ATMOSPHERE, Ἀτμόσφαιρα an Appendage of our Earth; consisting of a thin, fluid, elastic Substance, called Air, surrounding the Terraqueous Globe, to a considerable Height. See EARTH.

By atmosphere is usually understood the whole Mass or Assemblage of ambient Air: Though, among the more accurate Writers, the Atmosphere is restrained to that Part of the Air next the Earth, which receives Vapours and Exhalations; and is terminated by the Refraction of the Sun’s Light. See REFRACTION.



The further or higher Spaces, though perhaps not wholly destitute of Air, are supposed to be possessed by a finer Substance called Aether; and are hence denominated the Aetherial Region, or Space. See AETHER, HEAVEN, etc.

A late eminent Author considers the Atmosphere as a large Chemical Vessel, wherein the Matter of all the Kinds of sublunary Bodies is copiously floating; and thus exposed to the continual Action of that immense Furnace the Sun; whence innumerable Operations, Sublimations, Separations, Compositions, Digestions, Fermentations, Putrefactions, etc. See CHEMISTRY.

For the Nature, Constitution, Properties, Uses, Diversities, etc. of the Atmosphere, see the Article AIR.

We have a large Apparatus of Instruments, contrived for indicating and measuring the State and Alterations of the Atmosphere; as, Barometers, Thermometers, Hygrometers, Manometers, Anemometers, etc. see each under its proper Article, BAROMETER, THERMOMETER, HYGROMETER, etc. The Atmosphere insinuates itself into all the Vacuities of Bodies; and thus becomes the great Spring of most of the Mutations here below; as Generation, Corruption, Dissolution, etc.

See GENERATION, CORRUPTION, DISSOLUTION, etc. ‘Tis one of the great Discoveries of the modern Philosophers, that the several Motions attributed by the Ancients to a Fuga-vacui, are really owing to the Pressure of the Atmosphere.

See FUGA-VACUI, PUMP, PRESSURE, etc.

Weight of the ATMOSPHERE.

Organical Bodies are peculiarly affected by this Pressure: To this, Plants owe their Vegetation; and Animals their Respiration, Circulation, Nutrition, etc. See PLANT, ANIMAL, VEGETATION, CIRCULATION, etc. To this also we owe several considerable Alterations in the animal Oeconomy, with regard to Health, Life, Disease, etc. See HEALTH, etc. And hence, a Calculus of the precise Quantity of this Pressure, becomes a Point worthy of Attention.

Our Bodies, then, are equally pressed on by the incumbent Atmosphere; and the Weight they sustain is equal to a Cylinder of air, whose Base is equal to the Superficies of our Bodies. Now, a Cylinder of Air of the Height of the Atmosphere, is equal to a Cylinder of Water of the same Base, and 35 Foot high, or a Cylinder of Mercury, 29 Inches high; as appears from the Torricellian Experiments; as also from the Height to which Water ascends in Pumps, Syphons, etc. See TORRICELLIAN. See also PUMP, SYPHON, etc.

Hence it follows, that every Foot square of the Superficies of our Bodies, is pressed upon by a Weight of Air equal to 35 cubical Feet of Water; and a cubical Foot of Water, being found, by Experiment to weigh 76 Pound Troy Weight, therefore the Compas of a Foot square upon the Superficies of our Bodies, sustains a quantity of Air equal to 2660 Pound: For 76*35= 2660; and so many Foot square as the Superficies of our Body contains, so many times 2660 Pound does that Body bear.

Hence, if the Superficies of a Man’s Body, contain 15 square Feet; which is pretty near the Truth, he will sustain a Weight equal to 39900 Pound, for 2660*15=39900; which is above 13 Ton for the ordinary Load.

The Difference of the Weight of the Air which our Bodies sustain at one time more than at another, is also very great.—The whole Weight of Air which presses upon our Bodies when the Mercury is highest in the Barometer, is equal to 39900 Pounds. Whence, the Difference between the greatest and the least pressure of Air upon our Bodies, may be proved to be equal to 3982 Pounds.

