ATMOSPHERE
, a term used to signify the whole of the fluid mass, consisting of air, aqueous and other vapours, electric fluids, &c, which surrounds the earth to a considerable height, and partaking of all its motions, both annual and diurnal.
The composition of that part of our atmosphere properly called air, was till lately but very little known. Formerly it was supposed to be a simple, homogeneous, and elementary fluid. But the experiments of Dr. Priestley and others, have discovered, that even the purest kind of air, which they call dephlogisticated, is in reality a compound, and might be artificially produced in various ways. This dephlogisticated air, however, is but a small part of the composition of our atmosphere. By accurate experiments, the air we usually breathe, is composed of only one-fourth part of this dephlogisticated air, or perhaps less; the other three parts, or more, consisting of what Dr. Priestley calls phlogisticated, and M. Lavoisier mephitic air. |
Beside these sorts of air, it is obvious that the whole mass of the atmosphere contains a great deal of water, together with a vast heterogeneous collection of particles raised from all bodies of matter on the surface of the earth, by effluvia, exhalations, &c; so that it may be considered as a chaos of the particles of all sorts of matter confusedly mingled together. And hence the atmosphere has been considered as a large chemical vessel, in which the matter of all kinds of sublunary bodies is copiously floating; and thus exposed to the continual action of that immense surface the sun; from whence proceed innumerable operations, sublimations, separations, compositions, digestions, fermentations, putrefractions, &c.
There is, however, one substance, namely the electrical fluid, which is very distinguishable in the mass of the atmosphere. To measure the absolute quantity of this fluid, either in the atmosphere or any other substance, is perhaps impossible: and all that we know on this subject is, that the electric fluid pervades the atmosphere; that it appears to be more abundant in the superior than the inferior regions; that it seems to be the immediate bond of connection between the atmosphere and the water which is suspended in it; and that by its various operations, the phenomena of hail, rain, snow, lightning, and the other kinds of meteors are occasioned. See those respective articles; and see also Beccaria's Essay on Atmospheric Electricity, annexed to the English translation of his Artisicial Electricity.
Uses of the Atmosphere.—Theuses of the atmosphere are so many and great, that it seems indeed absolutely necessary, not only to the comfort and convenience of men, but even to the existence of all animal and vegetable life, and to the very constitution of all kinds of matter whatever, and without which they would not be what they are: for by it we live, breathe, and have our being; and by insinuating itself into all the vacuities of bodies, it becomes the great spring of most of the mutations here below; as generation, corruption, dissolution, &c; and without which none of these operations could be carried on. Without the atmosphere, no animal could exist, or indeed be produced; neither any plant, all vegetation ceasing without its aid; there would be neither rain nor dews to moisten the face of the ground; and though we might perceive the sun and stars like bright specks, we should be in utter darkness, having none of what we call day light or even twilight: nor would either fire or heat exist without it. In short, the nature and constitution of all matter would be changed and cease; wanting this universal bond and constituting principle.
By the mechanical force of the atmosphere too, as well as its chemical virtues, many necessary purposes are answered. We employ it as a moving power, in the motion of ships, to turn mills, and for other such uses. And it is one of the great discoveries of the modern philosophers, that the several motions attributed by the ancients to a fuga vaceui, are really owing to the pressure of the atmosphere. Galileo, having observed that there was a certain standard altitude, beyond which no water could be elevated by pumping, took an occasion from thence to call in question the doctrine of the schools, which ascribed the ascent of water in pumps, to the fuga vacui, and instead of it he happily substituted the hypothesis of the weight and pressure of the air. It was with him, indeed, little better than an hypothesis, since it had not then those confirmations from experiment, afterwards found out by his pupil Torricelli, and other succeeding philosophers, particularly Mr Boyle.
Nor have the attempts to fly or float through the air been altogether without success. F. De Lana thought he had contrived an aeronautic machine for navigating the atmosphere: and Sturmius, who examined it, asserted that it was not impracticable: though Dr Hook was of a different opinion, and detected the fallacy of the contrivance. Roger Bacon, long before, proposed something of the same kind. The great secret of this art is to contrive a machine so much lighter than the air, that it will rise up and float in the atmosphere, and together with itself, buoy up and carry men along with it. The principle on which it is to be effected is, by means of an air pump, to exhaust the air from a very thin and light, yet firm, metalline vessel. But the hopes of success in such an enterprize will appear very small if it be considered, that if a globe were formed of brass of the thickness only of 1/12 of an inch, such a globe would require to be about 277 feet in diameter to float in the air: and if, as De Lana supposes, the diameter of the globe were but 25 feet, the thickness of the metal could not exceed 1/33 of an inch. See Herman's Phoronomia pa. 158. However, what is not to be expected from metalline globes or shells, has now been successfully accomplished by the balloons of cloth, silk, or skin, of Montgolfier and others. See the article Aerostation, &c.
