1. PNEUMATICS. This science combines the phenomena which relate to the weight, pressure, and elasticity of the air or atmosphere that surrounds our globe. Some of the more essential mechanical properties of air, have been examined under the article AIR-PUMP; and its vitality has been fully illustrated in our view of pneumatic chemistry. Much, however, yet remains to be stated with regard to pneumatic equilibrium, which must of necessity occupy a prominent place in the present treatise.

2. Various conjectures have been formed with respect to the height of the atmosphere; and, as we know to a certainty the relative weight of a column of the atmosphere by the height to which its pressure will raise water or mercury in an empty tube, so different calculations have been founded on these data, to ascertain its extent as well as its density at different heights. If the air of our atmosphere were indeed every where of a uniform density, the problem would be very easily solved. We should in that case have nothing more to do than to find out the proportions between the height of a short pillar of air, and a small pillar of water of equal weight; and, having compared the proportion, which the height these bear to each other in the small, the same proportions would be certain to hold good in the great, between a pillar of water thirty-two feet high, and a pillar of air that reaches to the top of the atmosphere, the height of which we wish to know. Thus, for instance, we find a certain weight of water reaches one inch high, and a similar weight of air reaches seventy-two feet high this then is the proportion two such pillars bear to each other on the small scale. Now, if one inch of water is equal to seventy-two feet of air, to how much air will thirty-two feet of water be equal; by the common rule of proportion we readily find, that thirty-two feet, or 384 inches of water,will be equal to 331,776 inches, which makes something more than five miles, which would be the height of the atmosphere, was its density every where the same as at the earth, where seventytwo feet of air were equal to one inch of water. But this is not really the case; for the air's density is not every where the same, but decreases as the pressure upon it decreases; so that the air becomes lighter and lighter the higher we ascend; and at the upper part of the atmosphere, where the pressure is scarcely any thing at all, the air, dilating in proportion, must be expanded to a very great extent; and therefore the height of the atmosphere must be much greater than has appeared by the last calculation, in which its density was supposed to be every where as great as at the surface of the earth. In order, therefore, to determine the height of the atmosphere more exactly, geometricians have endeavoured to determine the density of the air at different distances from the earth. The following sketch will give an idea of the method which some have taken to determine this density which is preparatory to finding out the weight of the atmosphere more exactly. If we suppose a pillar of air to reach from the top of the atmosphere down to the earth's surface, and imagine it marked like a standard by inches, from the top to the bottom, and still further suppose that each inch of air, if

not at all compressed, will weigh one grain, the topmost inch then weighs one grain, as it suffers no compression whatever, the second inch is pressed by the topmost with a weight of one grain, and this, added to its own natural weight or density of one grain, now makes its density, which is ever equal to the pressure, two grains. The third inch by the weight of the two inches above it, whose weight united make three grains, and these added to its natural weight give it a density of four grains. The fourth inch is pressed by the united weight of the three above it which together make seven grains, and this added to its natural weight gives it a density of eight grains. The fifth inch being pressed by all the former fifteen, and its own weight added, gives it a density of sixteen grains, and so on descending downwards to the bottom. The first inch has a density of one, the second inch a density of two, the third inch a density of four, the fourth of eight, the fifth of sixteen, and so on. Thus the inches of air increase in density as they descend from the top, at the rate of 1, 2, 4, 8, 16, 32, 64, &c. Or if we reverse this, and begin at the bottom, we may say, that the density of each of these inches grows less upwards. If, instead of inches, we suppose the parts into which this pillar of air is divided to be extremely small, and like those of air, the rule will hold equally good in both. So that we may generally assert that the density of the air from the surface of the earth decreases in a geometrical proportion. This being understood, should we now desire to know the density of the air at any certain height, we have only first to find out how much the density of the air is diminished to a certain standard height, and thence proceed to tell how much it will be diminished at the greatest heights that can be imagined. At small heights the diminution of its density is by fractional or broken numbers. We will suppose at once that at the height of five miles the air is twice less dense than at the surface of the earth: at two leagues high it must be four times thinner and lighter, and at three leagues eight times thinner and less dense, and so on. In short, whatever decrease it received in the first step, it will continue to have in the same proportion in the second, third, and so on; and this, as was observed, is called geometrical progression.

