The atmosphere, or body of air encompassing the earth on all sides, is generally divided into three regions. The lowest region extends from the earth to the place where the air is no longer heated by the rays which the earth reflects: this region is the wannest. The middle region begins where the preceding one ends, and goes to the summit of the highest mountains, or even the highest clouds; this is the space where rain, hail, and snow are engendered: this region is much colder than the preceding one. The third region extends from the middle one to the utmost height of the atmosphere; whose limits have not been ascertained.[60] If the air were of an equal density throughout, the height of the atmosphere might be determined: but since the density of the air decreases with the pressure, it will be more rarefied and expanded the higher we go; and by this means the altitude of the atmosphere becomes indefinite, and terminates in pure ether. But though we cannot assign its real height, it is certain, from observations and experiments, that a distance of 45 or 50 miles is the utmost limit where the density is sufficient to refract the rays of light. For the beginning and ending of twilight show, that the height at which the atmosphere begins to refract the sun’s light is about 45 English miles; and therefore that may be reckoned the altitude of the air to the least degree of density.
The air is justly reckoned among the number of fluids, because it has all the properties by which a fluid is distinguished. It requires but little attention to be convinced of this. The air yields to the smallest force impressed on it; its parts are easily moved among themselves; it presses according to its perpendicular height, and its pressure is every where equal. That the air is a fluid consisting of such particles as have no cohesion among themselves, but easily glide over one another, and yield to the smallest impression, appears from the ease and freedom with which animals breathe in it, and move through it without any difficulty or sensible resistance. The ease with which it is penetrated, and driven about in every direction, and the motion of it in pipes and channels, however crooked and intricate, demonstrate its fluidity. It is also known to be a fluid, by the easy conveyance which it affords to sound.
Compressibility and elasticity are evident properties of air. Its elasticity was first ascertained by some experiments of Lord Bacon. The air nearest the earth is in a state of compression, occupying a smaller space than it otherwise would do, were it not compressed by the superincumbent air. It must therefore be in a state something resembling that of a quantity of fine carded wool thrown loosely into a deep pit; the lower strata supporting the weight of the upper strata, and being compressed by them; and so much the more compressed as they are further down, while the upper stratum only is in its unconstrained and most expanded state. If we should suppose this wool thrown in by a hundred weight at a time, it will be divided into strata of equal weights, but of unequal thickness, the lowest being the thinnest, and the superior strata gradually increasing in thickness.[61]
When the air is in a state of compression, we find that the same force with which we compressed it is necessary to keep it in its bulk; and that if we cease to press it together, it will swell out and regain its natural dimensions, which shows its elasticity. This distinguishes it essentially from such a body as a mass of flour, salt, and such like, which remains in the compressed state to which we reduce it. There is something therefore which opposes the compression of air, different from its simple impenetrability, and produces motion, by repelling the compressing body. As an arrow is gradually accelerated by the bow-string pressing it forward, and at the moment of its discharge is brought to a state of rapid motion; so the ball from a pop-gun or wind-gun is gradually accelerated along the barrel by the pressure of the air during its expansion from its compressed state, and finally quits it with an accumulated velocity. These two motions are indications perfectly similar to the elasticity of the bow and of the air.
Mr. Parkes observes, that atmospheric air in all states, and in all seasons, is permanently elastic. This elasticity arises from caloric being chemically combined with the solid substances of which it is composed. I say solid, because we have abundant evidence that oxygen and nitrogen are both capable of taking a solid form, and actually do, in many instances, exist in a state of solidity. Nitrogen is a component part of all animal substances, and exists in a solid state in all the ammoniacal salts. Oxygen takes the same state when it combines with metals and other combustibles; and in the composition of the nitrous salts they both take the same state of solidity. These facts surely evince that atmospheric air owes its fluidity to caloric.
Dr. Hales, by means of a press, condensed the air 33 times; and, afterwards, by forcing water in an iron globe, into 1,551 times less space than it naturally occupies. The dilation of the air, by virtue of its elastic force, is found to be very surprising. In experiments made by Mr. Boyle, it dilated to 10,000, and even, at last, in 13,679 times its space; and this altogether by its own expansive force, without the help of fire. In fact, it appears that the air we breathe is compressed by its own weight into at least the 13,679th part of the space it would occupy in vacuo. But if the same air be condensed by art, the space it would take up when most dilated, will be, according to the same author’s experiments, as 550,000 to 1.
