Page:Popular Science Monthly Volume 16.djvu/776

742 other reasons it is not unlikely that some of his cloud formations may be more lasting than ours. That his gaseous envelope is, however, at times subject to violent disturbances arising from a prodigious exertion of internal forces is proved by instances of sudden changes in the diameter of the disk. The Rev. T. W. Webb, in his "Celestial Objects" mentions as "inexplicable" an observation of Smyth, confirmed by similar observations of Maclear and Pearson, all being at different places, namely, that on June 26, 1828, Jupiter's second satellite, after fairly entering upon the disk, in the course of its revolution, was subsequently seen for four minutes outside it, and then suddenly vanished. More recently Secchi noticed a similar phenomenon; and the explanation can only be that Jupiter's atmosphere was suddenly blown out for some thousands of miles and retreated again. Secchi states that on April 2, 1874, he watched the first satellite as it was about to cross the planet's disk, which appeared "finely undulated." "When the satellite approached within its own diameter of the margin of the planet the latter sprang toward it, appeared to touch it, and immediately retired. This happened, backward and forward, until the satellite had plainly entered upon the planet; that is to say, for four or five minutes. . . . The satellite appeared fixed, and all the movement seemed to belong to the disk of the planet."

In October, 1879, Mr. Kidd, of Bramley, Guildford, saw, as is described in the "Observatory" for November, the second satellite first touch the disk, then appear separated from it, and finally pass behind it, but remain for some time visible through it. The "Observatory" for November also quotes the "Chicago Tribune," to the effect that observations at the Dearborn Observatory indicate that changes in the outline of the planet take place from day to day. Two sets of measures at the interval of a week are stated to have shown a difference in the direction of the major axis amounting to 5°.

When extensive belts or bright portions change rapidly, the storm effects must be immensely greater than in any of our hurricanes. Jupiter's motion at the equator is at the rate of about 28,000 miles an hour; his daily rotation is completed in a few seconds less than ten hours; and objects in Jupiter weigh about two and a half times as much as on our earth. When our winds move with a hurricane-speed of 85 miles an hour, they exert a pressure upon whatever they strike equal to 36 pounds per square foot. What, then, must be the force of a Jovian storm, moving much heavier matter than our air, at the rate of 300 miles an hour, as was observed on one occasion by Herschels?

On another occasion South saw a spot 22,000 miles long, and before a friend who was present could commence a sketch it had nearly all changed. There may in such cases be violent chemical action, a terrific clashing together of atoms, and the precipitation of solid oxides of metals, like the fumes produced by the burning of magnesium wire.