3. Upon the same principle it was attempted to calculate the height of the atmosphere; by carrying a barometer to the top of a high mountain, the density of the air at two or three dif ferent stations was easily ascertained. But so feeble are human efforts in endeavouring to comprehend and measure the works of the Creator, that this theory was soon demolished. It was found that the barometrical observations by no means corresponded with the density which, by other experiments, the air ought to have had; and it was therefore suspected that the upper parts of the atmosphere were not subject to the same laws or the same proportions as those which were nearer the surface of the earth. This process has, however, been fully examined under the article BAROMETER. Another still more ingenious method was therefore devised. Astronomers know to the greatest exactness the part of

the heavens in which the sun is at any one moment of time they know, for instance, the moment in which it will set, and also the precise time in which it is about to rise. They soon, however, found that the light of the sun was visible before its body, and that the sun itself appeared some minutes sooner above the horizon than it ought to do from their calculations. Twilight is seen long before the sun appears, and that at a time when it is several degrees lower than the horizon. There is then, in this case, something which deceives our sight; for we cannot suppose the sun to be so irregular in his motions as to vary every morning; for this would disturb the regularity of nature. The deception actually exists in the atmosphere by looking through this dense, transparent substance, every celestial object that lies beyond it is seemingly raised up, in a way similar to the appearance of a piece of money in a basin filled with water. Hence it is plain, that if the atmosphere was away, the sun's light would not be brought to view so long in the morning before the sun itself actually appears. The sun itself without the atmosphere would appear one entire blaze of light the instant it rose, and leave us in total darkness the moment of its setting. The length of the twilight, therefore, is in proportion to the height of the atmosphere: or let us invert this, and say that the height of the atmosphere is in proportion to the length of the twilight: it is generally found, by this means, to be about forty-five miles high, so that it was hence concluded either that that was the actual limit of the atmosphere, or that it must be of an extreme rarity at that height.

4. If a common drinking glass or tumbler be filled with water, and a piece of bladder tied closely over its mouth, on allowing it to sink to the bottom of a vessel of water, and to stand there with its mouth upwards, the bladder exhibits no sign of being pressed upon, although it bears on its upper side the whole weight of the water directly above it, as the water beneath the bladder resists just as strongly as that above presses: but if by means of a syringe or pump, the water be extracted from within the glass, the bladder itself will then have to bear the whole pressure of the water above it, and will probably be torn or burst, the degree of pressure, and consequently the depth of water in such a case might be ascertained by placing some support, by which the action could be measured under the bladder, to sustain it after the removal of the interior water. Now this phenomenon may be exactly copied in our atmosphere or sea of air. A glass held in the hand is immersed in the fluid air, and is as full of it as the other glass was of water: its mouth may be covered over with bladder, and no external pressure will be apparent, because there is a resistance from the air within just equal to the pressure of the air on the outside; but, if the air be extracted from under the covering by means of the air-pump, the bladder is first seen sinking down and becoming hollow from the weight of the air over it, and at last bursts inwards with a loud report. By placing a circular piece of wood under the bladder for it to rest upon, and a spring of known force to support the wood, we may ascertain very nearly the

