an upright and smooth pane of glass. At each step in advance, they procure a hold by the formation of a vacuum or air-tight space beneath their feet. The rapidity with which these vacuums or air-tightnesses are formed and destroyed, is an exceedingly interesting phenomenon in the economy of the animal, and cannot be rivalled by the utmost efforts of human skill. On a very moderate computation, a fly, in travelling six feet in the space of a minute, creates and destroys as many as 10,000 vacuums. When deprived of the outer extremities of its legs, on which the apparatus for adhesion is situ ated, a fly can walk without any apparent difficulty on a horizontal surface such as a table, but is quite incapable of adhering to the roof, or of climbing any upright surface. Limpets, snails, and some other crustaceous animals, adhere to rocks and stones, by causing a vacuum within their shells, which they accomplish by shrinking into a smaller bulk; by this simple contrivance, nature has effectually provided for their safe adhesion to their appropriate places of residence. THE AIR-PUMP. T R C Air may be artificially withdrawn from a containing vessel by means of an apparatus called the air-pump. This apparatus is usually small, for standing on a table, and consists chiefly of a glass jar called a receiver, placed mouth downwards over a flat surface, and with a small brass pump to draw the air from it. The annexed cut, fig. 19, represents an outline section of an air-pump, the working of which may be described. Ris the glass receiver standing on a flat and smooth plate SS, and fitting so exactly that no air can penetrate between the edges of the receiver and the plate. In the plate S SS, there is a channel AB issuing into the barrel of a pump. P is the piston of the pump, with its rod C above, which Fig. 19. is moved upwards and downwards by a handle and winch. The rod C works in a tight collar D. At the bottom of the pump there is a valve V, by which the air escapes, and is prevented from again entering. On depressing the piston, a portion of the contained air is to its position at the top, another column of air is adexpelled by the valve, and on raising the piston again mitted from the receiver into the pump, which is expelled in its turn. Thus, by a process of expulsion, the air in the receiver becomes at every stroke down The actual pressure varies from 14 lbs. to 15 lbs. according towards more rare, till at length a vacuum sufficient for circumstances. By various authorities it is stated at 14-7 lbs. all practical purposes is established. The valve V, which opens outwards, is kept forcibly shut at every rising of the piston by external pressure of the atmosphere. For convenience, we state it throughout in the text at 15 lbs. The body of a man has a surface of 2000 square inches, and ⚫ the pressure upon him is equal to 30,000 lbs. By means of the air-pump, a number of interesting experiments in pneumatics may be performed. For example, if a bladder, half full of air, and tightly tied at the neck, be placed under the receiver, and a vacuum then produced, the air in the bladder will expand by the removal of the external pressure, and seem as if ready to burst. Dried raisins, during a similar operation, will expand, and have all the plumpness of new fruit; and an egg, by the expansion of its confined air, will explode. Any small animals, such as mice, placed below the receiver and deprived of air, will immediately die, both from want of breath and the expansion of their bodies. The atmosphere serves to retard the falling of bodies of a light and porous nature; and, therefore, in the exhausted receiver of an air-pump, all such bodies descend with the same velocity as bodies of a heavy compact nature. A piece of coin and a feather let fall at the same instant of time, from a hook within the top of an exhausted receiver, will strike the bottom at the same moment. That atmospheric air is useful for the transmission of sound, in the absence of other media, is also exemplified by the air-pump. If we place a small bell in a receiver, in such a manner as to admit of being rung easily from the outside without admitting air into the inside, whilst the receiver is full of air the sound of the bell will be distinctly heard; but after the receiver has been exhausted, and although the bell be struck with the same force, the sound will be inaudible, or nearly so. If a small portion of air be admitted, it will be faintly heard, and it will gradually increase according to the quantity of air which is allowed to enter the receiver. Thus, we are indebted to the air as a medium for conveying to us the sound of each other's voices, and all the melodious notes which constitute music. The act of inspiring and expiring air resembles the alternating action of an air-pump. The air, on being drawn in through the appropriate tubes, fills the lungs, and the chest is expanded; having performed its office, the air is expelled in an impure condition, leaving a partial vacuum within, until another inspiration causes another expansion. A machine, called a condensing pump or syringe, is formed for the purpose of showing experiments with Fig. 20. bottom of the syringe there is a tube communicating with the interior of the receiver. When the piston is raised, a valve beneath opening inwards admits air into the cylinder of the syringe, and when it is depressed, this quantity of air is forced into the receiver; by the alternate raising and depressing of the piston, an immense quantity of air is forced into the receiver. PRESSURE OF AIR ON SOLIDS AND LIQUIDS. The pressure of the atmosphere affects all liquids as well as solid bodies. The load of the incumbent air is as sensibly exerted within any given mass of water as on the surface. Thus, atmospheric pressure keeps water and other liquids at the density they are usually seen to possess. If a glass be filled with water, and placed