88 THE POPULAR SCIENCE MONTHLY. 



masses, and, as the shadow had not so much greater a length at that 

 time, two months from opposition, as it had when the planet was much 

 nearer opposition, he infers that the true explanation of the appearance 

 has hardly been found. He appears to have overlooked the fact that 

 tlie assumption made in the explanation is not that Jupiter has a semi- 

 transparent atmosphere always equally translucent and penetrable to 

 the same depth by the solar rays. When the shadow was shorter 

 than it should have been, had the atmosphere been in the same condi- 

 tion as when Mr. Brett made his observation, it is probable that a 

 layer of clouds interrupted the rays, and thus the shadow was much 

 closer to the cloud-mass throwing it than it would have been had that 

 layer not been there, Mr, Knobel's paper contains very striking evi- 

 dence of the variability of Jupiter's atmosphere, or rather of the 

 clouds which float in it, " The greater distinctness of the satellites 

 when near the edge," he says, " is a curious phenomenon which has 

 been repeatedly observed by astronomers, but which seems to require 

 explanation." On an occasion described " the second satellite transited 

 a dark limb which was" (seemed) "most dark near the centre, and 

 fainter toward the edge, yet the satellite was almost invisible when 

 on the darkest part of the belt, and was bright and distinct when the 

 background of the belt was faintest." This practically proved that 

 on the occasion in question the dark, central part of the belt seemed 

 darker than it really was by contrast with the bright belts on either 

 side, while the edge seemed lighter than it really was by contrast 

 with the dark sky on which the planet was projected. In reality the 

 part near the edge must have been darker than the part near the 

 middle, or the satellite could not have appeared brighter when near 

 the edge. No doubt the darkness near the edge (which, by-the-way, 

 my friend Mr. Browning tested photometrically, and demonstrated, 

 at my suggestion, eight years ago) was due to transparency, the dark- 

 ness of the sky beyond being to some degree discernible througli the 

 edge. But this transparency is not always to be observed to the 

 same degree, or through the same extent of Jovian atmosphere as to 

 depth. Mr. Knobel proceeds, illustrating this the more effectively 

 that he does so unintentionally : "The third satellite, on March 25, 

 1874, appeared as a dark spot when in mid-transit, and on nearing the 

 edge appeared as a bright spot without trace of duskiness. But on 

 March 26, 1873" (observe the difference of years), "the fourth satel- 

 lite made the whole transit as a dark spot, and was not perceptibly 

 less dark at egress than in mid-transit." 



It appears to me demonstrated by the evidence thus far noted 

 that in a semi-transparent atmosphere of enormous depth, surrounding 

 Jupiter, there float vast cloud-masses, sometimes in layers, at others 

 in irregular heaps, at others having well-rounded forms. These cloud- 

 masses undergo sometimes remarkable changes of shape, often form- 

 ing or disappearing in a very short time, and thus indicating the infe- 



