There will thus come a time when the nebula has been replaced by a dense, central, nebulous mass, around which a greater or less number of smaller clouds or swarms are re volving in a region which still contains a con siderable amount of resisting material. And these smaller clouds will all move around the central cloud in the same direction, which will be that of the general rotation of the original nebula as a whole, for it can be shown that such bodies as began to move in a contrary direction, when the resistance was high, must have been speedily brought down upon the central mass. Investigation shows that a sec ond effect of a resisting medium must be to diminish the eccentricity (or increase the roundness), of the paths in which the bodies are moving. Thus the lesser clouds must steadily drop nearer the centre, their orbits at the same time becoming continually rounder, until they attain a region which has l*en swept clear, or nearly so, of resisting material. The contraction of .each smaller cloud into a planet with a possible system of satellites is explained by an exactly similar course of reasoning.
It is thus from the lesser clouds, or portions of greater condensation in the original nebula, that the planets have originated, and each has acquired a greater or less size according as its cloud in passing through the resisting medium has gathered up and added to itself a greater or less amount of this material. It can be shown that in whatever manner a small cloud may have been originally rotating, the effect of its colli sions with the scattered particles among which it moves will be to give it a forward rotation; that is, one in the same direction as its own motion around the central mass. And the more collisions it has sustained, and hence the larger it has grown, the more rapid should we ex pect its rotation to become. This is in accord ance with what is observed in the solar system, where Jupiter and Saturn rotate in less than half the time of the small planets. A similar investigation shows that all satellites formed near the smaller clouds while the system was still young should have small and rounded orbits, and their motion should be direct, but that there may also be more distant satellites moving in a retrograde direction in orbits which are highly eccentric.
The Origin of Spiral Nebulas.— A theory has been advanced, and quite fully developed from a mathematical standpoint, to account for the existence of spiral nebula: in general, and in particular for that of the original solar nebula, Which is thus supposed to have had a spiral form. It is supposed that these nebula; were caused by the close approach, or even by the actual collision, of two suns, for it can be shown that if such a close approach occurs the enormous gravitational attraction may tear one or both of these bodies apart into a roughly two-branched cloud, whose arms under the pull of the 'passing body may be drawn into curves strikingly similar to the outlines of cer tain of the well-known spiral nebula;. But there are grave and apparently insuperable objections to the hypothesis. In the first place, where the stars are most thickly gathered together (that is, in the direction of the Milky Way), almost none of these nebulae are found. It is 'here that the so-called New Stars predominate, but spiral nebulae are almost wholly confined to outside regions where the stars are few and the proba bility of a collision, or even of a near approach, of two suns is almost infinitely small. More
over it is certain that the spiral nebulae seen in the sky are of enormous size. Though the spectroscope shows that they are moving through space with speeds which range from 200 to 700 miles a second, they are so distant that even these great nations have not dis placed them appreciably on the sky (except possibly in one instance), even when measures which are separated by many years are com pared. In many cases they must have thou sands of times the extent of our solar system. Indeed, some astronomers believe that these nebulae, so far from being true nebulae, are, in reality, clusters of almost infinitely distant stars.
It is not indeed unlikely that any rotating nebula would as it contracted assume a more or less spiral form, and this may with some probability be assumed to have been the form of the prototype of the solar system. But the assumption of this, or of any other definite form, for the original nebula in no way affects the modern theory of the development of the solar system which has just been discussed.
The Earth-Moon System.— In the general process of evolution above described there are several local and minor modifications which must be made the subject of special inquiry. The system of our earth and moon, for ex ample, is wholly unlike that of any of the other planet-satellite systems, for while in these the moons are all minute bodies in comparison with the planet, our own moon is comparable in size with the earth and is indeed so large that it is perhaps better to regard these bodies as forming a sort of double-earth system than a planet attended by a satellite. It is uncertain whether the earth and moon originally formed a single body which afterward separated into two, or whether they separately condensed from neigh boring clouds of the original nebula, the moon later being drawn nearer to the earth and its orbit rounded by the action of the resisting medium. The former theory has been very fully investigated both by. Poincare and Darwin, and though the methods of attack in the two cases were wholly different, the results are in re markable agreement. By the former astrono mer the development of a rotating, tenuous mass, or cloud, was examined: it was shown mathematically that if the velocity of rotation exceeded a certain limit, two separate centres of condensation would appear within the rotat ing mass. As these separated the mass would take a pear-shaped form: the constriction around the neck of the pear increasing, there would finally result two bodies almost in con tact revolving about their common centre of gravity. At this time the length of the month must have been equal to the time of rotation of the earth; it has since been greatly length ened by the enormous bodily tides which the two closely adjacent, plastic masses must have raised in each other, for it can be demon strated that the effect of these tides is to rap idly push the two bodies apart and also to reduce the speed of rotation of each of them. The original energy of rotation of the moon being much less than that of the earth, its relative rotation was thus completely destroyed, so that now it turns always the same face toward us. In the same manner the relative rotation of Mercury with reference to the sun has been destroyed by the bodily tides produced on the planet while it was still in a nebulous or plastic condition.