under the receiver of an air-pump, the abstraction of the air, by the removal of the atmospheric pressure, will cause the water to expand or become less dense, and it will overflow the vessel in which it is contained. Water in its ordinary condition contains a certain quantity of particles of air mixed up with it. When the atmospheric pressure is lightened, these particles of air expand, and being of a less specific gravity than water, they mount to the top of the liquid in the form of small globules, and so fly off. The same effect is produced by expanding water by means of heat; the globules of air rise to the surface, and escape or remain attached to the inside of the vessel. Crystal bottles of water may be observed to be covered inside with small air-bells when the weather becomes suddenly light or warm. Water which has been boiled is comparatively free of air, and has an insipid flavour. Certain gases are generated in some liquors, such as in porter, beer, and champagne wine, and unless the bottles in which they are contained be of sufficient strength to endure the expansive tendency, they will burst. On drawing the cork from a bottle of one of these liquors, the confined gas or air is suffered to expand, and the contents gush forth, a mixture of froth and liquid. If the liquid remain in an open glass for a short time, a large portion of the long-confined gases escapes into the atmosphere, and the liquor seems flat or dead. A portion of confined air, however, still remains, in consequence of the atmospheric pressure. If we take a glass of ginger-beer which seems quite dead, and place it under the exhausted receiver of an air-pump, it will again froth and appear brisk. Some mineral waters on springing from the ground sparkle like beer. These most likely rise from great depths, where the incumbent pressure is considerable, and on attaining the surface of the earth they expand, and give forth the air pent up in their mass. If a bladder full of air be carried from a low situation to a great height, the contained air will expand, and the bladder will burst, the same as if placed under the exhausted receiver of an air-pump. If a bladder be filled with air at a great height, where the fluid is rare, and brought to a low situation, the contained air will be compressed by the more dense fluid without, and the bladder will appear as if only half or partially filled. The fluids in the animal and vegetable system are similarly affected by atmospheric pressure. Our bodies, for instance, would expand, and our blood-vessels probably be ruptured, if placed for a short time in a vacuum. On the same principle, any change in the density of the atmosphere has an effect on the animal frame. The atmospheric pressure, in ordinary conditions of the air, and at the level of the sea, as already stated, is equal to 15 lbs. to the square inch. If by any means, such as digging into the earth, we should go below the sea's level, the weight will be found to increase. In The elastic force of air so condensed is very great, and deep coal mines, for instance, the pressure of the atis employed for the projection of balls from an instru-mosphere is something more than 15 lbs. to the square ment called an air-gun. A certain quantity of compressed air is confined in a chamber at the inner end of the barrel, and when allowed to escape by touching a valve, a bullet is projected with a force resembling that of gunpowder. The explosive force of gunpowder itself is nothing else than the sudden disengagement of air from the particles of the powder. inch. The pressure diminishes in a similar degree as we ascend into the atmosphere. At every step upwards from the shore, the burden of the superincumbent mass lightens. At the height of three miles, one-half of the weight is lost; or, in other words, at that height the air is only half the density of air at the sea's level. The breathing apparatus of animals is suited to an atmospheric density and pressure such as is found at the sea's level, or at a moderate elevation above it. By ascending in the atmosphere, as in climbing hills, we are deprived of the quantity of air to which we have been accustomed; and when we reach a height of three miles, we in reality inhale only one-half of the weight of air into the lungs that we use at the sea's level. Consequently, those who ascend to great elevations experience difficulty in breathing, and feel an expansion in their blood-vessels and muscles by the removal of a portion of the ordinary pressure. All the joints in our bodies, particularly those of the knee and shoulder, are in a great measure held together by the external pressure of the atmosphere; and thus a principle in pneumatics compensates for a loading of muscular ligaments. presses with nothing but its own weight on the mercury of the cup. This weight of thirty inches of mercury is counterbalanced by the pressure of air on the surface of the mercury in the cup; and thus it is evident that the weight of the atmosphere is equivalent to the weight of thirty inches of mercury. If by any means we remove the atmospheric pressure from the mercury in the cup, the mercury in the tube will immediately sink into the cup. The circumstance of the column of mercury in the tube being narrow, and the surface of the mercury in the cup being broad, makes no difference in the experiment, because the pressure of elastic fluids is as their density, not as width of volume. The same result would occur if the surface of the mercury presented to the atmospheric pressure were only the width of the The height at which mercury stands in a tube of this kind, always bears reference to the incumbent weight of the atmosphere on the open and lower extremity of the column. If we increase the external pressure by artificial means, or by descending below the sea's level, the mercury rises; if we decrease it by artificial means, or by ascending into the atmosphere, or