The Difference of the Air's Weight, at different times, is measure by the different Height to which the eG is raised in the Barometer; and the greatest Variation of the Height of the Mercury being 3 Inches, a Column of Air of any assignable Base equal to the measure of a Cylinder of Mercury of the same Base, and of the Altitude of three Inches, will be taken off from the Pressure upon a Body of an equal Base, at such times as the Mercury is three Inches lower in the Barometer; so that every Inch square of the Surface of our Bodies, is pressed upon at one time more than another, by a Weight of Air equal to the Weight of 3 cubical Inches of Mercury.—Now a cubical Foot of Water being 76 Pound; a cubical Foot of Mercury must be 1064 Pound = 102144 Drams: And as 102144 Drams is to a cubical Foot, or which is all one, 1728 cubical Inches, so is 95 Drams, to one cubical Inch. So that a cubical Inch of Mercury being very near = 99 Drams; and there being 144 square Inches in a Foot square, therefore a Mass of Mercury of a Foot square, = 144 square Inches; and if three Inches high, must contain 432 cubical Inches of Mercury, which x 59 (the Number of Drams in a cubical Inch of Mercury) makes 25488 Drams.—And this Weight does a Foot square of the Surface of our Bodies, sustain at one time more than at another, Suppose again, the Superficies of a human Body = 15 Foot square; then would the Body sustain at one time more than at another, a Weight = 15 x 25488 Drams (= 3822 Ounces) = 3982+ Pound Troy. Hence, it is so far from being a wonder that we sometimes suffer in our Health, by a change of Weather; that it is the prearest, we don’t always do so.—For when we consider that our Bodies are sometimes pressed upon by near a Ton and a half Weight more than at other times, and that this Variation is often very sudden; ’tis surprising that every such Change does not entirely break the Frame of our Bodies to pieces. In effect, the Vessels of our Bodies being so much straightened by an increased Pressure, would stagnate the Blood up to the very Heart, and the Circulation would quite cease; if Nature had not wisely contrived, that when the Resistance to the circulating Blood is greatest, the Impetus, by which the Heart contracts, should be so too.—For, upon an increase of the Weight of the Air, the Lungs will be more forcibly expanded, and thereby the Blood more intimately broken and divided, so that it becomes fitter for the more fluid Secretions, such as that of the nervous Fluid; by which the Heart will be more strongly contracted. And the Blood’s Motion towards the Surface of the Body being obstructed, it will pass in greater quantity to the Brain, where the Pressure of the Air is taken off by the Cranium; upon which score also, more Spirits will be separated, and the Heart, on that account too, more enabled to carry on the Circulation, through all passable Canals, whilst some others, towards the Surface, are obstructed. See HEART, CIRCULATION, etc. The most considerable Alteration made in the Blood, upon the Air’s greater or lesser Pressure on the Surface of our Bodies, is its rendering the Blood more or less compact, and making it crowd into a less, or expand into a greater Space, in the Vessels it enters.—For the Air contained in the Blood, always keeps itself in Equilibrio with the external Air that presses upon our Bodies; and this it does by a constant Nifus to unbend itself, which is always proportionable to the compressing Weight by which it was bent: So that if the Compression or Weight of the circumambient Air be ever so little abated, the Air contained within the Blood unfolds its Spring, and forces the Blood to take up a larger Space than it did before. See BLOOD, HEART, CORPUSCLES, etc. The Reason we are not sensible of this Pressure, is well explained by Borelli, De motu animalium & Gravitate fac. prop. 29, etc.— After showing that Sand perfectly rammed in a hard Vessel is not capable by any means of being penetrated or parted, not even by a Wedge; and likewise that Water contained in a Bladder compressed equally on all Sides, cannot yield or give way in any Part: He proceeds, "In like manner, within the Skin of an Animal are contained a diversity of Parts, some hard, as Bones; others soft, as Muscles, Nerves, Membranes, etc.; others fluid, as Blood, Fat, etc. Now ’tis not possible the Bones should be broke or displaced in the Body, unless the Weight lay heavier on one Part than another, as we sometimes see in Porters. If the Pressure be subdivided, so that it lay equally all around, upwards, downwards, and sideways, and no Part of the Skin is exempt therefrom, it is evidently impossible any Fracture or Luxation should follow. The same may be observed of the Muscles and Nerves; which, though soft, yet being composed of solid Fibres, do mutually sustain each other, and resist the common Weight. The same holds of Blood, and the other Humours; and as Water does not admit any manifest Condensation, so the Animal Humours contained in their Vessels may suffer an Attrition from an impulse made in one or more particular Places; but can never be forced out of their Vessels by an universal Compression—It follows, that as none of the Parts undergo either Separation, Luxation, Contusion, or any other Change of Situation; ’tis impossible any Sense of Pain should ensue, which can only be the effect of a Solution of Continuity.” This is confirmed by what we see in Divers, etc.
See DIVING. The same is farther confirmed by Mr. Bayle; who including a young Frog in a Vessel half full of Water, and intruding so much Air as that the Water must sustain eight times the Weight it otherwise would; yet the Animalcule, notwithstanding the great tenderness of its Skin, did not seem to be at all affected thereby. For the Effects of the Removal of the Pressure of the Atmosphere, see Air-Pump. For the Cause of the Variations in the Weight and Pressure of the Atmosphere, see Barometer.