Salubrity of the Atmosphere. On the tops of mountains the air is generally more salubrious than in pits or very deep places. Indeed dense air is always more proper for respiration, as to the mere quality of density only, than that which is rarer. But then the air on mountains, though rarer, is freer from phlogistic vapours than that of pits; and hence it has been found that people can live very well on the tops of mountains, even when the air is but about half the density of that below. But it would seem that at some intermediate height between the two extremes, the air is the most salubrious and proper for animal life; and this height, according to M. de Saussure, is about 500 or 600 yards above the level of the sea.
Besides the difference arising from the mere difference of altitude, the salubrity of the atmosphere is greatly affected by many other circumstances. The air, when confined or stagnant, is commonly more impure than when agitated and shifted: thus, all close places are unhealthy, and even the air in a bed chamber is less salubrious in a morning, after it has been slept in, than in the evening. Dr White, in vol. 68 Philos. Trans. gives an account of experiments on this quality of the air, and remarks one instance when the air was particularly impure, viz September 13, 1777; when the barometer stood at 30.30, the thermometer at 69°; the air being then dry and sultry, and no rain having fallen for more than two weeks. A slight shock of an earthquake was perceived that day. In vol. 70 of the same Transactions Dr. Ingenhousz gives an account of some experiments on this head, made in various places and | situations: he finds,” That the air at sea, and close to it, is in general purer, and fitter for animal life, than the air on the land:” but the Doctor did not find much difference between the air of the towns and of the country, nor between one town and another. The Abbé Fontana, made nearly the same conclusious, from accurate experiments, asserting, “that the difference between the air of one country and that of another, at different times, is much less than what is commonly believed; and yet that this difference in the purity of the air at different times, is much greater than the difference between the air of the different places observed by him.” Finally M. Fontana concludes, that “Nature is not so partial as we commonly believe. She has not only given us an air almost equally good every where at every time, but has allowed us a certain latitude, or a power of living and being in health in qualities of air which differ to a certain degree. By this I do not mean to deny the existence of certain kinds of noxious air in some particular places; but only say, that in general the air is good every where, and that the small differences are not to be feared so much as some people would make us believe. Nor do I mean to speak here of those vapours and other bodies which are accidentally joined to the common air in particular places, but do not change its nature and intrinsical property. This state of the air cannot be known by the test of nitrous air; and those vapours are to be considered in the same manner as we should consider so many particles of arsenic swimming in the atmosphere. In this case it is the arsenic, and not the degenerated air, that would kill the animals who ventured to breathe it.”
Figure of the Atmosphere.—As the atmosphere envelops all parts of the surface of our globe, if they both continued at rest, and were not endowed with a diurnal motion about their common axis, then the atmosphere would be exactly globular, according to the laws of gravity; for all the parts of the surface of a fluid in a state of rest, must be equally removed from its centre. But as the earth and the ambient parts of the atmosphere revolve uniformly together about their axis, the different parts of both have a centrifugal force, the tendency of which is more considerable, and that of the centripetal less, as the parts are more remote from the axis; and hence the figure of the atmosphere must become an oblate spheroid; since the parts that correspond to the equator are father removed from the axis, than the parts which correspond to the poles. Besides, the figure of the atmosphere must, on another account, represent a flattened spheroid, namely because the sun strikes more directly the air which encompasses the equator, and is comprehended between the two tropics, than that which pertains to the polar regions: for, from hence it follows, that the mass of air, or part of the atmosphere, adjoining to the poles, being less heated, cannot expand so much, nor reach so high. And yet, notwithstanding, as the same force which contributes to elevate the air, diminishes its gravity and pressure on the surface of the earth, higher columns of it about the equatorial parts, all other circumstances being the same, may not be heavier than those about the poles.
