THE ORIGIN OF THE EARTH 417 



panded enough to embrace the whole space of the present system. 

 The nebula was assumed to have cooled by radiation of heat, and 

 in cooling to have shrunk. This shrinkage accelerated the rate 

 of rotation, and this in turn increased the equatorial bulge which 

 rotation developed. The progressive increase of cooling, rotation, 

 and bulging finally led to the separation of an equatorial ring. As 

 this ring also cooled and contracted, it was disrupted and its sub- 

 stance gathered into a planet whose orbit lay in the plane the ring 

 had occupied. A series of rings, separated in this way, gave rise 

 to the several planets in turn, while the central mass formed the 

 sun. The orbit of any planet bounds approximately the space 

 assigned to the nebula at the birth of that planet. At the time of 

 their formation, the several planets were thought to be hot, gaseous, 

 and rotating. Cooling and shrinkage increased the rate of their 

 rotation, and this caused a bulging of their equatorial zones, till 

 some of them, following the example of their parent body, shed rings 

 which became satellites. 



In support of this ingenious theory many harmonies in the 

 motions of the members of the solar system were cited, and in the 

 early days of the hypothesis, the phenomena of existing nebulae 

 were thought to give it much support, for among them, as then 

 known, there seemed to be nebulous aggregations in various stages 

 of development, from diffuse nebulous patches on the one hand, to 

 forms almost as concentrated as suns on the other. Critical 

 students of celestial mechanics have always recognized difficulties 

 in the hypothesis, but to those who did not follow the closer reason- 

 ing on which its truth or falsity must rest, the hypothesis seemed 

 to afford a plausible starting point for the history of the solar 

 system, and was accepted as if its foundations were firm. Without 

 a knowledge of celestial mechanics and of the molecular activities 

 of gases, it is scarcely possible to appreciate the full force of the 

 arguments which bear against it, but some of them may be stated 

 briefly. 



1. In the evolution of a gaseous nebula, it is highly improbable 

 that rings would be formed, for 4;he molecular activities would 

 cause the molecules to separate from the parent nebula one by one. 



2. There are grave difficulties in the contraction of a ring 



