Meteor ology.

Observa

this limit we are left to conjectures founded on the supposed divisibility of Matter; and if this be infinite, so also must be the extent of our atmosphere. For, if the density be throughout as the compressing force, then must a stratum of given thickness, at every height, be compressed by a superincumbent atmosphere, bearing a constant ratio to its own weight, whatever be its distance from the Earth. But if air consist of any ultimate particles no longer divisible, then must expansion of the medium composed of them cease at that distance, where the force of gravity downwards upon a single particle is equal to the resistance arising from the repulsive force

of the medium.

(94.) Mr. Faraday, when alluding to the admirable tions of Mr. argument of Dr. Wollaston, observes, that on passing Faraday. upwards from the Earth's surface, the air becomes more and more attenuated, in consequence of the gradually diminishing pressure of the superincumbent part, and its tension or elasticity is proportionally diminished; and when the diminution is such, that the elasticity is a force not more powerful than the attraction of gravity, a limit to the atmosphere must occur. The particles of the atmosphere there tend to separate with a certain force; but this force is not greater than the attraction of gravity, which tends to make them approach the Earth and each other; and as expansion would necessarily give rise to diminished tension, the force of gravity would then be the strongest, and, consequently, would cause contraction, until the powers were balanced as before.

Views of

Dr. Wollaston extended to the atmo

other

bodies.

(95.) Dr. Wollaston, however, has not confined his inquiries to the Earth's atmosphere; and, in extending his views to the probable existence of atmospheres round other bodies, he properly remarks, that since the spheres of law of definite proportions discovered by Chemists is the same for all kinds of Matter, whether solid, fluid, or elastic, if it can be ascertained that any one body consists of particles no longer divisible, we then can scarcely doubt that all other bodies are similarly constituted; and that we must conclude, without hesitation, that those equivalent quantities, which we have learned to appreciate by proportional numbers, do really express the relative weights of elementary atoms, the ultimate objects of Chemical research.

Absurdity ing to dis

of attempt

cover an

of infinite divisibility around the Moon,

(96.) In the first place, the views entertained by those, who, believing in the existence of a terrestrial atmosphere of indefinite extent, sought to discover the atmosphere existence of an atmosphere of a similar kind round the lunar spheroid, may, from the reasoning of Dr. Wollaston, be proved to be utterly fallacious. For, since the density of an atmosphere of infinite divisibility at the surface of the Moon would entirely depend on her gravitating force at that point, that density could not be greater than the density of our atmosphere at the point where the Earth's attraction is equivalent to the Moon's attraction at her surface. At this height, which by a simple computation is about 5000 miles above the Earth's surface, we obviously can have no perceptible atmosphere, and, consequently, should not expect to discern an atmosphere of similar rarity around the Moon. (97.) In the next place, the approach of Venus to the Sun proving Sun, if the latter body were surrounded by an atmothat no at- sphere of infinite divisibility, might be reasonably exmosphere pected to present some degree of retardation in her of infinite divisibility apparent motion; whereas, from the very precise and accurate observations of Captain Kater, no such retardasurrounds the latter. tion can be perceived. If we calculate at what apparent

Approach of

Venus to the

distance from the body of the Sun his attractive energy is equal to the gravitating influence at the surface of the Earth, it is there that his power would be sufficient to accumulate (from an infinitely divisible medium filling all space) an atmosphere* fully equal in density to our own, and, consequently, capable of producing a refraction of more than one degree in the passage of rays obliquely through it.

(98.) By considering the mass of the Sun as 330,000 times greater than that of the Earth, the distance at which his gravitating force will be equivalent to the force of gravity at the surface of the Earth, will be 330,000,

or, in other words, about 575 times the terrestrial radius; and if the radius of the Sun be 111.5 times that of the Earth, then will the distance here referred 575 to be or 5.15 times the solar radius. Now the 111.5' Sun's apparent semi-diameter on the day of observation (May 23d) having been 15'49", it follows that 15' 49" x 5.151° 21′ 29" was the distance from the Sun's centre, at which his gravitating force was just equivalent to the ordinary force of gravitation at the Earth's surface.

