Page:Popular Science Monthly Volume 4.djvu/455

Rh. Now, we know that the sun is intensely hot because we feel the heat that he emits, and recognize the intense lustre of his photosphere; so that we are not in danger of overlooking this important circumstance in his condition. Jupiter gives out no heat that we can feel, and assuredly Jupiter does not emit an intense light of his own. But, when we find that difficulties, precisely corresponding in kind, though not in degree, to those which we should encounter if we discussed the sun's condition in forgetfulness of his intense heat, exist also in the case of Jupiter, it appears manifest that we may safely adopt the conclusion that Jupiter is intensely heated, though not nearly to the same degree as the sun.

We have thus been led by a perfectly distinct an independent line of reasoning to the very conclusion which I have advocated elsewhere on other grounds, viz., that Jupiter is in fact a miniature sun as respects heat, though emitting but a relatively small proportion of light. I would invite special attention to the circumstance that the evidence on which this conclusion had been based was already cumulative. And now a fresh line of evidence, in itself demonstrative I conceive, has been adduced. Moreover, I have not availed myself of the argument, very weighty in my opinion, on which Mr. Mattieu Williams has based similar conclusions respecting the temperature of Jupiter, in his interesting and valuable work called "The Fuel of the Sun." I fully agree with him in regarding it as a reasonable assumption, though I cannot go so far as to regard it as certain, that every planet has an atmosphere whose mass corresponds with, or is even perhaps actually proportional to, the mass of the planet it surrounds. If we make such an assumption in the case of Jupiter, we arrive at conclusions closely resembling those to which I have been led by the above process of reasoning.

Thus many lines of evidence, and some of them absolutely demonstrative, in my opinion, point to the conclusion that Jupiter is an orb instinct with fiery energy, aglow it may well be with an intense light which is only prevented from manifesting itself by the cloudy envelope which enshrouds the planet.

But, so soon as we regard the actual phenomena presented by Jupiter in the light of this hypothesis, we find the means of readily interpreting what otherwise would appear most perplexing. Chief among the phenomena thus accounted for, I would place the recent color-changes in the equatorial zone of Jupiter.

What, at a first view, could appear more surprising than a change affecting the color of a zone-shaped region whose surface is many times greater than the whole surface of our earth. It is true that a brief change might be readily explained as due to such changes as occur in our own air. Large regions of the earth are at one time cloud-covered, and at another free from clouds. Such regions, seen from Venus or Mercury, would at one time appear white, and at the other would