weight and pressure of the air over it. The problem, however, can be solved more elegantly and accurately by means of the barometer already described. This phenomenon, illustrative of atmosheric pressure, is often shown by placing the hand on the mouth of a glass so as to cover it closely, and then extracting the air from underneath the hand, the weight of the atmosphere holds the hand down upon the mouth of the glass with considerable force. By means of the exhausting air-pump on the one hand, and of the condensing syringe on the other, all the most important facts dependent on atmospheric pressure, and its increase or diminution, may be strikingly shown. Thus to exhibit the effect of diminished pressure, water which is not heated by several degrees to the boiling point of ordinary low situations, but which would be boiling at the top of Mont Blanc, is caused to boil immediately by placing it under the receiver of an air-pump, and making a few strokes of the piston, to reduce the density of the air around it; and, if the exhaustion be rendered complete, the water will boil, even when less warm by 20° than the blood of animals: and at any temperature, however low, water even in a vacuum assumes rapidly the form of air, or condensible vapor, but without exhibiting the violent agitation of boiling. Other liquids as spirits or ether, from requiring inferior degrees of heat to separate their particles to aëriform distances, boil under the receiver of an air-pump at very low temperatures. Ether boils when as cold as freezing water. On the other hand, to exhibit the effect of increased pressure, if we confine the particles of a liquid by still more than a common atmospheric or equivalent pressure, degrees of heat higher than the common boiling point will be required to separate them. In the diving bell at sixty-eight feet under the surface, the boiling point of water is 272° instead of 212°, and at any other depth it is higher than 212° in proportion to the depth. The fact that liquids are driven off, or made to boil, at lower degrees of heat when the atmospheric pressure is lessened or removed, has recently been applied to some very useful purposes.

5. The process for refining sugar is to dissolve impure sugar in water, and, after clarifying the solution, to boil off or evaporate the water again that the dry crystallised mass may remain. Formerly this evaporation was performed under the atmospheric pressure; and a heat of 218° or 220° was required to make the syrup boil, by which degree of heat, however, a portion of the sugar was discolored and spoiled, and the whole product was deteriorated. The valuable thought occurred to Mr. Howard that the water might be dissipated by boiling the syrup in a vacuum or place from which the air was excluded, and therefore at a low temperature. This was done accordingly; and the saving of sugar and the improvement of quality have been such as to make the patent right, which secured the emoluments of the process to certain parties, worth many thousand pounds a year. The syrup, during this process, is not more heated that it would be in a vessel merely exposed to a summer sun. In the preparation of many medicinal sub

stances the process of boiling in vacuo is equally important. Many watery extracts from vegetables have their virtues impaired, or even destroyed, by a heat of 212°; but, when the water is driven off in vacuo, the temperature need never be higher than blood heat, and all the activity of the fresh plant remains in the extract. 6. It appears, by a paper in the fifth number of the Edinburgh Philosophical Journal, that the invention of the process of artificial desiccation under the receiver of an air-pump, and which has been ascribed to a modern philosopher, is due to Mr. Edward Nairne: a paper concerning which was published in the Philosophical Transactions for 1777. In this paper it is stated, on the authority of Mr. Cavendish, that water, whenever the pressure of the atmosphere on it is diminished to a certain degree, is immediately turned into vapor, and is as immediately turned back again into water. When the heat is at 72° of Fahrenheit's scale, it turns into vapor as soon as the pressure is no greater than that of threequarters of an inch of quicksilver, or about onefortieth of the usual pressure of the atmosphere; but, when the heat is only 41°, the pressure must be reduced to that of a quarter of an inch of quicksilver before the water turns into vapor. Hence it follows that, when the receiver is exhausted to the above mentioned degree, the moisture adhering to the different parts of the machine will turn into vapor and supply the place of the air, which is continually drawn away by the working of the pump, so that the fluid in the body to be dried, as well as in the receiver, will consist in a great measure of vapor. 7. If a piston move in a cylinder so as to be air-tight, and be provided with a valve which opens upwards, upon pressing the piston to the bottom of the cylinder, the air contained in the cylinder will be forced through the valve in the piston. Let us then suppose the piston in close contact with the botton and sides of the cylinder, all air having been excluded: upon attempting to draw the piston up, it will be found that very considerable force will be necessary; and that when sufficient effort has been used, and the piston has been brought to the top of the cylinder, if it be disengaged from the agent which drew it up, it will descend with great force and strike the bottom. This effect plainly indicates the weight of the air pressing on the upper surface of the piston. This is what is vulgarly called suction; as if there were some force within the cylinder which drew the piston to the bottom. But within the cylinder is nothing but empty space, and it is plainly unreasonable to ascribe to empty space any mechanical influence. That it is the weight of the incumbent atmosphere pressing on the upper surface of the piston which forces it to the bottom of the cylinder, is still further proved by the fact, that if the upper surface of the piston be increased, the force which presses it down will be also increased, and what is more will be increased in precisely the same proportion as the surface of the piston. In fact, it is found that, when all air or other elastic fluid has been expelled from beneath the piston, there will be a pressure amounting to about fifteen pounds on every square inch of the

upper surface of the piston; from which we may infer that a column of air, having a square inch for its base, and which extends from the surface of the earth to the top of the atmosphere, weighs about fifteen pounds. The atmospheric engine is a machine whose efficacy depends on the principle which we have been just explaining. In this machine the weight of the atmosphere is used as a first mover in pressing a piston to the bottom of a cylinder.

