ASTRONOMY 113 



solar sysem was evolved out of an immense nebula, a mass 

 of rare gas shaped like a flattened globe, of enormous dimen- 

 sions, very hot and having a slow rotation about an axis 

 through its center. By cooling and by gravitational attraction, 

 this nebula slowly contracted. Now, a necessary result of 

 its contration would be that it must rotate faster, and this 

 would make it more flattened, so that after a while it would 

 become shaped like a lens, or a disc thicker in the middle than 

 at the edges. Finally its rotation became so rapid that, owing 

 to what we call centrifugal force, a ring of gaseous matter 

 was left rotating alone, and the rest contracted away from it. 

 This ring finally gathered itself together at one point on its 

 circumference into a single rotating globe which became the 

 planet Neptune. The central mass, in the meantime, kept on 

 contracting and threw oflf other rings which formed the other 

 seven planets. These themselves threw off smaller rings which 

 formed the satellites of the planets. \\"hat was left of the 

 original nebula in the middle is the sun. This beautiful theory 

 has many strong points, and it is supported by the fact that 

 one of the planets, Saturn, still has several rings about it, as 

 if the formation of satellites in this case were not quite com- 

 plete. Consequently, the theory was for a long time thor- 

 oughly believed in. Unfortunately, we find that the rings of 

 Saturn are not rings of gaseous matter at all, but consist of 

 a great number of solid bodies each revolving about the planet. 

 Furthermore, careful mathematical analysis shows that a ring 

 could not of itself draw up into a single sphere at all, but 

 would remain more or less ring-shaped. There are other 

 points, too, in which the theory breaks down, and it fails 

 altogether to account for many of the facts. 



It is only recently, however, that the nebular hypothesis 

 has been overthrown, and since then a number of cosmogonic 

 hypotheses have been presented. One of these is due to 

 George Darwin, son of the famous Charles Darwin. He 

 showed that as the original nebula cooled and began to rotate 



