THE GREATEST PLANET. I 93 



bodies of our solar system were originally much hotter 

 than they are at present; they have all been steadily 

 cooling, and the process has been going on for uncounted 

 ages. The small bodies have cooled more rapidly than the 

 large ones. Thus the moon, which is one of the smallest 

 of the bodies in our system, appears to have parted with 

 almost all its heat. The sun, at the other extreme, on 

 account of his gigantic dimensions, amounting as they do 

 to more than a thousand times the size of Jupiter, which 

 is itself larger than all the rest of our system together, is 

 large enough to have retained so much heat that he still 

 glows with excessive fervour. 



Between the sun and the moon as the extreme limits 

 of size, may be ranked the other bodies of the system. 

 We find that many of the different bodies show more heat 

 or less heat, according to their size. Thus, our earth 

 berng so much larger than the moon, and so much smaller 

 than the sun, might be expected to have still retained 

 some of its heat, though the "glowing stage in the pro- 

 gress of cooling has long since been passed ; and this is what 

 we do actually find. Our globe shows many indications 

 of a high interior temperature. We have the well-known 

 phenomena of volcanoes, and of hot springs, to show us 

 that there are stores of heat beneath our feet, and our 

 belief in internal heat is confirmed by the gradual increase 

 of temperature that is found in every mine, the deeper it 

 penetrates into the earth. 



Jupiter, being 1,200 times as big as the earth, has not 



yet cooled to the same extent. It still retains vast stores 



of internal heat, which are sufficient to preserve in the 



vaporous form the oceans that shall one day roll upon its 



o 



