i88 IN STARRY REALMS. 



so that it can be regarded as one of the most accurately 

 known elements of our solar system. To express the 

 result in the form best adapted for our present line of 

 reasoning we may say that the mass of Jupiter is 310 

 times as great as the mass of the earth. 



Truly Jupiter is a noble globe. If we imagine a stu- 

 pendous pair of weighing scales constructed, and that 

 Jupiter was placed in one pan, we should find that it 

 would require 310 globes, each as ponderous as our earth, 

 to be placed in the other pan before the beam began to 

 turn. Yet while we are impressed with the gigantic 

 mass of Jupiter, we recall the fact that the planet is 1,200 

 times as large, and then we are driven to inquire why 

 it is that though Jupiter is 1,200 times as big as our 

 earth it is only 310 times as heavy. 



It is quite obvious that Jupiter must be a world of a 

 kind very different indeed from that on which we stand. 

 Our world is made up of rocks and minerals, largely also 

 of iron, and all these materials together produce a globe 

 of very high density, between five and six times the 

 weight of a globe of water of equal dimensions. But the 

 case with respect to Jupiter is widely different. If we 

 take the earth as a standard of comparison, we see that as 

 Jupiter exceeds us in bulk 1,200-fold, he should also be 

 about 1,200 times as heavy, if the materials of his con- 

 stitution were similar to and in the same state as those on 

 the earth. But he only weighs about a quarter as much 

 as this hypothesis would require. It therefore follows 

 that Jupiter cannot be organized on anything like the 

 same plan as our globe. His average density is only one- 

 fourth as much as ours. Jupiter only weighs about once 



