each such condition a nebulous ring might be thrown off by a further contraction of the whole central mass; and each nebulous ring would revolve, in obedience to Kepler's law, by a physical necessity implied in that very balance of forces by which the several rings were produced. Again, as this hypothesis involves no cause to produce any change in the first axis of rotation, each ring must revolve in the same direction and in the same plane. On this simple hypothesis the whole revolving nebula might pass into a system composed of a large central body surrounded by nebulous rings-all revolving in one direction, in one equatorial plane, and in periodic times corresponding to Kepler's law.

But the same cause which produced the contraction of the original mass must also produce a contraction of the nebulous rings. In this way each of them may be supposed to break up and roll into a spherical mass of more condensed nebulous matter. This change would, however, produce no dynamical force to disturb the original planes of movement; so that these secondary revolving nebula would continue to move in the common equatorial plane, and might finally pass into the condition of planetary bodies. But, before they reached this final state, these secondary nebula might pass through the critical conditions above indicated, and so might throw off a succession of rings; and these rings might in like manner undergo condensation, break up, and finally pass into the state of satellites; revolving, as before, in one direction and in a common equatorial plane. Thus, through progressive natural changes, we should end with a system having a central body surrounded by planets, and the planets, it might be, surrounded by satellites or ringsall obeying Kepler's law, and all revolving in one direction and in a common equatorial plane.

We may further remark, that however attenuated may have been the revolving nebulous mass, its parts may have possessed some tenacity which may have interfered with

the mathematical consistency of the result. Again, as each ring was thrown off, the attraction of the central mass must have been, in a small degree, diminished. This of itself would produce a slight perturbation of the first nebular movements. Other irregularities would arise out of the deviations from a perfectly spherical form in the revolving bodies, and the mutual attractions of the rings must have produced mutual perturbations in their movements. But the limits of such perturbations can be defined by calculation, and could not throw the system into inextricable confusion.

The planes of planetery movement might therefore slightly deviate from the equatorial plane of the revolving nebula, out of which they had been formed; and the circular orbits would, by physical necessity, pass into elliptical orbits of small excentricity, in which the several bodies must continue to revolve in obedience to Kepler's law.

Thus we may suppose our system to have passed through all its transformations. The equator of the primæval nebula becomes the ecliptic plane of the system, with the sun in its center and the several planets and satellites revolving round the sun in the same direction, nearly in the same plane, in orbits that are nearly circular, and all moving in obedience to Kepler's law.

The oblate figure of the earth, its central heat, and the crystalline structure of the lower portions of its crust, seem to prove a former condition approaching that of igneous fusion. But we should far transgress the bounds of fair induction were we to affirm that the earth had been once in a nebulous condition; and still more, were we to assume that it had ever been in the condition of a nebulous ring, thrown off by a revolving mass of igneous vapour. I have just alluded (in this Discourse) to such an hypothesis; but only as a speculation, and nothing is built upon it. If, by strict dynamical reasoning upon the Nebular Hypothesis, we had reached consequences opposed to the phenomena of the solar system, we should have proved the hypothesis to be false. But because we can deduce a series of conse

quences from this hypothesis, which conform approximately to the phenomena of our system, we by no means prove it to be true. All we can dare to assert is, that the hypothesis lends itself readily to many phenomena of the solar system: but many an ingenious hypothesis, put forth with all the confidence of truth and long received with favour, has in the end proved worthless and untrue. We must ever bear in mind one of the three golden rules of Newton"Causas rerum naturalium non plures admitti deberi, quam quæ et veræ sint, et earum phænomenis explicandis sufficiant." The hypothesis we have been discussing rests only on an assumed analogy; and, should that fail, once out of the rules of sound philosophy.

passes at

So far I have stated the reasonings which seem to give support to the Nebular Hypothesis. Let me now state some considerations that may be urged against it.

