434 THE POPULAR SCIENCE MONTHLY. 



I propose now to discuss certain considerations which appear to 

 me to indicate the nature and probable meaning of the phenomena 

 which have recently been observed in Jupiter. It seems to me that 

 these phenomena are full of interest, whether considered in themselves 

 or in connection with those circumstances on which I had been led to 

 base the theory that Jupiter is a planet altogether unlike our earth in 

 condition, and certainly unfit to be the abode of living creatures. 



I would first direct special attention to the facts which have been 

 ascertained respecting the atmosphere of Jupiter. 



It does not appear to have been noticed as a remarkable circum- 

 stance, that Jupiter should have an atmosphere recognizable from our 

 distant station. Yet, in reality, this circumstance is not only most 

 remarkable, but is positively inexplicable on any theory by which Ju- 

 piter is regarded as a world resembling our own. It is certain that, 

 except by the effects produced when clouds form and dissipate, our ter- 

 restrial atmosphere could not be recognized at Jupiter's distance with 

 any telescopic power yet applied. But no one who has studied Jupi- 

 ter with adequate means can for a moment fail to recognize the fact 

 that the signs of an atmosphere indicate much more than the mere 

 formation and dissipation of clouds. I speak here after a careful study 

 of the planet during the late opposition, with a very fine reflecting 

 telescope by Browning, very generously placed at my disposal by Lord 

 Lindsay ; and I feel satisfied that no one can study Jupiter for many 

 hours (on a single night) without becoming convinced that the cloud- 

 masses seen on his disk have a depth comparable with their length and 

 breadth. Now, the depth of terrestrial cloud-masses would at Jupiter's 

 distance be an absolutely evanescent quantity. The span of his disk 

 represents about 84,000 miles, and his satellites, which look little more 

 than points in ordinary telescopes, are all more than 2,000 miles in di- 

 ameter. I am satisfied that any one who has carefully studied the 

 behavior of Jupiter's cloud-belts will find it difficult to believe that 

 their depth is less than the twentieth part of the diameter of the least 

 satellite. Conceive, however, what the depth of an atmosphere would 

 be in which cloud-masses a hundred miles deep were floating ! 



It may be asked, however, in what sense such an atmosphere would 

 be inexplicable, or, at least irreconcilable with the theory that Jupiter 

 is a world like our earth. Such an atmosphere would be in strict pro- 

 portion, it might be urged, to the giant bulk of the planet, and such 

 relative agreement seems more natural than would be a perfect cor- 

 respondence between the depth of the atmosphere on Jupiter and the 

 depth of our earth's atmosphere. 



But it must not be forgotten that the atmosphere of Jupiter is at- 

 tracted by the mass of the planet ; and some rather remarkable con- 

 sequences follow when we pay attention to this consideration. Of 

 course a great deal must be assumed in an inquiry of the sort. Since, 

 however, we are discussing the question whether there can be any re- 



