of Jupiter and the sun. And we may infer that the other parts of nebula contain smaller aggregations not perceptible to us, out of which the smaller planets of the developing system are hereafter to be formed.

Side views of some of these nebula indicate a flatness of figure agreeing well with the general tendency of the members of the solar system towards the medial plane of that system. For the solar system may be described as flat, and if the nebula I have been dealing with (the spiral nebula with aggregations, were globular we could not recognise in them the true analogues of our solut system in the earlier stages of its history, But, thes telescope reveals nebula malaly umeyading in appearance to the great whirlpool weis & Song Vina, as i would appear it is a wealth open wate could be viewed weaty sig'

And here I may jasema, fad, scenes, a ve inferring pagressive danga vs but andage has off in 10** dirfing m Sagan si me

we need not at present attempt to pass back beyond the earliest stage of which we have any clear information.

Passing from the irregular nebulæ, in which we see chaotic masses of gaseous matter occupying millions of millions of cubic miles and scattered as wildly through space as clouds are scattered in a storm-swept air, we come to various orders of nebulæ in which we seem to find clear evidence of a process of evolution. We see first the traces of a central aggregation. This aggregation becomes more and more clearly defined, until there is no possibility of mistaking its nature as a centre having power (by virtue of the quantity of matter contained in it) to influence the motions of the matter belonging to the rest of the nebula. Then, still passing be it remembered from nebula to nebula, and only inferring, not actually witnessing, the changes described,-we see a subordinate aggregation, wherein, after a while, the greater portion of the mass of the nebula outside the central aggregation becomes gathered, even as Jupiter contains the greater portion of the mass of the solar system outside the central sun.* Next we see a second subordinate aggregation, inferior to the first, but comprising, if we judge from its appearance, by far the greater portion of what remained after the first aggregation had been formed,-even as Saturn's mass far exceeds the combined mass of all the planets less than himself, and so comprises far the greater portion of the solar system after account has been taken

* The mass of Jupiter exceeds, in the proportion of five to two, the

of Jupiter and the sun.* And we may infer that the other parts of nebulæ contain smaller aggregations not perceptible to us, out of which the smaller planets of the developing system are hereafter to be formed.

Side views of some of these nebulæ indicate a flatness of figure agreeing well with the general tendency of the members of the solar system towards the medial plane of that system. For the solar system may be described as flat, and if the nebulæ I have been dealing with (the spiral nebula with aggregations) were globular we could not recognise in them the true analogues of our solar system in the earlier stages of its history. But the telescope reveals nebulæ manifestly corresponding in appearance to the great whirlpool nebula of Lord Rosse, as it would appear if it is a somewhat flattened spiral and could be viewed nearly edgewise.

And here I may pause to note that, although, in thus inferring progressive changes where in reality we have but various forms of nebulæ, I have been adopting an assumption and one which no one can hope either to verify or to disprove, yet it must be remembered that these nebulæ by their very figure indicate that they are not at rest. If they consist of matter possessing the attribute of gravitation,—and it would be infinitely more daring to assert that they do not than that they do, then they must be undergoing processes of change. Nor can we conceive that discrete gaseous masses in whorls spirally

* The mass of Saturn exceeds, in the proportion of nearly three to one, the combined mass of all the planets smaller than himself.

arranged around a great central aggregation (taking one of the earlier stages) could otherwise change than by aggregating towards their centre, unless we admit motions of revolution (in orbits more or less eccentric) the continuance of which would necessarily lead, through collisions, to the rapid growth of the central aggregation, and to the formation and slower growth of subordinate gatherings.

I have shown elsewhere how the formation of our solar system, in the manner supposed, would explain what Laplace admitted that he could not explain by his theory, -the peculiar arrangement of the masses forming the solar system. The laws of dynamics tell us, that no matter what the original configuration or motion of the masses, probably gaseous, forming the nebula, the motions of these masses would have greater and greater velocity the nearer the masses were to the central aggregation, each distance indicating certain limits between which the velocities must inevitably lie. For example, in our solar system, supposing the central sun had already attained very nearly his full growth as respects quantity of matter, then the velocity of any mass whatever belonging to the system, would at Jupiter's distance be less than twelve. miles per second, whereas at the distance of the earth, the largest planet travelling inside the orbit of Jupiter, the limit of the velocity would be more than twice as great. Hence we can see with what comparative difficulty an aggregation would form close to the central one, and how

where the quantity of matter was still great but the average velocity of motion not too great. Such an aggregation once formed, the next important aggregation would necessarily lie far outside, for within the first there would now be two disturbing influences preventing the rapid growth of these aggregations. The third and fourth would be outside the second. Between the first aggregation and the sun only small planets, like the Earth and Venus, Mars, Mercury, and the asteroids, could form ; and we should expect to find that the largest of the four small planets would be in the middle of the space belonging to the family, (as Venus and the Earth are actually placed), while the much smaller planets Mercury and Mars travel next on either side, one close to the Sun and the other next to Jupiter, the asteroids indicating the region where the combined disturbing influences of Jupiter and the Sun prevented any single planet from being developed.

But I should require much more time than is now at my command to present adequately the reasoning on which the theory of accretion is based. And we are not concerned here to inquire whether this theory, or Laplace's theory of contraction, or (which I hold to be altogether more probable than either) a theory involving combined processes of accretion and contraction, be the true hypothesis of the evolution of the solar system. Let it suffice that we recognise as one of the earliest stages of our earth's history, her condition as a rotating mass of glowing vapour, capturing then as now, but far more actively then than now, masses of matter which approached