l8o LIGHT SCIENCE FOR LEISURE HOURS. 



been the result of long and profound investigation, had 

 been tested in every possible way, had been weighed 

 and not found wanting by all the ablest astronomers 

 the world has known. This is the theory that Jupiter 

 and Saturn are bodies in the same condition as our earth. 

 It is not easy to find any reasoning whatever 

 bearing upon this theory. It would seem almost that 

 so soon as Copernicus had shown that the planets da 

 not travel round the earth as a centre, but the earth 

 with the planets travel around the sun, the conclusion 

 was at once adopted that the earth and the planets are 

 of necessity bodies of the same nature ; and that, as no 

 one was at the pains to question this doctrine, it 

 became gradually regarded as one that had been 

 established by demonstrative evidence. The few 

 instances of anything like reasoning which I have been 

 able to find scattered here and there in books of 

 astronomy amount to what follows : First, because 

 Jupiter and Saturn are planets, and the earth is a 

 planet, therefore those planets are like the earth. 

 (This argument is open to the objection that it begs 

 the question, which is, Whether other planets resemble 

 the earth.) Jupiter and Saturn are globes like the 

 earth (also like the sun and moon). They rotate on 

 their axes, and therefore if they are inhabited worlds 

 like the earth, they have day and night, and in that 

 respect are like the earth. They circle around the 

 sun, and thus if they are worlds like the earth, they 

 are like the earth in having a year ; also in having 

 seasons, since their axes are not perpendicular to the 