Height of the Atmosphere.

The Height of the Atmosphere is a Point about which the modern Naturalists have been very solicitous—Had not the Air an elastic Power, but were everywhere 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 being above observed that the Weight of the Column of Air reaching to the Top of the Atmosphere is equal to the Weight of the Mercury contained in the Barometer; and the Proportion of Weight likewise being known between equal bulks of Air and Mercury; it were easy to find the Height of such Column, and consequently of the Atmosphere itself—For a Column of Air one Inch high, being to an equal Column of Mercury, as 1 to 10800; 'tis evident that 10800 such Columns of Air, that is, a Column 900 Foot high, is equal in Weight to one Inch of Mercury: And consequently the 30 Inches of Mercury sustained in the Barometer, require a Column of Air 27000 Foot high, on which footing the Height of the Atmosphere would only be 27000 Foot, or little more than five English Miles high. See TORRICELLIAN.

But the Air, by its elastic Property, being liable to expand and contract; and it being found by repeated Experiments in England, France, and Italy, that the Spaces it takes up, when compressed by different Weights, are reciprocally proportional to those Weights themselves; or, that the Air takes up the less Space, the more it is pressed: It follows, that the Air in the upper Regions of the Atmosphere where the Weight is so much less, must be much rarer than nearer the Surface of the Earth; and consequently that the Height of the Atmosphere must be much higher than is above assigned.

If we suppose the Height of the whole Atmosphere divided into innumerable equal Parts; the Density of the Air in each of the said Parts being as its Quantity, and the Weight of the Atmosphere being also as the Quantity of the whole incumbent Air; 'tis evident the Weight of the incumbent Air is everywhere as the Quantity of Air contained in the subjacent Part, which makes a Difference between each two contiguous Parts of incumbent Air.—Now, we have a Theorem in Geometry, that where the Differences of Magnitudes are geometrically proportional to the Magnitudes themselves, those Magnitudes are in continual Arithmetical Proportion: Wherefore if, according to the Supposition, the Altitude of the Air by the continual Addition of the new Parts into which it is divided, do increase in a continual Arithmetical Proportion; its Density will be diminished, or which amounts to the same, its Gravity increased, in continual Geometrical Proportion.

From such a Series it is easy by making two or three Barometrical Observations of the Rarity of the Air at two or three different Heights; to find its Rarity at any other Height, or the Height corresponding to any Rarity; and consequently the Altitude of the whole Atmosphere, supposing the utmost Degree of Rarity known beyond which the Air cannot go. See the Articles Barometer, Series, Progression, etc. See also GREG. Astron. Phys. Geom. Log. 5. Prop. 3. and Halley in Philosoph. Transactions, No. 181. It must not be here omitted, that some Observations made by Cassini and his Associates, seem to render this Method precarious.—In continuing the Meridian Line of the Observatory at Paris, they measured the Altitudes of several Mountains with great accuracy; noting the Height of the Barometer at the Top of each; and found, that the Rarities of the Air as you ascend from the Level of the Earth, are much greater than they ought to be, according to this Proportion.

Suspecting, therefore, the justness of the Experiments, the Royal Academy made divers others, under great Dilatations of Air, far exceeding the Rarities found on the Tops of the Mountains; the Result whereof was, that they all exactly answered the Proportion of the incumbent Weights. Whence it should follow, that the higher Air about the Tops of Mountains, is of a different Nature, and observes a different Law from that near the Earth.

The Reason hereof may be owing to the great Quantity of gross Vapours and Exhalations here, more than there; which Vapours being less Elastic and not capable of so much Rarefaction as the pure Air above: The Rarefactions of the pure Air increase in a greater Ratio than the Weights diminish—M. Fontenelle, however, from some Experiments of M. de la Hire, accounts for the Phenomenon in a different Manner; alleging, that the elastic Power of Air is increased by the admixture of Humidity therewith; and consequently, that the Air near the Tops of Mountains, being moister than that below, becomes thereby more Elastic, and rarifies in a greater Ratio than naturally and in a drier State it would—But Dr. Jurin shows, that the Experiments produced to support this System, are by no means conclusive. Append. ad Varen. Geograph.

