view of the principal phenomena by which it is sustained; and thirdly, consider the most prominent of the objections which have been urged against it.

As a group, our solar system is comparatively isolated in space; the distance of the nearest fixed star being at least seven thousand times that of Neptune, the most remote known planet. Besides the central or controlling orb, it contains, so far as known at present, sixty-seven primary planets, twenty-one satellites, three planetary rings, and nearly eight hundred comets. In ta king the most cursory view of this system of bodies, we cannot fail to notice the following interesting facts in regard to the motions of its various members:

1. The sun rotates on his axis from west to east.

2. The primary planets all move nearly in the plane of the sun's equator.

3. The orbital motions of all the planets, primary and secondary, except the satellites of Uranus and perhaps those of Neptune, are in the same direction with the sun's rotation.

4. The direction of the rotary motions of all the planets, primary and secondary, in so far as has been observed, is identical with that of their orbital revolutions; viz., from west to east.

5. The rings of Saturn revolve about the planet in the same direction,

6. The planetary orbits are all nearly circular.

7. The cometary, is distinguished from the planetary portion of the system by several striking characteristics: the orbits of com ets are very eccentric and inclined to each other, and to the ecliptic at all possible angles. The motions of a large proportion of comets are from east to west. The physical constitution of the latter class of bodies is also very different from that of the former; the matter of which comets are composed being so exceedingly attenuated, at least in many instances, that fixed stars have been distinctly visible through what appeared to be the densest portion of their substance.

None of these facts are accounted for by the law of gravitation. The sun's attraction can have no influence whatever in determining either the direction of a planet's motion, or the eccentricity of its orbit. Iu other words, this power would sustain a planetary body moving from east to west, as well as from west to east; in an orbit having any possible degree of inclination to the plane of the sun's equator, no less than in one coincident with it; or, in a very eccentric ellipse, as well as in one differing but little from a circle. The consideration of the coincidences which we have enumerated led LaPlace to conclude that their expla nation must be referred to the mode of our system's formation— a conclusion which he regarded as strongly confirmed by the contemporary researches of Sir William Herschel. Of the numer

ous nebulæ discovered and described by that eminent observer, a large proportion could not, even by his powerful telescope, be resolved into stars. In regard to many of these, it was not doubted that glasses of superior power would show them to be extremely remote sidereal clusters. On the other hand, a considerable number were examined which gave no indications of resolvability. These were supposed to be a species of self-luminous, nebulous matter-the chaotic elements of future stars. The great number of these irresolvable nebulæ, scattered over the heavens and apparently indicating the various stages of central condensation, very naturally suggested the idea that the solar system, and perhaps every other system in the universe, originally existed in a similar state. The sun was supposed by Laplace to have been an exceedingly diffused, rotating nebula, of spherical or spheroidal form, extending beyond the orbit of the most distant planet; the planets as yet having no separate existence. This immense sphere of vapor, in consequence of the radiation of heat and the continued action of gravity, became gradually more dense, which condensation was necessarily attended by an increased angular velocity of rotation. At length a point was thus reached where the centrifugal force of the equatorial parts was equal to the central attraction. The condensation of the interior meanwhile continuing, this zone was detached, but necessarily continued to revolve around the central mass with the same velocity that it had at the epoch of its separation. If perfectly uniform throughout its entire circumference, which would be highly improbable, it would continue its motion in an unbroken ring, like that of Saturn; if not, it would probably collect into several masses, having orbits nearly identical. "These masses should assume a spheroidal form, with a rotary motion in the direction of that of their revolution, because their inferior particles have a less real velocity than the superior; they have therefore constituted so many planets in a state of vapor. But if one of them was sufficiently powerful, to unite successively by its attraction, all the others about its centre, the ring of vapors would be changed into one spheroidal mass, circulating about the sun, with a motion of rotation in the same direction with that of revolution."* Such, according to the theory of Laplace, is the history of the formation of the most remote planet of our system. That of every other, both primary and secondary, would be precisely similar.

If it be said that the small eccentricities of the planetary orbits, the approximate coincidence of their planes with that of the solar equator, and the uniformity of direction in which the planets move, are ultimate facts that the final cause of these arrange

* Harte's Translation of Laplace's System of the World, vol. ii, note vii.

ments was the stability of the system; and that we transgress the legitimate domain of scientific research in attempting their explanation ;—we ask by what rule in philosophy the arrangements in question are determined to be ultimate facts. Even granting their final cause to have been the stability of the system, we are by no means to conclude that they are nccessarily unsusceptible of explanation. "Final causes," says Whewell,* are to be excluded from physical inquiry; that is, we are not to assume that we know the object of the Creator's design, and put this assumed purpose in the place of a physical cause. We are not to think it a sufficient account of the clouds that they are for watering the earth-to take Bacon's examples-or 'that the solidness of the earth is for the station and mansion of living creatures.' The physical philosopher has it for his business to trace clouds to the laws of evaporation and condensation; to determine the magnitude and mode of action of the forces of cohesion and crystallization by which the materials of the earth are made solid and firm. This he does, making no use of the notion of final causes; and it is precisely because he has thus established his theories independently of any assumption of an end, that the end, when, after all, it returns upon him, and cannot be evaded, becomes an irresistible evidence of an Intelligent Legislator. He finds that the effects, of which the use is obvious, are produced by most simple and comprehensive laws; and when he has obtained this view, he is struck by the beauty of the means, by the refined and skillful manner in which the useful effects are brought about;-points different from those to which his researches were directed."

As the question, then, to which the cosmogony of Laplace proposes a solution, is a legitimate one, we shall proceed to consider some of the evidences by which the theory is supported.

