COSMIC EVOLUTION 



solar nebula, which once extended as far as the 

 orbit of Neptune, began to shrink, the atoms 

 which composed it tended, in accordance with the 

 law of conservation of momentum, to arrange 

 themselves in a number of planes, of which one 

 was the most frequented, and was called the prin- 

 cipal plane. 



When we learn the origin of the nebula we shall 

 know what conditions determine the presence and 

 position of the principal plane. But "the great 

 ages onward roll," and the influence of gravita- 

 tion causes the atoms in these various planes to 

 attract one another, so that ultimately the whole 

 substance of the nebula is disposed in one plane, 

 which is, approximately, of course, the principal 

 plane already described. 



The chaos has now been resolved into a flat 

 object, nearly all the atoms of which are now 

 revolving in the same direction as do planets 

 and nearly all the satellites of the solar system 

 around their common centre of gravity, which in 

 our case is now represented by the sun. But there 

 is another most important difference between the 

 chaotic or primitive nebula and the flattened spiral 

 nebula to which it has yielded. 



Time was when we thought it probable that a 

 nebula was merely a star-cluster, too distant for 

 terrestrial telescopes to resolve into its constituent 

 stars. No advance in the construction of tele- 

 scopes could ever have answered this objection; 

 but a new astronomy arose, which left the telescope 

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