Meteorology.

the atmo

(99.) But the approach of Jupiter's Satellites to that Strong conPlanet, instead of being retarded by refraction, is well firmation known to be perfectly uniform, till they appear in afforded by the apactual contact; showing that there is not that extent proach of of atmosphere surrounding Jupiter, which that body Jupiter's should attract to itself from an infinitely divisible Satellites to medium filling all space. For since the mass of Jupiter, that Jupiter is full 309 times that of the Earth, the dissphere of tance at which his attraction would become equivalent that planet to the ordinary terrestrial gravity, must be as 309, is not one or about 17.6 times the Earth's radius. And since his of infinite divisibility. diameter is nearly eleven times that of the Earth, we 17.6 shall have 1.6 times his own radius, for the dis11 tance from his centre, at which an atmosphere, equal in density to our own, should occasion a refraction exceeding one degree. To the fourth Satellite, this distance would subtend an angle of about 3° 37'; so that an increase of density equivalent to 3 times the density of our ordinary atmosphere, would be more than sufficient to render the fourth Satellite visible to us when behind the centre of the Planet, and, consequently, to make it appear on all sides of the Planet at the same time, or rather as a luminous ring surrounding the entire disc of the Planet.

(100.) Now, though with regard to the argument respecting the Solar atmosphere, some degree of doubt may be entertained in consequence of the possible effects of heat which cannot be appreciated, it is evident that no error from this source can be apprehended in regard to Jupiter; and as this Planet has certainly not its due share of an infinitely divisible atmosphere, the universal prevalence of such a medium cannot be maintained; while, on the contrary, all the phenomena accord entirely with the supposition that the Earth's atmosphere is of finite extent, limited by the weight of conclusion

General

(101.) Mr. Ivory, in a Paper in the Philosophical Transactions for 1823, on Astronomical Refractions, in treating of this highly interesting question, conceives a cylinder of air to extend indefinitely in a vertical direction, and to be divided into equal parts of a moderate length, so that the density of every division may be considered as uniform; and by abstracting the diminution of Gravity and the increase of the centrifugal force, which are inconsiderable at the distance of 200 or 300 miles from the Earth's surface, the weight of air in every portion of the cylinder will be proportional to its density. If now, continues Mr. Ivory, we admit the elastic force to be proportional to the density, as it would be in an atmosphere of uniform temperature, it will follow, that the weights of the several divisions of the cylinder will vary in the same proportion as their elasticities. But in the lowest part of the cylinder, the weight of the small quantity of air contained in one division, is incomparably less than its elastic force, which is an equipoise to the whole atmosphere; and the same thing will therefore be true of every portion of the cylinder, however high it is placed. Hence an atmosphere constituted as we have supposed, must necessarily be infinite in its extent. For if it were finite, since there is no pressure at the surface, the weight of a volume of air situated there would be in equilibrium with its elastic force, whereas it has been proved that the former is always an inconsiderable part of the

latter.

(102.) But in the foregoing reasoning, says Mr. Ivory, a cause has been neglected which diminishes the elasticity of the air as we ascend above the Earth's surface, without affecting the force of Gravity in any degree. In the higher parts of the atmosphere a continually increasing degree of cold is found to prevail, the effect of which is to contract all bodies in their dimensions; and therefore, by the operation of this cause, as we ascend in the atmosphere, the expansive force of a given volume of air is constantly diminished and brought nearer to an equality with its weight. To estimate this effect with greater precision, let p', z', t, denote the Barometric pressure, the density, and the temperature by the centigrade Thermometer at the Earth's surface; and let the same letters, without the accent, denote corresponding elements at any height

be set to its approach to zero. But when

1 + β t 1+ Bť

is evanescent, or when t=266°, the elastic force of the air will cease, and gravity will stop the further dilatation of the atmosphere. This reasoning, observes Mr. Ivory, is independent of the law of the densities; and it proves both that the atmosphere may be finite in its extent, and that it may have a finite density at its upper surface.