In the Transactions of the Royal Irish Academy for 1788 Mr Kirwin has an ingenious dissertation on the figure, height, weight, &c, of the atmosphere. He observes that, in the natural state of the atmosphere, that is, when the barometer would every where, at the level of the sea, stand at 30 inches, the weight of the atmosphere, at the surface of the sea, must be equal all over the globe; and in order to produce this equality, as the weight proceeds from its density and height, it must be lowest where the denfity is greatest, and highest where the density is least; that is, highest at the equator and lowest at the poles, with several intermediate gradations.
Though the equatorial air however be less dense to a certain height than the polar, yet at some greater heights it must be more dense: for since an equatorial and polar column are equal in total weight or mass, the lower part of the equatorial column, being more expanded by heat &c than that of the polar, must have less mass, and therefore a proportionably greater part of its mass must be found in its superior section; so that the lower extremity of the superior section of the equatorial column is more compressed, and consequently denser, than the corresponding part of the polar column. The same thing is to be understood also of the extra-tropical columns with respect to each other, where differences of heat prevail.
Hence, in the highest regions of the atmosphere, the denser equatorial air, not being supported by the collateral extra-tropical columns, gradually flows over, and rolls down to the north and south.
These superior tides consist chiefly of inflammable air, as it is much lighter than any other, and is generated in great plenty between the tropics; it furnishes the matter of the auroræ borealis and australis, by whose combustion it is destroyed, else its quantity would in time become too great, and the weight of the atmosphere annually increased; but its combustion is the primary source of the greatest perturbations of the atmosphere.
Weight or Pressure of the Atmosphere.—It is evident that the mass of the atmosphere, in common with all other matter, must be endowed with weight and pressure; and this principle was asserted by almost all philosophers, both ancient and modern. But it was only by means of the experiments made with pumps and the barometrical tube, by Galileo and Torricelli, that we came to the proof, not only that the atmosphere is endued with a pressure, but also what the measure and quantity of that pressure is. Thus, it is sound that the pressure of the atmosphere sustains a column of quicksilver, in the tube of the barometer, of about 30 inches in height; it therefore follows, that the whole pressure of the atmosphere is equal to the weight of a column of quicksilver, of an equal base, and 30 inches height: and because a cubical inch of quicksilver is found to weigh nearly half a pound averdupois, therefore the whole 30 inches, or the weight of the atmosphere on every square inch of surface, is equal to 15 pounds. Again, it has been found that the pressure of the atmosphere balances, in the case of pumps &c, a column of water of about 34 1/2 feet high; and, the cubical foot of water weighing just 1000 ounces, or 62 1/2 pounds, 34 1/2 times 62 1/2, or 2158lb, will be the weight of the column of water, or of the atmosphere on a base of a square foot; and consequently the 144th part of this, or 15lb, is the weight of the atmosphere on a square | inch; the same as before. Hence Mr Cotes computed that the pressure of this ambient fluid on the whole surface of the earth, is equivalent to that of a globe of lead of 60 miles in diameter. And hence also it appears, that the pressure upon the human body must be very considerable; for as every square inch of surface sustains a pressure of 15 pounds, every square foot will sustaín 144 times as much, or 2160 pounds; then, if the whole surface of a man's body he supposed to contain 15 square feet, which is pretty near the truth, he must sustain 15 times 2160, or 32400 pounds, that is nearly 14 1/2 tons weight, for his ordinary load. By this enormous pressure we should undoubtedly be crushed in a moment, if all parts of our bodies were not filled either with air or some other elastic fluid, the spring of which is just sufficient to counterbalance the weight of the atmosphere. But whatever this fluid may be, it is certain that it is just able to counteract the weight of the atmosphere, and no more: for, if any considerable pressure be superadded to that of the air, as by going into deep water, or the like, it is always severely felt let it be ever so equable, at least when the change is made suddenly; and if, on the other hand, the pressure of the atmosphere be taken off from any part of the human body, as the hand for instance, when put over an open receiver, from whence the air is afterwards extracted, the weight of the external atmosphere then prevails, and we imagine the hand strongly sucked down into the glass.