if the atmosphere is rarefied by heat, the mercury falls. A consideration of the effects of atmospheric pres-tube. sure, and its variability at different elevations, also the alterations in pressure caused by the expansion or lightening of the air by heat, and its increased density by cold and moisture, tends to explain the remarkable influence which change of climate has upon the human constitution. Thus, the inhabitants of countries possessing a light dry atmosphere are usually more lively than those of countries with a heavy moist climate. PRESSURE ON MERCURY-THE BAROMETER. The pressure of the atmospheric column, at any given point, may be weighed with considerable exactness, by balancing it against an opposite column of mercury, water, or other liquid. The pressure of 15 lbs. to the square inch at the ocean's level is found by experiment to be equal to the weight of a column of mercury of 30 inches in height, a column of water 33 feet in height, or a column of oil 37 feet in height. In other words, the burden of the whole of our atmosphere is equivalent to an ocean of mercury covering the earth to a height of 30 inches, an ocean of water to a height of 33 feet, or an ocean of oil to a height of 37 feet. The fact of such being the degree of atmospheric pressure admits of easy proof, by means of a glass tube upwards of thirty-two inches in length, and a cup half filled with mercury, as represented in fig. 21. The tube is close at its upper end at B, but open at its lower extremity, which is immersed in the mercury below the surface level CPD. The tube having in the first place been filled with pure mercury, a finger is placed on its open end to prevent the egress of the liquid, and thus held, the lower end of the tube is turned downwards, and plunged into the vessel of mercury, when the finger is removed from the orifice. The mercury in the c tube will now be observed to fall to E, or the height of about thirty inches above the surface CP D, and there it will remain. The question now arises, Why the Fig. 21. mercury in the tube does not run out altogether into the cup, instead of standing to a height of thirty inches in the tube? The explanation of the phenomenon is, that from E to B in the tube is a vacuum, and therefore the mercury at its upper extremity is entirely free of atmospheric pressure there is no superincumbent weight to push it out. The column of mercury EP * It is known that travellers, and even their practised guides, often fall down suddenly as if struck by lightning, when approaching lofty summits, on account chiefly of the thinness of the air which they are breathing, and some minutes elapse before they recover. In the elevated plains of South America, the inhabitants have larger chests than the inhabitants of the lower regions-another admirable instance of the animal frame adapting itself to the circumstances in which it is placed.-Arnott's Physics. 46 This very obvious connexion between the rising and falling of mercury in a tube, and the atmosphere, has suggested the construction of an instrument called the barometer (a word from the Greek, signifying weight and measure), by which the effects of atmospheric pres sure may be accurately known. The barometer in common use consists of a narrow glass tube upwards of thirty inches in length, and bent upwards at its lower extremity, as represented in fig. 22. The mercury is introduced into the tube with great care, so that a perfect vacuum exists at the upper extremity. The surface of the mercury in the bent part is open to the action of the atmosphere, and buoys up a small plummet or float F, to which a thread is attached; the thread proceeds upwards to a small pulley G, over which it goes, and terminates in a small ball W. The friction of the thread on the pulley turns a small index H, which points to figures on the surrounding dial. Commonly, the whole apparatus, except the dial-plate, is concealed in an ornamental frame. A B OP B Fig. 22. Barometers of this description are adjusted in such a manner that the smallest rising or falling of the mer. cury from atmospheric action affects the index on the dial, and shows the degree of pressure. In common circumstances, the mercury ranges from 29 to 30 inches. It seldom sinks so low as 28, or rises to 31. When it falls, an indication is given of diminished pressure, and as diminished pressure causes the air to expand, and consequently to be sensibly cooled, moisture is liable to be precipitated in the form of rain. Hence a fall in the mercury of the barometer is considered a prognostic of rain or wet weather, and a rise the reverse. The dial of the barometer is marked accordingly. The barometer, besides being a weather-glass, is used as an instrument for measuring the heights of mountains, or heights attained in balloons, above the level of the sea. As the entire atmosphere sustains thirty inches of ascend, the pressure will become less, and a less body mercury in the tube, it follows that at every step as we of mercury be sustained. It is found that at the height of five hundred feet the mercury has sunk half an inch. But the fall does not proceed in this ratio as we go upwards, because a half of the whole atmosphere is within about three miles, and the other half expanded to an altitude of about fifty miles. Hence, on gaining a height of three miles, the mercury is found to have sunk to fifteen inches, or one half; and on gaining a height of four miles, to twelve inches. Barometers for measuring heights are constructed with a determined scale, marked along the tube of mercury, and by consulting it as we ascend, we learn the height of any spot that we may reach. Perfect exactness, however, is not to be expected in this mode of measurement, because the atmospheric pressure is liable to variation from temperature, and the mercury is liable to contraction or expansion from the same cause. To guard against error, a