(103.) But it may be objected, that the effect of temperature on the air's elasticity has been verified only to a certain extent; and that in the case of air of a great rarity, and subjected to extreme degrees of cold, the law of dilatation and contraction may be very different from what it has been proved to be in the limited range of our experiments. This observation is probably well founded, but it will not destroy the force of what has been advanced. We know that air always gives out heat when it is compressed into a less volume, and absorbs heat when it expands. As long, therefore, as that fluid retains its elasticity, so long, we must conclude, will temperature continue to modify the changes of bulk which that force produces. The law of dilatation and contraction may, no doubt, undergo some change in different circumstances, but every expansion must be productive of cold, and every new degree of cold must diminish the elastic force of a given volume of air. Gravity continuing to act with nearly the same energy, while the elastic force of the air is continually diminished, these two forces will at length become equivalent, and will counterbalance one another, which is all that is necessary for imposing a limit to the extent of the atmosphere. We have proved, says Mr. Ivory, that air, if it were confined by the action of Gravity alone, would extend indefinitely into space; and it is not unreasonable to consider the effect of temperature as a contrivance for accurately attaching to the terrestrial globe a fluid so necessary in every point of view to the economy of nature.

(104.) Since it is found, continues Mr. Ivory, that all elastic fluids follow the same laws in regard to heat and pressure, the foregoing reasoning will be found equally true, whether we conceive the atmosphere as composed of one homogeneous fluid, or as a collection of many elastic gases and vapours, however much they may differ from one another in Specific Gravity.

of the

(105.) It may even be possible, adds Mr. Ivory, to His conform some reasonable conjecture as to the actual height jecture of the finite atmosphere. Gay Lussac ascended in a respecting balloon to the altitude of 3816 English fathoms, or the altitude nearly four miles and a quarter above the level of the Earth's at Seine at Paris; the proportion of the heights of the mosphere. Barometer in the balloon and at the surface of the Earth being 0.467 nearly, which is therefore the relative elasticity of the air. The temperatures, as observed at the extremities of the elevation, were 30°.8, and -9°.5 on the centigrade scale; and if we increase 0.467 to what it would have been, had the temperature remained unchanged during the ascent, we shall find 0.500 for the density of the air at the height ascended, in parts of the density at the surface of the Earth. Thus, in the decreasing scale of elasticities, the diminution is from 1 to 0.467; but in the decreasing scale of densities, it is only from 1 to 0.500.

Meteor- The quantities of the one scale continually fall behind ology. those of the other at a rate that must bring them to zero, whatever be the gradation of the latter. If we divide 3816 fathoms, the whole height ascended, by 40°.3 the difference of temperature, the elevation for depressing the Thermometer one degree will come out equal to 95 fathoms; and if we suppose that the same rate prevails in all parts of the atmosphere, the whole height will be 266 × 95 fathoms, or nearly 29 miles. The observations of the twilight show that this is less than the true altitude; and hence we must infer, that the Thermometer falls at a slower rate in the higher, than in the lower parts of the atmosphere. But, taking the observed rate of 95 fathoms for the first 40 degrees, and allowing, on an average, a double, or even a triple, elevation for the remaining 226°, we shall still find that the atmosphere will extend only to a moderate height above the Earth's surface.

Mr. Dalton's views respecting the finite

(106.) Mr. Dalton, in a Paper on the constitution of the atmosphere, published in the Philosophical Transactions for 1826, and before alluded to, extent of the has also referred to its finite extent. He commences atmosphere. his observations by referring to the 23d Proposition of the IId Book of the Principia, wherein Newton demonstrates, that if homogeneous particles of Matter were endued with a power of repulsion in the inverse ratio of their central distances, that, collectively, they would form an elastic fluid agreeing with atmospheric air in its mechanical properties. Newton does not, however, infer from this demonstration that elastic fluids must necessarily consist of such particles; and his argument requires that the repulsive power of each particle terminates, or very nearly so, in the adjacent particles.