(1). It is not brought before us as a sure induction from fact or observation; but rather belongs to a class of speculations, which, whether true or false, go beyond our material knowledge. Men have ever been willing to desert the beaten. track of natural science, and, instead of ascending step by step to the conception of laws by which separate phenomena are bound together, have dared to speculate about the beginning of things-to put themselves in the Creator's place, and to make a world after their own conceits. Such speculations have perhaps never done any good, and often do much mischief. Heat has a continual tendency to diffuse itself; yet the hypothesis commences by supposing its concentration in many large, but definite, portions of space. Whence came this concentration? It is not suggested by any conception we can rather in direct opposition to it. incandescent nebulæ have emanated from the fiat of Creative Will, and contained within themselves the germs which (when guided by the second causes ordained by God) produced in due time all the beauty and harmony of the natural universe. There may, perhaps, be no moral objec

form of heat, but seems But, it may be said, the

tion to such a view, provided it trench not on the phenomena of organic life; for it takes not from us our belief in Providence and final causes; and God, so far as our senses are concerned, appears only to work by second causes: but the whole speculation carries us out of the province of natural science, and is condemned by Bacon in some of the sentences quoted in the early part of this Note.

(2). The planets do not revolve in one plane. Can their different obliquities to the ecliptic plane be accounted for by the perturbations of the nebular rings? If not, we have a difficulty in the way of the hypothesis.

(3). The density of the planets does not conform to any law we might expect from the nebular hypothesis. This is an unexplained difficulty.

(4). The satellites of Uranus revolve not, like the other bodies, from west to east, but from east to west, and their orbits are inclined at great angles to the ecliptic plane. Here, then, we have a very great unexplained difficulty.

(5). Of all bodies in our system the comets are most nearly in a nebulous condition. They move in obedience to Kepler's law; but they cut through the ecliptic plane at various angles and in highly excentric orbits, which baffle all our conceptions of analogy with condensed rings thrown off by a revolving body.

(6). The formation of nebulous rings, on certain assumed conditions of condensation, has been explained by strict mechanical reasoning, and illustrated by direct experiment: but the formation of planetary nebulæ, by the breaking up of nebulous rings, has neither been explained by strict mechanical reasoning, nor illustrated by any direct experiment.

(7). Lastly, The original hypothesis of Sir W. Herschel has not been confirmed by the progress of discovery.

The apparently nebulous light of the "milky-way" is produced by confluent streams of light proceeding from innumerable telescopic stars. In like manner multitudes of smaller nebulæ have been resolved into luminous points.

They are only great clusters of stars at immeasurable distances from the eye. Hence arises a question suggested by Newton's second rule, "Effectuum naturalium ejusdem generis eædem sunt cause." May not all the apparent nebulae be formed by the confluent light of stars? If many of them have been "resolved," might not all of them become "resolved," had we telescopes of sufficient power? Should astronomers be enabled to give a positive answer to this question, there will be an end of the Nebular Hypothesis; for it will have no analogy to rest upon and, since the last edition of this Discourse was printed, the truth of such an answer has, to say the least of it, been made probable by the discoveries of Lord Rosse; who, by help of his gigantic Reflector, has succeeded in resolving into luminous points several of those remote masses of sidereal light, which had been appealed to as giving the surest evidence of a true nebulous condition.

However great the names connected with it, and however favourable its acceptance with the scientific world, the Nebular Hypothesis verges not, as has been asserted, "on the region of ascertained truth." It is at present nothing better than a splendid vision; and before it can be received as a physical reality, it must be supported by better evidence, and placed on a firmer foundation than that whereon it is at present made to rest.

NOTE (E), p. 31.

As I am anxious that this passage, and the one immediately following it on the same page, should not be misunderstood, I will endeavour, in the simplest words I can find, to explain my meaning.

The preceding Discourse offers to the reader no scheme of ideal perfection; but, on the contrary, gives a short comment on the physical, classical, and moral studies of our academic course; and against certain parts of them