Page:Popular Science Monthly Volume 11.djvu/102

92 differ less in velocity from the superincumbent atmosphere into which they were driven than would masses expelled from higher latitudes. (It is probable that the same explanation applies also in the case of the sun.)

This conclusion, that the spots of Jupiter have rapid rates of relative motion, would of itself be of singular interest, especially when we remember that the larger white spots represent masses of cloud 5,000 or 6,000 miles in diameter. That such masses should be carried along with velocities so enormous as to change their positions relatively to each other, at a rate sometimes of more than 150 miles per hour, is a startling and stupendous fact. But it appears to me that the fact is still more interesting in what it suggests than in what it reveals. The movements taking place in the deep atmosphere of Jupiter are very wonderful, but the cause of these movements is yet better worthy of study. We cannot doubt that deep down below the visible surface of the planet—that is, the surface of its outermost cloud-layers—the fiery mass of the real planet. Outbursts, compared with which the most tremendous volcanic explosions on our earth are utterly insignificant, are continually taking place beneath the seemingly quiescent envelope of the giant planet. Mighty currents carry aloft great masses of heated vapor, which, as they force their way through the upper and cooler strata of the atmosphere, are converted into visible cloud. Currents of cool vapor descend toward the surface, after assuming, no doubt, vorticose motions, and sweeping away over wide areas the brighter cloud-masses, so as to form dark spots on the disk of the planet. And, owing to the various depths to which the different cloud-masses belong, and whence the up-rushing currents of heated vapor have had their origin, horizontal currents of tremendous velocity exist, by which the cloud-masses of one belt or of one layer are hurried swiftly past the cloud-masses of a neighboring belt, or of higher or low cloud-layers. The planet Jupiter, in fact, may justly be described as a miniature sun, vastly inferior in bulk to our own sun, inferior to a greater degree in heat, and in a greater degree yet in lustre, but to be compared with the sun—not with our earth—in size, in heat, and in lustre, and, lastly, in the tremendous energy of the processes which are at work throughout his cloud-laden atmospheric envelope.

Since the above article was written, news has been received by the Astronomical Society that Mr. Todd, a well-known observer of Adelaide, New South Wales, has been able to trace the motions of satellites behind the parts of the planet near the edge, or, in other words, through those parts of the planet's atmosphere which have hitherto been regarded as belonging to the mass of the planet itself.