8. But there is a still more conclusive argument that it is the weight of the atmosphere which presses down the piston. If, by a valve in the bottom of the cylinder, the air be admitted below the piston, it will no longer be pressed down, or rather it will be pressed both upwards and downwards by equal forces, and will be indifferent as to its ascent or descent, except so far as the weight of the piston itself will produce the effect. This is owing to a property of air, by which it presses equally in every direction, which we shall explain more fully hereafter.

9. Many effects with which we are familiar, and which often excite our curiosity, are accounted for by the gravitation of the atmosphere. If the nozzle and the valve-hole of a pair of bellows be stopped, it will be found that a very considerable force will be necessary to separate the boards. This is owing to the air not being permitted to enter at the usual apertures, to resist the pressure of the atmosphere on the external surfaces of the boards. Shell-fish which adhere to rocks, snails, and other animals, have a power by muscular exertion of expelling the air from between the surface of the rock and the surface which they apply to it, in consequence of which they are pressed upon the rock by the atmosphere with a force of about fifteen pounds for every square inch in the surface of contact. The same cause enables flies and other animals to walk on a perpendicular plane of glass or on the lower surface of an horizontal plane, apparently suspended by their feet, and with their bodies downwards. This has lately been proved to arise from a power of expelling the air from between their feet and the surface on which they tread, so as to obtain a pressure from the atmosphere proportionate to the magnitude of the soles of their feet.

10. It may hardly be necessary to state that the animal body is made up of solids and fluids, and that the atmospheric pressure affects it accordingly. It is, however, at first view difficult to conceive how the human frame can bear a pressure of fifteen pounds on every square inch of its surface, while the person who supports the weight remains altogether insensible to its intluence; but such is the fact. Experiment proves that his not feeling the fluid pressure is owing to its being perfectly uniform all around. If a pressure of the same kind be even many times greater, such for instance as fishes bear in deep water, or as a man supports in the diving bell, it must in both cases after the lapse of a few minutes pass unnoticed. Fishes are at their ease in a depth of water where the pressure around will instantly break or burst inwards almost the strongest empty vessel that can be sent down; and men walk on earth without discovering a

11. The fluid pressure on the bodies of animals, thus unperceived under ordinary circumstances, may be rendered instantly sensible by a little artificial arrangement. In water, for instance, an open tube partially immersed becomes full to the level of the water around it, and the water contained in it is supported, as already explained, by what is immediately below its mouth-now a flat fish resting closely against the mouth of the tube would evidently be bearing on its back the whole of this weight, perhaps 100 lbs., but the fish would not thereby be pushed away, nor would it even feel its burden, because the upward pressure of the water immediately under it would just counterbalance, while the lateral pressure around would prevent any crushing effects of the mere upward and downward forces, but if, while the fish continued in the supposed situation, the 100 lbs. of water were lifted from off its back by a piston in the tube, the opposite upward pressure of 100 lbs. would at once crush its body into the tube and destroy it. At a less depth, or with a smaller tube, the effect might not be fatal, but there would be a bulging or swelling of the substance of the fish into the mouth of the tube. In air and in the human body a perfectly analogous case is exhibited. A man, without pain or any peculiar sensation, lays his hand closely on the mouth of a vessel containing air, but, the instant that the air is withdrawn from within the vessel, the then unresisted pressure of the air on the outside fixes the hand upon the vessel's mouth, causes it to swell or bulge into it, and makes the blood ooze from any crack or puncture in the skin.