The difference in the weight of the air which our bodies sustain at one time more than another, is also very considerable, from the natural changes in the state of the atmosphere. This change takes place chiefly in countries at some distance from the equator; and as the barometer varies at times from 28 to 31 inches, or about one tenth of the whole quantity, it follows that this difference amounts to about a ton and a half on the whole body of a man, which he therefore sustains at one time more than at another. On the increase of this natural weight, the weather is commonly fine, and we feel ourselves what we call braced and more alert and active; but, on the contrary, when the weight of the air diminishes, the weather is bad, and people feel a listlessness and inactivity about them. And hence it is no wonder that persons sufser very much in their health, from such changes in the atmofphere, especially when they take place very suddenly, for it is to this circumstance chiefly that a sensation of uneasiness and indisposition is to be attributed; thus, when the variations of the barometer and atmosphere are sudden and great, we feel the alteration and effect on our bodies and spirits very much; but when the change takes place by very slow degrees, and by a long continuance, we are scarcely sensible of it, owing, undoubtedly, to the power with which the body is naturally endowed, of accommodating itself to this change in the state of the air, as well as to the change of many other circumstances of life, the body requiring a certain interval of time to effect the alteration in its state, proper to that of the air &c. Thus, in going up to the tops of mountains, where the pressure of the atmosphere is diminished two or three times more than on the plain below, little or no inconvenience is felt from the rarity of the air, if it is not mixed with other noxious vapours &c; because that, in the ascent the body has had sufficient time to accommodate itself gradually to the slow variation in the state of the atmosphere: but, when a person ascends with a balloon, very rapidly to a great height in the atmosphere, he feels a difficulty in breathing and an uneasiness of body; and the same is soon felt by an animal when inclosed in a receiver, and the air suddenly drawn or pumped out of it. So also, on the condensation of the air, we feel little or no alteration in ourselves, except when the change happens suddenly, as in very rapid changes in the weather, and in descending to great depths in a diving bell, &c. I have often heard the late unfortunate Mr. Spalding speak of his experience on this point: he always found it absolutely necessary to descend with the bell very slowly, and that only from one depth to another, resting a while at each depth before he began to descend farther: he first descended slowly for about 5 or 6 fathom, and then stopped a while; he felt an uneasiness in his head and ears, which increased more and more as he descended, till he was obliged to stop at the depth above mentioned, where the density of the air was nearly doubled; having remained there a while, he felt his ears give a sudden crack, and after that he was soon relieved from any uneasiness in that part, and it seemed as if the density of the air was not altered. He then descended other 5 fathoms or 30 feet more, with the same precaution and the same sensations as before, being again relieved, in the same manner, after remaining awhile stationary at the next stage of his descent, where the density of the air was tripled. And thus he continued proceeding to a great depth, always with the same circumstances, repeated at every 5 or 6 fathoms, and adding the pressure of one more atmosphere at every period of the progress.
It is not easy to assign the true reason for the variations that happen in the gravity of the atmosphere in the same place. One cause of it however, either immediate or otherwise, it seems, is the heat of the sun; for where this is uniform, the changes are small and regular; thus between the tropics it seems the change depends on the heat of the sun, as the barometer constantly sinks about half an inch every day, and rises again to its former station in the night time. But in the temperate zones the barometer ranges from 28 to near 31 inches, shewing, by its various altitudes, the changes that are about to take place in the weather. If we could know therefore, the causes by which the weather is influenced; we should also know those by which the gravity of the atmosphere is affected. These may perhaps be reduced to immediate ones, viz, an emission of latent heat from the vapour contained in the atmosphere, or of electric fluid from the same, or from the earth; as it is observed that they both produce the same effect with the solar heat in the tropical climates, viz, to rarefy the air, by mixing with it, or setting loose a lighter fluid, which did not before act in such large proportion in any particular place.
With regard to the alteration of heat and cold in the atmosphere, many reasons and hypotheses have been given, and many experiments made; as may be seen by consulting the authors upon this subject, viz, M. Bouguer's observations in Peru, Lambert, De Luc, Saussure's journeys on the Alps, Sex's and Darwin's | experiments in vol. 78 Philos. Trans. This last gentleman hence infers, “There is good reason to conclude that in all circumstances where air is mechanically expanded, it becomes capable of attracting the fluid matter of heat from other bodies in contact with it. Now, as the vast region of air which surrounds our globe is perpetually moving along its surface, climbing up the sides of mountains, and descending into the valleys; as it passes along it must be perpetually varying the degree of heat according to the elevation of the country it traverses: for, in rising to the summits of mountains, it becomes expanded, having so much of the pressure of the superincumbent atmosphere taken away; and when thus expanded, it attracts or absorbs heat from the mountains in contiguity with it; and, when it descends into the valleys and is compressed into less compass, it again gives out the heat it has acquired to the bodies it comes in contact with. The same thing must happen to the higher regions of the atmosphere, which are regions of perpetual frost, as has lately been discovered by the aerial navigators. When large districts of air, from the lower parts of the atmosphere, are raised two or three miles high, they become so much expanded by the great diminution of the pressure over them, and thence become so cold, that hail or snow is produced by the precipitation of the vapour: and as there is, in these high regions of the atmosphere, nothing else for the expanded air to acquire heat from after it has parted with its vapour, the same degree of cold continues till the air, on descending to the earth, acquires its former state of condensation and of warmth. The Andes, almost under the line, rests its base on burning sands: about its middle height is a most pleasant and temperate climate covering an extensive plain, on which is built the city of Quito; while its forehead is encircled with eternal snow, perhaps coeval with the mountain. Yet, according to the accounts of Don Ulloa, these three discordant climates seldom encroach much on each other's territories. The hot winds below, if they ascend, become cooled by their expansion; and hence they cannot affect the snow upon the summit; and the cold winds that sweep the summit, become condensed as they descend, and of temperate warmth before they reach the fertile plains of Quito.”
