184. THE POPULAR SCIENCE MONTHLY. 



scribed appears particularly interesting, and that relates to the sup- 

 posed resemblance between the larger planets, and more especially 

 Jupiter, and the sun. Everybody knows that Jupiter has a conspicu- 

 ous dark-colored belt on each side of his equator, for those belts are 

 one of the commonest objects of celestial sight-seeing. Saturn too 

 has belts similarly situated, although they are less conspicuous than 

 those of Jupiter. All the trustworthy evidence we have points to 

 the conclusion that these huge planets are yet in a state which has 

 more points of resemblance to the condition of a sun than to that of 

 a cool and solid globe. There can be little doubt that Jupiter is sur- 

 rounded by a cloud-laden atmosphere of great depth, and that his 

 geological development, so to speak, is in a stage much earlier than 

 any whose former existence is recorded in the present rock strata of 

 the earth. In other words, Jupiter probably has not yet a continuous 

 solid crust, even if the formation of such a crust has been begun. 

 But, accepting the nebular hypothesis, we must conclude that Jupiter 

 is gradually cooling and contracting, and that eventually he will have 

 as solid a surface as the earth's. ' He seems, then, to be in a transition 

 state between a luminous sun and an opaque world, and, if so, his pres- 

 ent condition may throw light upon the future condition of the sun, 

 just as the moon throws light upon the future condition of our own 

 earth. For this reason it may be interesting for the reader to compare 



Fig. 4. 



with the figures representing the belt of sun-spots seen last summer a 

 picture of Jupiter and his belts, shown in Fig. 4. It is, of course a 

 long step from the string of separate spots in one case to the unbroken 

 bands in the other, and yet it is easily seen that some resemblance 