12. These last few lines closely describe the surgical operation of cupping, the essential circumstances of which are the application of a cup or glass with a smooth blunt lip to the skin of any part, and the extraction by a syringe or other means of a portion of the air from within the cup. It may facilitate to some minds the exact comprehension of this phenomenon, to consider the similar case of a small bladder or bag of India rubber full of any fluid, and pressed between the hands on every part of its surface except one, at which part it swells, and even bursts if the pressure be strong enough. In cupping, the whole body except the surface under the cup is squeezed with a force of fifteen pounds on the square inch, while in that one situation the pressure is diminished according to the degree of exhaustion in the cup, and the blood consequently accumulates there. The mere application of a cup with exhaustion constitutes the operation called dry cupping; to obtain blood, the cup is removed, and the tumid part is cut into by the simultaneous stroke of a number of lancet points, and the cup is afterwards used as before, so that the blood may rush forth under the diminished pressure. The partial vacuum in the cup may be produced either by a syringe or by burning a little spirit in the cup, and applying it while the momentary dilatation effected by the heat has driven out the greater part of

13. There is an effect of the atmospheric pressure on the living body which is rarely thought of although of much importance, viz. its keeping all the parts about the joints firmly together, by an action similar to that on the Magdeburgh hemispheres. The broad surfaces of bone forming the knee joint, for instance, even if not held together by ligaments, could not, while the capsules surrounding the joint remain air-tight, be separated by a force of less than about 100 lbs. In the loose joint of the shoulder, this support is of greater consequence. When the shoulder or other joint is dislocated, there is no empty space left, as might be supposed, but the soft parts around are pressed in to fill up the natural place of the bone. When a thigh bone is dislocated the deep socket called the acetabulum instantly becomes like a cupping glass, and is filled partly with fluid and partly with the soft solids. In all joints it is the atmospheric pressure which keeps the bones in such steady contact that they work smoothly and without noise.

14. A quantity of air or gas shut up in any vessel, and compressed, is equally affected throughout, and its tendency to escape from the pressure is equal in all directions, as is proved by the force necessary to keep similar valves close wherever placed. Hence the hydrostatic press and hydrostatic bellows, which depend for their action on this law, may be worked by air or gas, as they are by a liquid. Owing to this law, air, when allowed, will always rush from where there is more to where there is less pressure. The suddenness with which any compression made on part of a confined aeriform fluid is communicated throughout the whole is strikingly seen in the simultaneous increase in light of all the gas burners over an extensive building, or even over a town, at any instant when the force supplying the gas is augmented.

15. It must be observed, in this place, that very great condensation requires great force, and therefore small syringes. It is therefore convenient to have them of various sizes, and to begin with those of a larger diameter which operate more quickly, and when the condensation becomes fatiguing to change the syringe for a smaller. For this reason, and in general to make the condensing apparatus more convenient, it is proper to have a stop-cock interposed between the syringe and the vessel, or as it is usually called the receiver. This consists of a brass pipe, which has a well-ground cock in its middle, and a hollow screw at one end, which receives the nozzle-screw of the syringe, and a solid screw at the other end, which fits the screw of the receiver.

16. It is known that when air or any other elastic fluid is dilated, by enlarging the space in which it is enclosed, cold is produced. Messrs. Welter and Gay Lussac, who were engaged in researches concerning the heat disengaged by the gasses, when their volume is varied under very different pressures, discovered several important facts, one of the most singular of which is as follows :—The

air which escapes from a vessel by blowing through an aperture, under any pressure whatever, does not alter in temperature although it expands on issuing from the vessel. Hence it should seen to result, that heat is produced in the blowing of the air, and that this heat is so much the more considerable as the difference of pressure producing the blast is greater, so that the heat exactly compensates for the cold produced by expansion. This fact would explain the heat produced, when air enters into a vacuum, or into a space occupied by air at a less pressure. It would likewise explain why the blast of the Chremnitz machine, with a column of water produces cold, and freezes water, while the air of the Chaillot engines, where the pressure is invariable and equal to 2.6 atmospheres, does not alter the thermometer.