Height and Density of the Atmosphere. Various attempts have been made to ascertain the height to which the atmosphere is extended all round the earth. These commenced soon after it was discovered by means of the Torricellian tube, that air is endued with weight and pressure. And had not the air an elastic power, but were it every where of the same density, from the surface of the earth to the extreme limit of the atmosphere, like water, which is equally dense at all depths it would be a very easy matter to determine its height from its density and the column of mercury which it would counterbalance in the barometer tube: for, it having been observed that the weight of the atmosphere is equivalent to a column of 30 inches or 2 1/2 feet of quicksilver, and the density of the former to that of the latter, as 1 to 11040; therefore the height of the uniform atmosphere would be 11040 times 2 1/2 feet, that is 27600 feet, or little more than 5 miles and a quarter. But the air, by its elastic quality, expands and contracts; and it being found by repeated experiments in most nations of Europe, that the spaces it occupies, when compressed by different weights, are reciprocally proportional to those weights themselves; or, that the more the air is pressed, so much the less space it takes up; it follows that the air in the upper regions of the atmosphere must grow continually more and more rare, as it ascends higher; and indeed that, according to that law, it must necessarily be extended to an indefinite height. Now, if we suppose the height of the whole divided into innumerable equal parts; the quantity of each part will be as its density; and the weight of the whole incumbent atmosphere being also as its density; it follows, that the weight of the incumbent air, is every where as the quantity contained in the subjacent part; which causes a difference between the weights of each two contiguous parts of air. But, by a theorem in arithmetic, when a magnitude is continually diminished by the like part of itself, and the remainders the same, these will be a series of continued quantities decreasing in geometrical progression: therefore if, according to the supposition, the altitude of the air, by the addition of new parts into which it is divided, do continually increase in arithmetical progression, its density will be diminished, or, which is the same thing, its gravity decreased, in continued geometrical proportion. And hence, again, it appears that, according to the hypothesis of the density being always proportional to the compressing force, the height of the atmosphere must necessarily be extended indefinitely. And, farther, as an arithmetical series adapted to a geometrical one, is analogous to the logarithms of the said geometrical one; it follows therefore that the altitudes are proportional to the logarithms of the densities, or weights of air; and that any height taken from the earth's surface, which is the difference of two altitudes to the top of the atmosphere, is proportional to the difference of the logarithms of the two densities there, or to the logarithm of the ratio of those densities, or their corresponding compressing forces, as measured by the two heights of the barometer there. This law was first observed and demonstrated by Dr. Halley, from the nature of the hyperbola; and afterwards by Dr. Gregory, by means of the logarithmic curve. See Philos. Trans. N°. 181, or Abridg. vol. 2, p. 13, and Greg. Astron. lib. v, prop. 3.
