INHABITED WORLDS. 267 



moon, and the planeis once existed as a sort of 

 cloud or luize, spread over an enormously wide 

 area — as far, in fact, as the orbit of Neptune, the 

 most distant of the known bodies belone^injjf to 

 our system. Slowly the gravitative force of the 

 entire mass drew it together toward its common 

 centre, at the spot now occupied by our great 

 luminary, the sun. As the cloud condensed and 

 gathered in its hem, it cast off rings, one after 

 another, wliich formed the materials in the fulness 

 of time for the various planets. Each such planet 

 began its existence as a little sun, a minor mass of 

 red-hot matter, which gradually cooled by radia- 

 tion on its outer surface, though still remaining at 

 a very high temperature in its inner core. Obser- 

 vation, in fact, leads us to believe that Jupiter and 

 Saturn, the two great giant planets, immensely 

 bigger than our own earth, are still in a state of 

 fiery commotion, subject to terrilic throes and 

 cyclones, and enjoying even now their hot and 

 boisterous planetary youth. Indeed, since large 

 masses would take much longer to cool down than 

 small ones, this is just what we should reasonably 

 expect to be the case ; the big [)lanets ought natu- 

 rally to retain their heat longest, while the snialler 

 ones ought of course to present every appearance 

 of a chilly exterior in exact proportion to their 

 minuteness of size. The moon, our own insignifi- 

 cant satellite, shows us in fact the very opposite 