17. A great variety of experiments have been made to determine the elasticity of air. Those made by M. de Luc, general Roy, Mr. Trembly and Sir George Shuckbourgh, are by far the most accurate; but they are all confined to a very moderate degree of rarefaction. The general result has been that the elasticity of rarefied air is very nearly proportional to its density. We cannot say with confidence that any regular deviation from this law has been observed, there being as many observations on one side as on the other: but we think that it is not unworthy the attention of philosophers to determine it with precision in the cases of extreme rarefaction, where the irregularities are most remarkable. The great source of error has hitherto arisen from the adhesion of the mercury when the impelling forces are very small and other fluids can hardly be used, because they either coat the inside of the tube and diminish its capacity, or they are converted into vapor, which alters the law of elasticity.

18. Let us, upon the whole, assume the Boylean law, viz. that the elasticity of the air is proportional to its density. The law deviates not in any sensible degree from the truth in those cases which are of the greatest practical importance, that is, when the density does not much exceed or fall short of ordinary air.

19. Let us now see what information this gives us with respect to the action of the particles on each other. The investigation is extremely easy. A force eight times greater than the pressure of the atmosphere will compress common air into the eighth part of its ordinary bulk, and give it eight times its common density; and in this case we know that the particles are at half their former distance, and that the number which are now acting on the surface of the piston employed to compress them are quadruple the number which act on it when it is of the common density. Therefore, when the eightfold compressing force is distributed over a four-fold number of particles, the portion of it which acts on each is double. In like manner, when a compressing force of twenty-seven is employed, the air is compressed into one twenty-seventh of its former bulk, the particles are at one-third of their former distance, and the force is distributed among nine times the number of particles: the force on each is therefore three.

In short, let be the distance of the particles, the number of them in any given vessel, and therefore the density, will be as a, and the number pressing by their elasticity on its whole internal surface will be as r2. Experiment shows that the compressing force is as 3, which, being distributed over the number as 2, will give the force on each as r. Now this force is in immediate equilibrium with the elasticity of the particles immediately contiguous to the compressing surface. This elasticity is therefore as r: and it follows from the nature of perfect fluidity, that the particle adjoining to the compressing surface presses with an equal force on its adjoining particles on every side. Hence we must conclude that the corpuscular repulsions, exerted by the adjoining particles, are inversely as their distances from each other, or that the adjoining particles tend to recede from each other with forces inversely proportional to their distances.

20. Sir Isaac Newton was the first who reasoned in this manner from the phenomena. Indeed he was the first that had the patience to reflect on the phenomena with any precision. His discoveries in gravitation naturally gave his thoughts this turn, and he very early hinted his suspicions that all the characteristic phenomena of tangible matter were produced by forces which were exerted by the particles at small and insensible distances: and he considered the phenomena of air as affording an excellent example of this investigation, and deduced from them the law which we have now demonstrated; he says that air consists of particles which avoid the adjoining particles with forces inversely proportional to their distances from each other.' From this he deduces (in the second book of the Principia) several beautiful propositions, determining the mechanical constitution of the atmosphere. But it must be noticed that he limits this action to the adjoining particles, and this is a remark of immense consequence, though not attended to by the numerous experimenters who adopt the law. It is plain that the particles are supposed to act at a distance, that this distance is variable, and that the forces diminish as the distances increase. A very ordinary air-pump will rarefy the air 125 times. The distance of the particles is now five times greater than before; and yet they still repel each other; for air of this density will still support the mercury in a syphon-gage at the height of 0.24 or 2 of an inch; and a better pump will allow this air to expand twice as much, and still leave it elastic. Thus we see that, whatever is the distance of the particles of common air, they can act five times farther off. The question now becomes whether, in the state of common air, they really do act five times farther than the distance of the adjoining particles; while the particle a acts on the particle b with the force 5, does it also act on the particle c with the force 2.5, on the particle d with the force 1.669, on the particle e with the force 1.25, on the particle f with the force 1, on the particle g with the force 0-8333, &c.

21. Sir Isaac Newton shows, in the plainest manner, that this is by no means the case; for if this were the case, he makes it appear that the