It is now easy, from the foregoing property, and two or three experiments, or barometrical observations, made at known altitudes, to deduce a general rule to determine the absolute height answering to any density, or the density answering to any given altitude above the earth. And accordingly, calculations were made upon this plan by many philosophers, particularly by the French; but it having been found that the barometrical observations did not correspond with the altitudes as measured in a geometrical manner, it was suspected that the upper parts of the atmospherical regions were not subject to the same laws with the lower ones, in regard to the density and elasticity. And indeed, when it is considered that the atmosphere is a heterogeneous mass of particles of all sorts of matter, some elastic, and others not, it is not improbable but this may be the case, at least in the regions very high in the atmosphere, which it is likely may more copiously abound with the electrical fluid. Be this however as it may, it has lately been discovered that the law | above given, holds very well for all such altitudes as are within our reach, or as far as to the tops of the highest mountains on the earth, when a correction is made for the difference of the heat or temperature of the air only, as was fully evinced by M. De Luc, in a long series of observations, in which he determined the altitudes of hills both by the barometer, and by geometrical measurement, from which he deduced a practical rule to allow for the difference of temperature. See his Treatise on the Modifications of the Atmosphere. Similar rules have also been deduced from accurate experiments, by Sir George Shuckburgh and General Roy, both concurring to shew, that such a rule for the altitudes and densities, holds true for all heights that are accessible to us, when the elasticity of the air is corrected on account of its density: and the result of their experiments shewed, that the difference of the logarithms of the heights of the mercury in the barometer, at two stations, when multiplied by 10000, is equal to the altitude in English fathoms, of the one place above the other; that is, when the temperature of the air is about 31 or 32 degrees of Fahrenheit's thermometer; and a certain quantity more or less, according as the actual temperature is different from that degree.
But it may here be shewn, that the same rule may be deduced independent of such a train of experiments as those above, merely by the density of the air at the surface of the earth alone. Thus, let D denote the density of the air at one place, and d the density at the other; both measured by the column of mercury in the barometrical tube: then the difference of altitude between the two places, will be proportional to the log. of D - the log. of d, or to the log. of D/d. But as this formula expresses only the relation between different altitudes, and not the absolute quantity of them, assume some indeterminate, but constant quantity h, which multiplying the expression log. D/d, may be equal to the real difference of altitude a, that is, . Then, to determine the value of the general quantity h, let us take a case in which we know the altitude a which corresponds to a known density d; as for instance, taking a = 1 foot, or 1 inch, or some such small altitude: then because the density D may be measured by the pressure of the whole atmosphere, or the uniform column of 27600 feet, when the temperature is 55°; therefore 27600 feet will denote the density D at the lower place, and 27599 the less density d at 1 foot above it; consequently , which, by the nature of logarithms, is nearly nearly; and hence we find h = 63551 feet; which gives us this formula for any altitude a in general, viz, fathoms; where M denotes the column of mercury in the tube at the lower place, and m that at the upper This formula is adapted to the mean temperature of the air 55°: but it has been found, by the experiments of Sir Geo. Shuckburgh and General Roy, that for every degree of the thermometer, different from 55°, the altitude a will vary by its 435th part; hence, if we would change the factor h from 10592 to 10000, because the difference 592 is the 18th part of the whole factor 10592, and because 18 is the 24th part of 435; therefore the change of temperature, answering to the change of the factor h, is 24°, which reduces the 55° to 31°. So that, fathoms, is the easiest expression for the altitude, and answers to the temperature of 31°, or very nearly the freezing point: and for every degree above that, the result must be increased by so many times its 435th part, and diminished when below it.
From this theorem it follows, that, at the height of 3 1/2 miles, the density of the atmosphere is nearly 2 times rarer than it is at the surface of the earth; at the height of 7 miles, 4 times rarer; and so on, according to the following table:
| Height in miles. | Number of times rarer. |
| 3 1/2 | 2 |
| 7 | 4 |
| 14 | 16 |
| 21 | 64 |
| 28 | 256 |
| 35 | 1624 |
| 42 | 4096 |
| 49 | 16384 |
| 56 | 65536 |
| 63 | 262144 |
| 70 | 1048576 |
Hence we may perceive how very soon the air becomes so extremely rare and light, as to be utterly imperceptible to all experience; and that hence, if all the planets have such atmospheres as our earth, they will, at the distances of the planets from one another, be so extremely attenuated, as to give no sensible resistance to the planets in their motion round the sun for many, perhaps hundreds or thousands of ages to come. Even at the height of about 50 miles, it is so rare as to have no sensible effect on the rays of light: for it was found by Kepler, and De la Hire after him, who computed the height of the sensible atmosphere from the duration of twilight, and from the magnitude of the terrestrial shadow in lunar eclipses, that the effect of the atmosphere to reflect and intercept the light of the sun, is only sensible to the altitude of between 40 and 50 miles: and at that altitude we may collect, from what has been already said, that the air is above 10000 times rarer than at the surface of the earth. It is well known that the twilight begins and ends when the centre of the sun is about 18 degrees below the horizon, or only 17° 27′, by subtracting 33′ for | refraction, which raises the sun so much higher than he would be. And a ray coming from the sun in that position, and entering the earth's atmosphere, is refracted and bent into a curve line in passing through it to the eye. M. de la Hire took great pains to demonstrate, that, supposing the density of the atmosphere proportional to its weight, this curve is a cycloid: he also says, that if the ray be a tangent to the atmosphere, the diameter of its generating circle will be the height of the atmosphere; and that this diameter increases, till at last, when the rays are perpendicular, it becomes infinite, or the circle degenerates into a right line. This reasoning supposes that the refracting surface of the atmosphere is a plane; but since it is in reality a curve, he observes that these cycloids become in fact epicycloids. But Herman detected the error of M. de la Hire, and shewed that this curve is infinitely extended, and has an asymptote. And it is observed by Dr. Brook Taylor, in his Methodus Increm. pa. 168, &c, that this curve is one of the most intricate and perplexed that can well be proposed. The same ingenious author computes, that the refractive power of the air is to the force of gravity at the surface of the earth, as 320 millions to 1.
