Page:Popular Science Monthly Volume 4.djvu/57

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formed is measurable by the actual difference in dimensions between the globe and its parent cloud-mass, and of course the larger the cloud-mass the greater this difference would necessarily be. It is equally certain that the heat generated by the gathering-in of meteoric matter would be so much the greater according as the quantity of matter gathered and gathering was greater; for the heat is produced by the downfall of such matter on the globe it helps to form, and the greater the mass of that globe the greater is its attracting might, the greater the velocity it generates in the falling meteors, and therefore the greater the heat produced when they are brought to rest.

Saturn, then, would originally be much hotter than our earth, on any theory of the evolution of our solar system—and there are few astronomers who doubt that the solar system was wrought by processes of evolution to its present condition. But not only would Saturn be much hotter than the earth, but, owing to his enormous size, he would part with his heat at a much slower rate. On both accounts we should infer that at this present time Saturn is much hotter than the earth—in other words, since our earth still retains no inconsiderable proportion of its original heat, Saturn may be assumed to be in a state of intense heat. What his actual heat may be is not so easily determined. We shall presently show reasons for believing that an inferior limit, below which his heat does not lie, is indicated by the fact that he still possesses inherent luminosity. On the other hand, a superior limit is indicated by the fact that his inherent luminosity is not great, and that, in all probability, the thicker cloud-zones of Saturn prevent the passage of the greater part of his light.[1]

We should infer, then, that Saturn in some respects resembles the sun, though of course the very same reasoning which teaches us to believe that Saturn is very much hotter than the earth, leads us also to the conclusion that it is not nearly so hot as the sun. Now, thus viewing Saturn, we should be led to expect, apart from all telescopic evidence to that effect, that he would resemble the sun in certain general features. For instance, we might expect that he would have spot-zones, while his equatorial zone would be free from spots; or, if it were thought that so close a resemblance was not to be looked for, then we might still expect that his equatorial zone, like the sun's, would be distinguished from the rest of his surface by some well-marked peculiarity. This is the case. The equatorial zone of Saturn is distinguished by a peculiar brightness from the rest of his surface, insomuch that the late Prof. Nichol was led to regard this zone as the

  1. To prevent misapprehension, it may be as well to remind the reader that the apparent continuity of Saturn's cloud-belts by no means implies that they are really continuous, and it is on a priori grounds highly improbable that they are so; openings in his cloud-zones two or three hundred miles in length and breadth would be quite undiscernible at Saturn's enormous distance.