Principle on which he founds a

(107.) From the Scholium to this Proposition, Newton was evidently aware of the difficulty of conceiving how the repulsive action of such particles could terminate so abruptly as his supposition demands; but in order to show that such cases exist in nature, he finds a parallel one in Magnetism,

(108.) Following up his reasoning Mr. Dalton observes, that on the hypothesis of the density of any limit to the atmosphere diminishing in Geometrical progression atmosphere. to intervals of ascent in Arithmetical progression, every atmosphere must be unlimited, or of infinite extent. But if an atmosphere is constituted of particles on the Newtonian hypothesis, it must have a limit; and which limit will exist where the repulsion of two particles becomes equal to the weight of one of them.

Comparison

of the relative heights of two atmospheres.

(109.) We have no data, continues Mr. Dalton, from which to determine the absolute height above the surface of the Earth to which any one atmosphere can ascend; but we can form a pretty accurate comparison of the relative heights to which two atmospheres would ascend, especially if the relative weights of their atoms be known.

(110.) For instance, says he, we know that the diameter of an elastic particle of carbonic acid is nearly, or exactly, the same as that of a particle of hydrogen under the same pressure; also that their weights are as 20 to 1. At two miles' elevation, the elasticity of an atmosphere of carbonic acid gas is diminished one half; and at 40 miles' elevation, that of hydrogen is diminished one half. Now let it be supposed that at 30 miles' elevation the carbonic acid atmosphere ceases to exist, or terminates, at which elevation its elasticity must be, according to the Geometrical progression,

of 15 x 40 = 600 miles; also the diameters of the particles of the two gases are still equal at those elevations, because they vary as the cube roots of the elasticities inversely; that is, if the diameters of the particles of carbonic acid and hydrogen at the surface of the Earth be denoted by 1, that of carbonic acid at 30 miles will be represented by 3/33000, and that of hydrogen at 600 miles' elevation will also be /33000. But by the hypothesis, this distance is capable of supporting a weight as 20, (namely, the weight of one atom of carbonic acid ;) the hydrogen atmosphere, therefore, must be further elevated, till it is capable of supporting a weight only as 1, (namely, the weight of an atom of hydrogen;) this will take place when the elasticity is still further diminished in the ratio of 203 to 13, or 8000 to 1. Hence, we shall have to extend the atmosphere about 13 × 40 = 520 miles further before it can terminate, or the height of 1120 miles. In this estimate we have not taken into consideration the variable force of Gravity. At the height of 1400 miles the force of Gravity is reduced one half, nearly; on this account the elevation of the hydrogen atmosphere will be increased between one and two hundred miles more, so as to make it amount to twelve or thirteen hundred miles. The variation of temperature in ascending does not materially affect Mr. Dalton's views.

(111.) Thus it appears, that, upon the assumption made by the Manchester Philosopher, the hydrogen atmosphere must be 40 times the altitude of the carbonic acid atmosphere. If he had assumed the utmost height of the carbonic acid atmosphere less than 30 miles, the disproportion of the two heights would have been still greater; and if more than 30 miles, it would have been less; but, in this case, the absolute difference would be greater.

(112.) By applying these principles to the terrestrial atmosphere, composed as it is of azote, oxygen, carbonic acid, and aqueous vapour, and fixing the limit of altitude in a full atmosphere (of 30 inches of mercury) of oxygen gas at 45 miles, Mr. Dalton finds that of an atmosphere of the same gas of 6.3 inches of mercury will be found to be about 38 miles, the atom of oxygen being 7; and that of azotic gas of 23.7 inches weight will be found 54 miles, if the atom of azote be taken as 5; but if the atom of azote be double this weight, as is supposed by many, but Mr. Dalton thinks without sufficient reason, then the height of the azotic atmosphere will be only 44 miles. The very fine and attenuated carbonic acid atmosphere must ascend to the height of 10 miles, if a full atmosphere of this gas ascend to 30 miles; and that of aqueous vapour to the height of 50 miles, allowing the Specific Gravity of steam to be .625, and the weight of its atom 8.

Meteor ology.

(113.) Mr. Luke Howard, in alluding to this in- Views of teresting subject, remarks, that the surface of the Mr. Luke atmosphere is less elevated, and better defined, than Howard. many imagine. A portion of air, says he, rarefied by means of the air-pump, does indeed exhibit an elasticity, which seems limited only by the imperfection of the instrument. For the most minute residuum still