Considering the extreme rarity of the atmosphere at only 40 or 50 miles in height, it seems to be surprizing that some meteors should be enflamed at such great heights as they have been observed at. A very remarkable one of this kind was observed by Dr. Halley in the month of March 1719, the altitude of which he computed at between 69 and 73 1/2 English miles; its diameter 2800 yards, or more than a mile and a half; and its velocity about 350 miles in a minute. Others, apparently of the same kind, but whose altitude and velocity were still greater, have been observed; particularly that very remarkable one, of August 18th 1783, whose distance from the earth could not be less than 90 miles, its diameter at least as large as the former, while its velocity was certainly not less than 1000 miles in a minute. Now, from analogy of reasoning, it seems very probable, that the meteors which appear at such great heights in the air, are not essentially different from those which are seen on or near the surface of the earth. The difficulty with regard to the former is, that at the great heights above-mentioned, the atmosphere ought not to have any density sufficient to support flame, or to propagate sound; and yet such meteors are commonly succeeded by one explosion or more, and it is said are even sometimes accompanied with a hissing noise as they pass over our heads. The meteor of 1719 was not only very bright, seeming for a short time to turn night into day, but was attended with an explosion heard over all the island of Britain, causing a violent concussion in the atmosphere, and seeming to shake the earth itself: And yet, in the regions in which this meteor moved, the air ought to have been 300 thousand times rarer than the air we breathe, or 1000 times rarer than the vacuum commonly made by a good air-pump. Dr. Halley offers a conjecture, indeed, that the vast magnitude of such bodies might compensate for the thinness of the medium in which they moved. But appearances of this kind are, by some others, attributed to electricity; though the circumstances of them cannot be reconciled to that cause; for the meteors move with all different degrees of velocity; and though the electrical fire easily pervades the vacuum of an airpump, yet it does not in that case appear in bright well-desined sparks, as in the open air, but rather in long streams resembling the aurora borealis; and from some late experiments it has been concluded that the electric fluid cannot even penetrate a perfect vacuum.
Of the Refractive and Reflective Power of the ATMOSPHERE. It has been observed above, that the atmosphere has a refractive power, by which the rays of light are bent from the right lined direction, as in the case of the twilight; and many other experiments manifest the same virtue, which is the cause of many phenomena. Alhazen, the Arabian, who lived about the year 1100, it seems was more inquisitive into the nature of refraction than former writers. But neither Alhazen, nor his follower Vitello, knew any thing of its just quantity, which was not known, to any tolerable degree of exactness, till Tycho Brahe, with great diligence, settled it. But neither did Tycho nor Kepler discover in what manner the rays of light were refracted by the atmosphere. Tycho thought the refraction was chiefly caused by dense vapours, very near the earth's surface: while Kepler placed the cause wholly at the top of the atmosphere, which he thought was uniformly dense; and thence he determined its altitude to be little more than that of the highest mountains. But the true constitution of the density of the atmosphere, deduced afterwards from the Torricellian experiment, afforded a juster idea of these refractions, especially after it appeared, by a repetition of Mr. Lowthorp's experiment, that the refractive power of the air is proportional to its density. By this variation in the density and refractive power of the air, a ray of light, in passing through the atmosphere, is continually refracted at every point, and thereby made to describe a curve, and not a straight line, as it would have done were there no atmosphere, or were its density uniform.