thermometer, as well as a barometer, is consulted in ascending heights, and the indications of both instruments, according to a scale established by experiment, determine the degree of elevation. Thus, for a diminution of one degree of temperature between 0 and 32 degrees, the mercury in the barometer falls 0.0034 of an inch, and between 32 degrees and 52 degrees it rises 0-0033 of an inch. PRESSURE ON WATER-PUMPS. The effect of atmospheric pressure on water is observable in various contrivances in the arts. Fill a glass to the brim with water, and lay a piece of paper over the whole surface of the liquid: then turn the glass carefully upside down, holding on the paper by the hand; the water will now remain in the glass, being upheld by the pressure of the atmosphere against the paper. Glass fountains of water for bird-cages, ink-holders, and reservoirs of oil for lamps, are constructed on the principle of the liquid being upheld by atmospheric pressure. The apparatus for lifting water from wells, forming the common sucking-pump, acts on the principle of removing the atmospheric pressure from a column of the liquid, thus causing a vacuum in the pump, and allowing the atmospheric pressure on the surface of the liquid in the well to force up and balance the column of liquid. The form of pump used for forcing water to a height above the ground, as in the case of fire-engines or portable forcingpumps for gardens, is different from the common suction-pump. The object in the forcing-pump is to lift water to a certain height by the formation of a vacuum, and then to inject it with violence into the air. The action of the forcingpump apparatus is represented in fig. 24. The piston A sucks the water by its upward motion; but on depressing it, the valve B A B Fig. 24. is closed, and the water is consequently forced through the pipe C. In the case of supplying water to the boiler of a steam-engine, it is necessary to employ a forcing-pump, in order to overcome the pressure of steam within the boiler. The force with which the water is injected overcomes the tendency which the steam has to rush out. to pass through a vessel of waste steam. SYPHONS. Cold or moderately warm water can only be lifted by a pump. If the water be above a certain temperature, about 150 degrees at the utmost, the sucker cannot form a perfect vacuum, because, in the attempt to do so, the water yields a steam or vapour which fills the space; in other words, by removing the atmospheric pressure by the piston, the water begins to vaporise as if about to boil. When a pump is made to operate upon hot water, it labours in vain to raise the liquid. This circumstance limits the heat of water injected into the boilers of steam-engines; or if the water is injected at a high temperature, it must reFig. 23 represents the outline of a common sucking-ceive its heat between the pump and the boiler. This pump. It consists of a cylinder, furnished with a piston is sometimes done, by causing the tube from the pump A made to fit air-tight. In this piston there is a valve opening upwards, not seen in the cut. When the piston is raised, the air is rarefied more and more at each stroke in that portion of the cylinder through which it has moved upwards, and the pressure of the air upon the surface of the water on the outside of the tube forces the fluid into it. The valve B is at the same time opened upwards, and the water after several strokes rushes in above it. When the upward stroke of the piston B Fig. 23. is complete, it is again depressed-the water passes through the valve in the piston, and on the next stroke, it is discharged at the spout. It is evident, that, when the piston is sunk downwards, the water cannot be again forced out of the pump, because the valve at the bottom is pressed down, and prevents its escape. Water may in this manner be lifted by a pump to any height, but in each case the lower or fixed valve in the pump must be less than 34 feet from the surface of the water. It is, however, disadvantageous to lift water from great depths by this means. In such cases it is usual to employ a succession of pumps one above another. It is customary to call pumps hydraulic machines; properly speaking, they are both hydraulic and pneumatic machines, for water is raised by them in a great measure through the agency of atmospheric pres sure. Atmospheric pressure is very conspicuous in the case of the syphon. Fig. 25. A syphon is a tube bent in a particular manner, and is used for drawing off liquors from casks, or water from reservoirs. One kind of syphon is represented in fig. 25, and consists of a tube bent into two equal limbs, each open at the extremity. If such a syphon be filled with water and inverted, so as to turn the two orifices downwards, the liquid will not run out, but remain suspended in the tube, because the pressure of the column of water within is not so great as the pressure of the air without, and thus its escape outwards is prevented. If one end be put into a vessel of water, the vessel will be emptied down to a level with the orifice. It is evident that, when one end of the syphon is inserted in water, the pressure of the atmosphere upon the surface of the water impels the liquid through the tube, and it could be forced upwards to an elevation of above thirty feet, or the height to which water rises in a vacuum. The diagram represents an instrument of this kind furnished with two cups, firmly attached to the ends, which, by retaining a portion of the liquid, keeps the syphon always full and ready for use. Syphons are more commonly made with a long and short limb, as in fig. 26. On inserting the short limb into a vessel of liquid, and drawing the air out of the tube at the mouth A, the liquid will rush out in a stream, and continue flowing till the vessel is emptied. The pressure upwards into the tube at A is the excess of the atmospheric pressure above the vertical pres |