the nearly equal times of revolution of all the solid planets, and for the smaller period of rotation, which the larger planets show. 15. In what manner, however, portions are to be detached from such a rotating mass, so as to form solid planets on the one side, and watery planets on the other, and how these planets, so detached, are to be made to revolve round the Sun, in orbits nearly circular, we have no hypothesis ready to explain. And perhaps we may say, that no satisfactory, or even plausible, hypothesis to explain these facts, has been proposed: for the Nebular Hypothesis, the only one which is likely to be considered as worthy any notice on this subject, is too imperfectly worked out, as yet, to enable us to know, what it will or will not account for. According to that hypothesis, the nebular matter of a system, having originally a rotatory motion, gradually contracts; and separating, at various distances from the centre, forms rings; which again, breaking at some point of their circumference, are, by the mutual attraction of their parts, gathered up into one mass; which, when cooled down, so as to be opaque, becomes a planet; still revolving round the luminous mass which remains at the centre. That such a process, if we suppose the consistency, and other properties, of the nebulous matter to be such as to render it possible, would produce planetary masses revolving round a sun in nearly circular orbits, and rotating about their own axes, seems most likely; though it does not appear that it has been very clearly shown.[9] But no successful attempt has been made to deduce any laws of the distances from the centre, times of rotation, or other properties of such planets; and therefore, we cannot say that the nebular hypothesis is yet in any degree confirmed. 16. The Theory which we have ventured to propose, of the Solar System, agrees with the Nebular Hypothesis, so far as that hypothesis goes; if we suppose that there is, at the centre of the exterior planets, Jupiter, Saturn, Uranus, and Neptune, a solid nucleus, probably small, of the same nature as the other planets. Such an addition to our theory is, perhaps, on all accounts, probable: for that circumstance would seem to determine, to particular points, the accumulation of water and vapors, to which we hold that those planets owe the greater part of their bulk. Those planets then, Jupiter, Saturn, and the others, are really small solid planets, with enormous oceans and atmospheres. The Nebular Hypothesis, in that case, is that part of our Hypothesis, which relates to the condensation of luminous nebular matter; while _we_ consider, further, the causes which, scorching the inner planets, and driving the vapors to the outer orbs, would make the region of the earth the only habitable part of the system.