WHAT IS JUPITER DOING f 739 



at the time of any formation the geologists can reach would not no- 

 ticeably approach the enormous difference that separates the condition 

 of our earth from that of either Jupiter or Saturn. It is possible that 

 they now represent stages which our earth passed through in remote 

 times, and they may be undergoing changes that are approximating 



Fig. 2.— Spots on Jupiter obsekved anu drawn bt AIh E. L. Trouvelot.— Observation of 

 December 23, 18~S, with the sliadow ol'the third satellite. 



them to our present condition. It is, however, probable that, while 

 there are analogies and resemblances in the life-histories of all the 

 heavenly bodies, there are also individual peculiarities and diversities 

 not less important or less striking. 



Jupiter's diameter is about eleven times that of our earth, and his 

 mean density is about a quarter that of the earth, or about a third 

 more than water. Now, a bulky body may be composed of heavy 

 materials, and still, as a whole, be light, like an iron ship or a lump of 

 pumice-stone, that will float in water. The pumice-lump is light on 

 account of its vesicular formation, so that the mass consists of heavy 

 feldspathic material and the air it contains. Extract the air, and the 

 pumice loses its floating power, though still far from heavy in propor- 

 tion to its bulk. Most of the earth's crust is formed of solids much 

 heavier than water. Granites are more than two and a half times 

 heavier than water, slaty rocks much about the same, and so are ordi- 

 nary limestones, the variations of all being from about 2*5 to 2-9, The 

 ironstone group contains denser minerals ; red hematite ha-s a specific 

 gravity of 4*5 ; magnetic ironstone, 4-5 to 5*2, etc. ; and many other 

 ores are heavy. 