1. The nebular hypothesis furnishes a very simple explanation of the motions and arrangements of the planetary system. In the first place, it is evident that the separation of a ring would take place at the equator of the revolving mass, where of course the centrifugal force would be greatest. These concentric rings—and consequently the resulting planets-would all revolve in nearly the same plane. It is evident also that the central orb must have a revolution on its axis in the same direction with the progressive motions of the planets. Again: at the breaking up of a ring, the particles of nebulous matter more distant from the sun would have a greater absolute velocity than those nearer to it, which would produce the observed unity of direction in the rotary and orbital revolutions. The motions of the satellites are explained in like manner. The hypothesis, moreover, accounts satisfactorily for the fact that the orbits of the planets are all nearly circular.

* Bridgewater Treatise, vol. ii, p. 180.

And finally, it presents an obvious explanation of the rings of Saturn. It would almost seem, indeed, as if these wonderful annuli had been left by the Architect of Nature, as an index to the

creative process.

The argument derived from the motions of the various members of the solar system is not new, having been forcibly stated by Laplace, Pontécoulant, Nichol, and other astronomers. Its full weight and importance, however, have not, we think, been duly appreciated. That a common physical cause has determined these motions, must be admitted by every philosophic mind. But apart from the nebular hypothesis, no such cause, adequate both in mode and measure, has ever been suggested;-indeed none, it seems to us, is conceivable. The phenomena which we have enumerated demand an explanation, and this demand is met by the nebular hypothesis. It will be found, therefore, when closely examined, that the evidence afforded by the celestial motions is sufficient to give the theory of Laplace a very high degree of probability.

2. The fact that this theory of the genesis of the solar system may be extended to the binary and multiple systems among the so-called fixed stars, may be urged as no inconsiderable evidence in its favor. Unity, no less than variety, is characteristic of nature's works. All the diversified and apparently disconnected phenomena of the universe have their roots in a few general laws. Whatever, therefore, leads us higher in the process of generalization may be presumed to have some foundation in truth.

3. Numerous geological facts appear to harmonize with the hypothesis under consideration.Fossil organic remains, and their absence in the earlier rocks, both indicate that the temperature of the earth's surface was formerly much higher than at present, and that the decrease was slow and gradual. In places, for instance, where ferns do not now exceed three feet in height, those of former periods are found to have been from forty to fifty, or even seventy feet high. Now in regard to existing plants, it is well-known that their number, as well as their size and luxuriance of growth, gradually increase as we advance from high latitudes towards the equator. In both these respects a similar increase is observable in descending through the successive strata of the ancient world, until we reach the oldest Secondary rocks. These facts are doubtless to be referred to the same cause, namely, a gradual change of temperature. A comparison of fossil animals with those now existing, leads to a similar result. This high temperature of the earth's surface during the earlier stages of its physical history, is attributed by most geologists to a central heat,* which diffused itself through

* A different theory in regard to the ancient high temperature of the earth has been developed by the celebrated Poisson. Starting with the fact-established be

out the entire mass. That the interior of our planet is in a state of igneous fluidity, or has, at least, an extremely high temperature, is now very generally admitted. As we descend from the surface we find a regular increase of heat, varying indeed for different localities, partly no doubt, on account of the different conducting powers of the rocks which constitute the crust, but averaging about one degree for every fifty feet.* This has been ascertained from a great variety of experiments upon rocks and springs in mines, and upon the water issuing from Artesian wells. It is true, the depth to which man has been able to penetrate, is comparatively small; but assuming the same rate of increase to continue, it has been calculated that the point at which all known rocks would be in a state of fusion must be considerably less than one hundred miles from the surface, and possibly less than fifty. Reasons are not wanting for regarding this molten mass as the seat of present volcanic activity,† as well as the source of that great upheaving power by which not only mountains, but islands and continents have been elevated.

Geologists are generally agreed that there is conclusive evidence of the primitive igneous fluidity of the unstratified rocks of the earth's crust. Indeed it can scarcely be doubted that the solidification of the outer portions of our globe was a mere cooling by radiation from the surface. "There is no small reason," says Professor Hitchcock, "to suppose that the globe underwent numerous changes previous to the time when animals were placed upon it; that, in fact, the time was when the whole matter of

yond doubt by modern observations-that the solar system has a progressive motion in space, he supposes that in the sweep of its mighty orbit it has passed through regions of very different temperatures, and that the heat of former periods-the residuum of which is still found at great depths beneath the surface-was received ab extra while in a portion of space much richer in stars, and having therefore a higher temperature, than that through which it is at present passing. "The physical doubts which have reasonably been entertained against this extraordinary cosmical view, (which attributes to the regions of space that which probably is more dependent on the first transition of matter condensing from the gaseo-fluid into the solid state,) will be found collected in Poggendorff's Annalen, bd. xxxix, S. 93–100.” Humboldt's Cosmos, vol. i. p. 165.

* See Proceedings of the American Association for the Advancement of Science, Tenth Meeting, Part II. p. 102.

+ "The similarity of lava, wherever found, and the close agreement as to composition and physical characters of the basalt of ancient epochs and of that still bursting through and intersecting the walls of modern volcanoes, are proofs that all such eruptions have a common origin, and are due, as well as the accompanying physical phenomena of earthquakes, to forces acting on the still liquid portion of the earth." Portlock's Rudimentary Geology, p. 70.

The researches of Hopkins and Hennessy have led to different results in regard to the thickness of the earth's crust. See Trans. of the Roy. Soc. for 1839, p. 381; for 1840, p. 193; for 1342, p. 43; for 1851, Part II, p. 495. Also, this Journal for March, 1852, p. 271, and for May, 1853, p. 126. For "some suggestions in Explanation of the primitive Incandescent condition of the Earth and the other Planets," se Monthly Notices of the Roy. Astro. Soc. for January 13th, 1854.