Page:Popular Science Monthly Volume 1.djvu/299

Rh cause in our books on astronomy we commonly see a set of concentric circles at regularly increasing distances, assigned as the paths of the several planets of the solar system. And besides, there yet remains, in the modern teaching of astronomy, a perceptible trace of the ancient astronomical systems, in which Saturn and Jupiter, Mars, Venus, and Mercury, played parts of equal importance.

Let it be carefully remembered, then, that the four planets which circle nearest the sun—the family of which our earth is a member—differ in all their characteristics from the outer family (also consisting of four planets) to which Jupiter belongs. The whole of the inner family—the whole of the space within which its members travel—could be placed between the paths of Jupiter and his next neighbor Saturn, with a clear space many millions of miles wide on either side. The actual area between the paths of Jupiter and Saturn exceeds nearly thirty times the whole area within which the four lesser planets pursue their paths. And, when we consider the dimensions of the four inner planets, we find a like disproportion. Four circles representing these orbs can be enclosed within a circle representing Uranus, the smallest of the four outer planets; yet even this circumstance does not adequately represent the enormous disparity between the two families of planets; for, in fact, the volume of Uranus exceeds the combined volume of all the inner planets upward of thirty times. We might adduce many other illustrations of the complete dissimilarity between the inner and outer families of planets; but what has been already stated will suffice for our present purpose. It will be evident that, in considering the members of one or other family, we must be prepared to meet with relations which differ not merely in degree, but in kind. We may thus, at the outset, dismiss from our thoughts the idea that the planet Jupiter is necessarily to be regarded as an inhabited world merely because the only planet we are actually acquainted with is inhabited. The latter circumstance may be an excellent reason for regarding Mars or Venus as the abode of life; but the analogy can no more be extended to Jupiter than to the fixed stars, which certainly are not inhabited worlds. We must, in fact, consider the physical habitudes of Jupiter independently of all conceptions based upon terrestrial analogies. Studied thus, he will be found, as we conceive, to hold a position in the scheme of creation differing considerably from that which has been assigned to him, until of late, in treatises on astronomy.

It is necessary briefly to state the dimensions, mass, and general characteristics of the planet, before proceeding to discuss its probable physical condition.

Jupiter has a diameter exceeding the earth's rather more than ten times, and a volume exceeding hers 1,230 times. It is not far from the truth to say that Jupiter's dimensions exceed the earth's in very nearly the same degree that those of the sun exceed Jupiter's. But his mass, though gigantic compared with the earth's, does not altogether