It is only by means of the experiments made with pumps,[62] and the barometrical tube, by Galileo and Torricelli, that we came to the proof, not only that the atmosphere is endued with weight and pressure, but also of the measure and quantity of that pressure. The rise of water in a pump was formerly attributed to the horror that nature had of a vacuum. This absurd notion was refuted about the middle of the seventeenth century, by the following occurrence. The Duke of Florence, having occasion to raise water to the height of 50 or 60 feet, ordered a common pump to be made for that purpose; but when it was completed, the workmen were astonished to find that it would not work. The matter was referred to Galileo, but he was unable to account for it in any way. All they were able to determine was, that water would not rise in a common pump more than 32 or 35 feet. The fact remained inexplicable till philosophers caught the idea of atmospheric pressure; since when, the suspension of mercury in the barometer, and water in a pump, have been well understood.[63]
That the air is a heavy body, has been demonstrated by a variety of experiments. The air next the earth is more dense than that at a distance, because, as it is of an elastic or springy nature, it is pressed down by the whole weight of the superincumbent air. Its general force of gravity appears, from its surrounding the earth, and always accompanying it in its orbit round the sun. As the matter of which the air is composed is always variable, so likewise will its weight or gravity be, as barometers of various kinds and structure evince. The weight of the air at the earth’s surface, is found by the quantity of mercury that the atmosphere balances in the barometer; in which, at a mean state, the mercury stands 29½ inches high. And if the tube were a square inch wide, it would at that height contain 29½ cubic inches of mercury, which is just 15 pounds weight; and so much weight of air every square inch of the earth’s surface sustains; and every square foot, as containing 144 inches, must sustain a pressure of 2,160. At this rate, a middle-sized man, whose surface is about 15 square feet, must sustain a weight of 32,400 pounds, or 16 tons; for the air, like other fluids, presses equally upwards, downwards, and sideways, in every direction. But because this enormous weight bears equally on all sides, and is counterbalanced by the spring of air diffused through all parts of the body, it is not in the least felt by us.[64]
By this enormous pressure we should undoubtedly be crushed in a moment were not all parts of our bodies filled either with air or some other elastic fluid, whose spring is just sufficient to counterbalance the weight of the atmosphere. The human body is a bundle of solids, hard or soft, filled or mixed with fluids, and there are few or no parts of it which are empty. All communicate either by vessels or pores; and the whole surface is a sieve through which the insensible perspiration is performed. The whole extended surface of the lungs is open to the pressure of the atmosphere; every thing therefore is in equilibrio: and if free or speedy access be given to every part, the body will not be damaged by the pressure, however great, any more than a wet sponge would be deranged by plunging it any depth in water. The pressure is instantaneously diffused by means of the incompressible fluids with which the parts are filled: and if any parts are filled with air or other compressible fluids, these are compressed till their elasticity balances the pressure. Besides, all our fluids are acquired slowly, and gradually mixed with that proportion of air which they can dissolve or contain. The whole animal has grown up in this manner from the first vital atom of the embryo. For such reasons the pressure can occasion no change of shape by squeezing together the flexible parts; nor any obstruction by compressing the vessels or pores.
Sometimes the air is so heavy and elastic as to support the mercury in the tube at the height of 31 inches nearly; at other times it is so light and unelastic, as to suffer it to fall as low as 28 inches. The difference between these two altitudes is three inches, that is, about 1-9th of the whole weight of the atmosphere. Our bodies, therefore, are sometimes pressed with a weight one-ninth more than at other times, that is, with about 3,360 pounds more weight at one time than another. This has considerable effect on our feelings, and consequently on our health, but we are apt to ascribe this effect to a wrong cause. When we feel ourselves dull and languid, we think it is owing to the air being too thick and heavy about us. But it is just the reverse: the air is then too light and thin, as is evident from the mercury’s sinking in the barometer, and its not bearing up the clouds: it is seldom dense enough at two miles height to bear them up.[65] The weight of the air is proved by its supporting the clouds and vapors which we so frequently see floating in it; in the same manner that the swimming of a piece of wood indicates the weight of the water which supports it.