84 THE POPULAR SCIENCE MONTHLY. 



Of course, it is just possible that a great dark rift, such as I have 

 desribed, might appear tiius to change in position without any actual 

 transference of the bordering cloud-masses. Mr. Webb, speaking of 

 a number of phenomena, of which those presented by the great rift of 

 1860 are but a few, says that "they prove an envelope vaporous and 

 mutable like that of the earth, without, however, necessarily inferring" 

 (? implying) "the existence of tempestuous winds: even in our own 

 atmosphere, when near the dew-point, surprising changes sometimes 

 occur very quietly: a cloud-bank observed by Sir John Herschel, 

 April 19, 1827, was precipitated so rapidly that it crossed the whole 

 sky from east to west at the rate of at least 300 miles per hour; and 

 alterations far more sudden are conceivable where everything is on a 

 gigantic scale." It does not seem to me altogether probable that 

 more rapid alterations would affect cloud-banks covering millions of 

 square miles than occasionally affect terrestrial cloud-banks cover- 

 ing pei'liaps a few tens of thousands of square miles; on the con- 

 trary, as small terrestrial clouds change relatively in a far more rapid 

 way than large ones, and these than cloud-masses covering a county 

 or a country, so it would seem that the changes affecting our largest 

 cloud-layers would be relatively far more rapid than those affecting 

 cloud-masses which could (many times over) enwrap the whole frame 

 of this earth on which we live. But apart from that, and apart also 

 from the important consideration that all such processes as evapora- 

 tion and condensation, so far as the sun brings them about, should 

 proceed far more sluggishly in the case of a planet like Jupiter 

 than in that of our earth, which I'eceives some twenty-seven times 

 as much heat from the sun (mile for mile of surface), it is utterly 

 incredible that precipitation should have occurred so steadily and 

 swiftly along one edge of tlie great rift, and condensation with such 

 exactly equal steadiness and swiftness on the opposite edge, that, 

 while the rift as a whole shifted its position during a hundred Jovian 

 nights and days at the rate of 150 miles per hour, its sides should 

 nevertheless remain parallel all the time. Such processes may be 

 spoken of as possible, in the same sense that it is possible that a coin 

 tossed fifty times in succession should always show the same face ; but 

 we do not reckon such possibilities among scientific contingencies. 



But the motion of great rounded masses in the atmosphere of Ju- 

 piter is still more decisive as to the existence not only of a very deep 

 atmosphere, but also as to the swift motions taking place in that atmos- 

 phere. 



I would, in the first place, note that the very existence of belts in 

 the Jovian atmosphere, and especially of variable belts, implies the 

 great depth at which the real surface of the planet must lie below the 

 visible cloud-layers. Atmospheric belts can only be formed where 

 there are differences of rotational velocity. In the case of our own 

 earth we know that the trade-wind zone and the counter-trade zone 



