any knowledge, and we must refer their structure at once to the fiat of Omnipotence.

Granting the formation of a single sun by the nebular theory, and we account at once for the formation of all other suns and systems throughout all space, and according to the advocates of this theory, the comets have their origin in masses of nebulous matter occupying positions intermediate between two or more great centres, and held nearly in equilibrio, until, finally, the attraction of some one centre predominates, and this uncondensed filmy mass commences slowly to descend towards its controlling orb This theory would seem to be sustained, so far as a single truth can sustain any theory, by the fact that the comets come into our system from all possible directions, and pursue their courses around the sun either in accordance with, or opposed to, the direction in which the planets circulate. Their uncondensed or nebulous condition results from the feeble central attraction which must necessarily exist in bodies composed of such small quantities of matter. Moreover, in some cases at least, there is reason to believe, that in their perihelion passage they are entirely dissipated into vapor by the power of the sun's heat, and may thus revolve for ages, going through alternate changes of solidification and evaporation.

But whence come the enormous trains of light which sometimes attend these wandering bodies ?--The last return of Halley's comet has furnished the data for the positive illustration of this mysterious subject. Sir John Herschel, after a careful and most elaborate examination of all the physical characteristics of this comet, comes to the conclusion that the

figure of the comet, envelope and tail, could not be a figure of equilibrium under the law of gravitation. He is therefore compelled to bring in a repulsive force to explain the phenomena.

I cannot do better than to quote his own language in this bold introduction of a new power.-" Nor let any one be startled at the assumption of such a repulsive force as here supposed. Let it be borne in mind that we are dealing (in the tails of comets) with phenomena utterly incompatible with our ordinary notions of gravitating matter. If they be material in that ordinary received sense which assigns to them only inertia and attractive gravitation, where, I would ask, is the force which can carry them round in the perihelion passage of the nucleus, in a direction pointing continually from the sun, in the manner of a rigid rod, swept round by some strong directive power, and in contravention to all the laws of planetary motion, which would require a slower angular motion of the more remote particles, such as no attraction to the nucleus could give them, be it ever so intense? The tail of the comet of 1680, in five days after its perihelion passage, extended far beyond the earth's orbit, having, in that brief interval, shifted its angular position nearly 150°. Where can we find, in its gravitation either to the sun, or to the nucleus, any cause for this extravagant sweep?

"But again, where are we to look (if only gravity be admitted) for any reasonable account of its projection outward from the sun, putting its angular motion out of the question? Newton calculated that the matter composing its upper extremity quitted the nu

cleus only two days previous to its arriving at this enormous distance."

Herschel argues the inadequacy of gravitation to account for these wonderful phenomena. The velocity with which the matter composing the tail shot forth from the head of the comet, from the sun, was far greater than that which the sun could impress on a body falling to it, even from an infinite distance.An energy of a different kind from gravitation, and far more powerful, must exist, to produce such results. If, then, we are forced to the admission that a power exists in the sun capable of repelling matter of a certain quality existing in comets, a way is opened for the explanation of some of the most difficult problems with which the mind has been obliged to contend.

The diminution of the periodic time of Encke's comet has led some astronomers to adopt the idea of the existence of a resisting medium. But in case the sun possesses the power of repelling the matter of comets in their perihelion passage, a part of the matter thus repelled may be driven entirely beyond the attractive influence of the nucleus, and be irrecoverably lost. In this case, a diminution of mass would inevitably involve a like diminution of periodic time, a contraction of the orbit, and all the phenomena presented by this mysterious object. Herschel even thinks it possible, on this theory, to account for the separation of Biela's comet into two distinct objects, and it appears to me that it presents the most reasonable explanation of the luminous appearance seen at certain seasons of the year, called the zodiacal light.— This phenomenon appears to be a ring of nebulous

matter surrounding the sun, and some of whose par ticles are sustained at a much greater distance than could be accounted for by gravitation. Admitting the repulsive power already adverted to, there is no difficulty in understanding how this nebulous ring may be sustained at a vast distance from the sun.

Here we freely admit that we enter the confines of the unknown. We have left the solid ground of truth and certainty, and are pushing our investigations into the dim twilight of the invisible and uncertain. But as antiquity predicted that the time would come when the comets would be traced in their career, their periods revealed, and their orbits ascertained, so we may confidently hope that, at no very distant day, all the mysteries which hang around these chaotic worlds will be fully revealed, and a knowledge of their physical condition shall reward the long study and deep research of the human mind.

LECTURE IX.

THE SCALE ON WHICH THE UNIVERSE IS BUILT.

THUS far our attention has been directed to an examination of the achievements of the human mind within the limits of our own peculiar system. We have swept outward from the sun through the planetary worlds, until we have reached the frontier limits of this mighty family. Standing upon the latest found of all the planets, at a distance of more than 3,000,000,000 of miles from the sun, we are able to look backwards, and examine the worlds and systems which are all embraced within the vast circumference of Neptune's orbit. An occasional comet, overleaping this mighty boundary, and flying swiftly past us, buries itself in the great abyss of space, to return after its "long journey of a thousand years," and report to the inhabitants of earth the influences which have swayed its movements in the invisible regions whither it speeds its flight.

The magnificence and complexity of the great system of planets and satellites and comets which constitute the sun's retinue; the immense magnitude of some of these globes; their periods of revolution, and reciprocal action, would seem to furnish a sufficient exercise, not only for the highest intellectual efforts, but for the entire energy which the human mind can exert. But the whole of this stupendous scheme, as

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