The atmosphere, or air, has also a reflective power; and this power is the means by which objects are enlightened so uniformly on all sides. The want of this power would occasion a strange alteration in the appearance of things; the shadows of which would be so very dark, and their sides enlightened by the sun so very bright, that probably we could see no more of them than their bright halves; so that for a view of the other halves, we must turn them half round, or if immoveable, must wait till the sun could come round upon them. Such a pellucid unreflective atmosphere would indeed have been very commodious for astronomical observations on the course of the sun and planets among the fixed stars, visible by day as well as by night; but then such a sudden transition from darkness to light, and from light to darkuess, immediately upon the rising and setting of the sun, without any twilight, and even upon turning to or from the sun at noon day, would have been very inconvenient and offensive to our eyes. However, though the atmosphere | be greatly assistant in the illumination of objects, yet it must also be observed that it stops a great deal of light. By M. Bouguer's experiments, it seems that the light of the moon is often 2000 times weaker in the horizon, than at the altitude of 66 degrees; and that the proportion of her light at the altitudes of 66 and 19 degrees, is about 3 to 2; and the lights of the sun must bear the same proportion to each other at those heights; which Bouguer made choice of, as being the meridian heights of the sun, at the summer and winter solstices, in the latitude of Croisic in France. Smith's optics. Rem. 95.
For the Atmosphere of the sun, moon, and planets, see the respective articles.
Atmosphere of Solid or Consistent Bodies, is a kind of sphere formed by the effluvia, or minute corpuscles, emitted from them. Mr. Boyle endeavours to shew, that all bodies, even the hardest and most coherent, as gems, &c, have their atmospheres.
Atmosphere, in Electricity, denotes that medium which is conceived to be diffused over the surface of electrified bodies, and to some distance around them, and consisting of effluvia issuing from them; by which, other bodies immerged in it become endued with an electricity contrary to that of the body to which the atmosphere belongs. This was first noticed at a very early period in the history of this science by Otto Guericke, and afterwards by the academicians del Cimento, who contrived to render the electric atmosphere visible, by means of smoke attracted by a piece of amber, and gently rising from it, but vanishing as the amber cooled. Dr. Franklin exhibited this electric atmosphere with greater advantage, by dropping rosin on hot iron plates held under electrified bodies, from which the smoke arose and encompassed the bodies, giving them a very beautiful appearance. But the theory of electric atmospheres was not well explained and understood for a considerable time; and the investigation led to many curious experiments and observations. The experiments of Mr. Canton and Dr. Franklin prepared the way for the conclusion that was afterwards drawn from them by Mess. Wilcke and Epinus, though they retained the common opinion of electric atmospheres, and endeavoured to explain the phenomena by it. The conclusion was, that the electric fluid, when there is a redundancy of it in any body, repels the electric fluid in any other body, when they are brought within the sphere of each other's influence, and drives it into the remote parts of the body, or quite out of it, if there be any outlet for that purpose.
By Atmosphere, M. Epinus says, no more is to be understood than the sphere of action belonging to any body, or the neighbouring air electrified by it. Sig. Beccaria agrees in the same opinion, that electrified bodies have no other atmosphere than the electricity communicated to the neighbouring air, and which goes with the air, and not with the electrified bodies. Mr. Canton also, having relinquished the opinion that electrical atmospheres were composed of effluvia from excited or electrified bodies, maintained that they only result from an alteration in the state of the electric fluid contained in it, or belonging to the air surrounding these bodies to a certain distance; for instance, that excited glass repels the electric fluid from it, and consequently beyond that distance makes it more dense; whereas excited wax attracts the electric fluid existing in the air nearer to it, making it rarer than it was before. In the course of experiments that were performed on this occasion, Mess. Wilcke and Epinus succeeded in charging a plate of air, by suspending large boards of wood covered with tin, with the flat sides parallel to one another, and at some inches asunder: for they found, upon electrifying one of the boards positively, that the other was always negative; and a shock was produced by forming a communication between the upper and lower plates. Beccaria has largely considered the subject of electric atmospheres, in his Artificial Electricity, pa. 179 &c, Eng. edit. See also Dr Priestley's Hist. of Electricity, vol. ii. sect. 5. and Cavallo's Electricity, pa. 241.
Atmosphere, Magnetic, &c, is understood of the sphere within which the virtue of the magnet, &c, acts.