Be this as it will, the Rarities of the Air at different Heights proving not to bear any constant Proportion to the Weights wherewith they are pressed; Experiments made with Barometers at the Feet and Tops of Mountains, cannot give the Height of the Atmosphere, since our Observations are all made near the Earth; whereas the greater Part of the Atmosphere is far beyond; and the farther from us, the farther does it seem to recede from the Nature and Laws of ours.—M. de la Hire, therefore, after Kepler, has recourse to a more simple and secure Way of ascertaining the Height of the Atmosphere; viz. from the consideration of the Crepuscula. 'Tis allowed by Astronomers, that when the Sun is 18° below the Horizon, we begin or cease to see the Twilight: Now the Ray whereby we do it, can be no other than a horizontal Line, or a Tangent to the Earth in the Place where the Observer is. But this Ray cannot come directly from the Sun, which is under the Horizon, and must therefore be a Ray reflected to us by the last inner and concave Surface of the Atmosphere. We are to suppose that the Sun when 18° below the Horizon, emits a Ray which is a Tangent to the Earth, and strikes upon this last Surface of the Atmosphere, and is thence reflected to our Eye, being still a Tangent, and horizontal.—If there were no Atmosphere, there would be no Crepusculum; and consequently if the Atmosphere were not so high as it is, the Crepusculum would begin and end when the Sun is at a less Distance from the Horizon than 18°, And contrarily—Hence we gather, that the largeness of the Arch by which the Sun is depressed when the Crepusculum begins or ends, determines the Height of the Atmosphere. We are to note however, that 32 Minutes must be subtracted from the Arch of 18°, for the Refraction, which raises the Sun so much higher than he would be; and 16’ more for the Height of the upper Limb of the Sun, which is supposed to send the Ray, above his Centre, which is supposed to be 18° low. The remaining Arch, therefore, which determines the Height of the Atmosphere, is only 17° 12’. Two Rays, one direct, and the other reflected, but both Tangents to the Earth, must necessarily meet in the Atmosphere, at the Point of Reflection, and comprehend an Arch between them of 17° 12’, whereof they are Tangents—Hence it follows from the Nature of the Circle, that a Line drawn from the Centre of the Earth, and cutting the Arch in two, will go to the Point of Concurrence of those two Rays; and as it is easy finding the excess of this Line over the Semidiameter of the Earth, which is known; 'tis easy to find the Height of the Atmosphere, which is only that excess—On this Principle M. de la Hire discovers the Height of the Atmosphere to be 37223 Fathoms, or near 17 French Leagues. The same Method was made use of by Kepler, who only rejected it, because it gave the Height of the Atmosphere 20 times greater than he otherwise allowed it.

It must be added, that in this Calculus, the direct and reflected Rays are supposed to be right Lines; whereas in fact they are Curves, formed by the perpetual Refraction the Rays undergo in pressing through a Series of different Densities of Air—Computing then, upon them as two similar Curves; or rather as a single Curve, one extreme whereof is a Tangent to the Earth: Its Vertex equally distant from both Extremes, determines the Height of the Atmosphere; which therefore, will be found somewhat lower than in the former Case; the Point of Concurrence of two right Lines, which are here only Tangents to the Curve, the one at one end, and the other at the other; being higher than the Vertex of the Curve. On this footing, M. de la Hire finds the Atmosphere 35362 Fathoms, or 16 Leagues. Mem. de l’Acad. Roy. des Sciences, An. 1713. See the Articles Refraction, Crepusculum, etc.

Atmospheres of the Heavenly Bodies.

Lunar Atmosphere.—That the Moon is surrounded, like the Earth, with a changeable Atmosphere, see evinced under the Article Moon. The Reality of the Atmospheres of the other Planets, see also under the Article Planets.

For the Atmospheres of Comets, and the Sun, see Comet and Sun.—See also MACULAE; Tail, etc. Atmosphere of solid, or consistent Bodies, is a kind of Sphere formed by the Effluvia, or minute Corpuscles emitted from them. See SPHERE and EFFLUVIA. Mr. Boyle endeavors to show that all Bodies, even the hardest and most coherent, as Gems, etc., have their Atmospheres. See GEM.—See also MAGNET, MAGNETISM, etc.