82 THE POPULAR SCIENCE MONTHLY. 



though the sun pours but about a twenty-seventh part of the heat on 

 Jupiter, and but about a hundredth part on Saturn, which we receive 

 from his rays. The outline of Jupiter, as indicated by the apparent 

 position of a satellite close to his disk, expands and contracts through 

 thousands of miles, yet the theory that Jupiter is still intensely hot 

 must not for a moment be entertained, though the expansion and con- 

 traction of the solid crust of a cool planet through so enormous a 

 range would vaporize a portion of its mass exceeding many times the 

 entire volume of our earth. Saturn is seen by Sir W. Herschel and 

 Sir J. Herschel, by Sir G. Airy, Cooliclge, the Bonds, and a host of 

 other observers, to assume from time to time the square-shouldered 

 aspect, a change which to be discernible from our distant standpoint 

 would imply the expansion and contraction of whole zones of Sat- 

 urn's surface through 4,000 or 5,000 miles at least ; yet it is better to 

 believe that these stupendous changes have affected the solid crust of 

 a planet like our earth than to admit the possibility that the outline 

 we measure is not that of the planet itself, but of layers of cloud 

 i-aised to a vast height in the deep atmosphere surrounding a planet 

 still glowing with its primeval fires. 



The phenomena I am now about to consider belong to the same 

 category. They are utterly inexplicable, or only explicable by the 

 most sensational assumptions as to the processes taking place on Jupi- 

 ter, if we adopt the old theory of Jupiter's condition; while if we 

 regard Jupiter as an intensely-heated planet surrounded by and entirely 

 concealed within a cloud-laden atmosphere several thousand miles in 

 depth, they at once admit of the most simple and natural explanation. 



It has, of course, long been known that the belts of Jupiter are 

 phenomena of his atmosphere, not of his surface. The belts of light- 

 est tint have been regarded as belts of cloud, and the darker belts as 

 either the real surface of the planet seen between the cloud-belts, or 

 else as lower cloud-layers, appearing darker because in shadow. Ac- 

 cordingly, when features of the belts have been watched in their rota- 

 tional circuit, it has been clearly recognized that the rotation deter- 

 mined in this way is not necessarily or probably the true rotation of 

 the planet itself. Further, it has been proved, beyond all possibility 

 of question, that some at least among the spots upon the planet's 

 belts have a motion of their own ; for whenever two spots in different 

 Jovian latitudes have been observed, it has been almost constantly 

 noticed that the one nearer the equator has had a greater rotation 

 rate than the other. Again, it has sometimes liappcned that instead 

 of two spots, in different latitudes, a well-defined dark streak or open- 

 ing, having its two extremities in different latitudes, has remained 

 long enough to be observed during several rotations of the planet. 

 In these cases it has been observed that the end of the streak nearest 

 the equator has traveled fastest, not only absolutely, but in longitude, 

 insomuch that the position of the streak has notably altered. 



