THE BOUNDARIES OF ASTRONOMY. 243 



portions of the ring would be much, denser than others, and the denser portion 

 would gradually attract the rarer portions around it, until instead of a ring we 

 should have a single mass, composed of a nearly solid center, surrounded by an 

 immense atmosphere of fiery vapor. This condensation of the ring of vapor 

 around a single point would have produced no change in the amount of rotary 

 motion originally existing in the ring ; the planet surrounded by its fiery atmos- 

 phere would therefore be in rotation, and would be, in miniature, a reproduction 

 of tbe case of the sun surrounded by his atmosphere with which we set out. In 

 the same way that the solar atmosphere formed itself first into rings, and then 

 these rings condensed into planets, so, if the planetary atmosphere were suffi- 

 ciently extensive, they would form themselves into rings, and these rings would 

 condense into satellites. In the case of Saturn, however, one of the rings was 

 so perfectly uniform that there could be no denser portion to draw the rest of 

 the ring around it, and thus we have the well-known rings of Saturn. 



It will thus be seen that one of the principal features in the solar 

 system for which the nebular theory has been invoked is the fact that 

 the planets all revolve round the sun in the same direction. It will 

 therefore be natural to take up first the discussion of this subject, and 

 to inquire how far the common motion of the planets can be claimed 

 in support of Laplace's nebular theory. The value of this argument is 

 very materially influenced by another consideration of a somewhat 

 peculiar character. If it were quite immaterial to the welfare of the 

 planetary system whether all the planets moved the same way, or 

 whether some moved one way and some another, then the nebular hy- 

 pothesis would be entitled to all the support which could be derived 

 from the circumstances of the case. Take, for instance, the eight 

 principal planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, 

 Uranus, Neptune. All these planets move in the same way around 

 the sun. The chances against such an occurrence are 127 to 1. The 

 probability that the system of eight planets have been guided to move 

 in the same direction by some cause may be taken to be 127 to 1. If 

 we include the two hundred minor planets, the probability would be 

 enormously enhanced. The nebular theory seems a reasonable explana- 

 tion of how this uniformity of movements could arise, and therefore 

 the advocates of the nebular theory may seem entitled to claim, all this 

 high degree of probability in their favor. There is, however, quite a 

 different point of view from which the question may be regarded. 

 There are reasons which imperatively demand that the planets (at all 

 events the large planets) shall revolve in uniform directions, which lie 

 quite outside the view taken in the nebular theory. If the big planets 

 did not all revolve in the same direction, the system would have per- 

 ished long ago, and we should not now be here to discuss the nebular 

 or any other hypothesis. 



It is well known that, in consequence of the gravitation which per- 

 vades the solar system, each of the planets has its movements mainly 

 subordinated to the attraction of the sun. But each of the planets 

 attracts every other planet. In consequence of these attractions the 



