440 THE POPULAR SCIENCE MONTHLY. 



show whatever color the actual surface of the ground might possess 

 when viewed as a whole. But it seems altogether impossible to ex- 

 plain in this way a change or series of changes occupying many years, 

 as in the case of the recent color-changes of Jupiter's belt. Let me 

 not be misunderstood. I am not urging that the changes in Jupiter 

 are not due to the formation and dissipation of clouds in his atmos- 

 phere. On the contrary, I believe that they are. What seems to me 

 incredible is, the supposition that we have here to deal with such 

 changes as occur in our own air in consequence of solar action. 



I do not lose sight of the fact that the Jovian year is of long dura- 

 tion, and that whatever changes take place in the atmosphere of 

 Jupiter through solar action might be expected to be exceedingly 

 slow. Nay, it is one of the strongest arguments against the theory 

 that solar action is chiefly in question, that any solar changes would 

 be so slight as to be in effect scarcely perceptible. It is not commonly 

 insisted upon in our text-books of astronomy — in fact, I have never 

 seen the point properly noticed anywhere — that the seasonal changes 

 in Jupiter correspond to no greater relative change than occurs in our 

 daily supply of solar heat from about eight days before to about eight 

 days after the spring or autumn equinox. It is incredible that so 

 slight an effect as this should produce those amazing changes in the 

 condition of the Jovian atmosphere which have unquestionably been 

 indicated by the varying aspect of the equatorial zone. It is manifest 

 that, on the one hand, the seasonal changes should be slow and slight 

 so far as they depend on the sun, and, on the other, that the sun can- 

 not rule so absolutely over the Jovian atmosphere as to cause any par- 

 ticular atmospheric condition to prevail unchanged for years. 



If, however, Jupiter's whole mass is in a state of intense heat — if 

 the heat is in fact sufficient, as it must be, to maintain an effective re- 

 sistance against the tremendous force of Jovian gravitation — we can 

 understand any changes, however amazing. We can see how enor- 

 mous quantities of vapor must continually be generated in the lower 

 regions to be condensed in the upper regions, either directly above 

 the zone in which they were generated, or north or south of it, ac- 

 cording to the prevailing motions in the Jovian atmosphere. And, 

 although we may not be able to indicate the precise reason why at 

 one time the mid zone or any other belt of Jupiter's surface should ex- 

 hibit that whiteness which indicates the presence of clouds, and at 

 another should show a coloring which appears to indicate that the 

 glowing mass below is partly disclosed, we remember that the diffi- 

 culty corresponds in character to that which is presented by the phe- 

 nomena of solar spots. We cannot tell why sun-spots should wax 

 and wane in frequency during a period of about eleven years ; but 

 this does not prevent us from adopting such opinions as to the con- 

 dition of the sun's glowing photosphere as are suggested by the be- 

 havior of the spots. 



