MOVEMENTS OF JUPITER'S CLOUD-MASSES. 85 



owe their origin to the difference of absolute rotational velocity 

 between the equatorial parts of the earth and parts in high latitudes. 

 In the case of Jupiter the difference of this kind is not sufficient to 

 account for the observed belts partly because there are many, 

 partly because they are variable, but principally because Jupiter is so 

 much larger than the earth that 'much greater distances must be trav- 

 ersed ia passing from any given latitude to another where the rota- 

 tional velocity is so many miles per hour more or less. Combining 

 with these considerations the circumstance that the solar action which 

 causes the atmospheric movements from one latitude to another in 

 the case of our earth is reduced to one twenty-seventh part only of 

 its terrestrial value in the case of Jupiter, we must clearly look to 

 some other cause for the difference of absolute rotational velocity 

 necessary to account for the belts of Jupiter. 



Now, it seems to me that we are thus at once led to the conclu- 

 sion that the cloud-masses forming the belts of Jupiter are affected 

 by vertical currents, up-rushing motions carrying them from regions 

 nearer the axle, where the absolute motion due to rotation is slower, 

 to regions farther from the axis, where the motion due to rotation 

 is swifter, and motions of down-rush carrying them from regions f 

 swifter to regions of slower rotational motion. This view seems cer- 

 tainly encouraged by what we find when we come to study more 

 closely the aspect of the Jovian belts. The white spots some small, 

 some large which are seen to form from time to time along the chief 

 belts present precisely the appearance which we should expect to find 

 in masses of vapor flung from far down below the visible cloud-sur- 

 face of Jupiter, breaking their way through the cloud-layers, and 

 becoming visible as they condense into the form of visible vapor in 

 the cooler upper regions of the planet's atmosijliere. Then, again, 

 the singular regularity with which in certain cases the great, rounded 

 white clouds are set side by side, like rows of eggs upon a string, is 

 much more readily explicable as due to a regular succession of up- 

 rushes of vapor, from the same region below, than as due to the simul- 

 taneous up-rush of several masses of vapor from regions set at uniform 

 distances along a b^lt of Jupiter's surface. The latter supposition is 

 indeed artificial and improbable in the highest degree, and in several 

 distinct respects. It is unlikely that several up-rushes should occur 

 simultaneously, unlikely that regions whence up-rush took place should 

 be set at equal distances from each other, unlikely that they should 

 lie along the same latitude parallel. On the other hand, the occur- 

 rence of up-rush after up-rush from the same region of disturbance, at 

 nearly uniform intervals of time, is not at all improbable. The 

 rhythmical succession of explosions is a phenomenon, indeed, alto- 

 gether likely to occur under certain not improbable conditions as, 

 for instance, when each explosion affords an excess of relief, if one 

 may so speak, and is therefore followed by a reactionary process, in 



