ORIGIN AND DEVELOPMENT OF THE EARTH 313 



weaker than in large ones, it is clear that small bodies are less 

 likely to hold an atmosphere than large ones. Thus the moon 

 seems to be quite devoid of an atmosphere to-day. If the 

 earth had gravity enough to hold an atmosphere at the outset, 



FIG. 316. Spiral nebula. 



(Photograph by 

 Observatory.) 



Ritchey at the Yerkes 



it probably had one. If its gravity was not sufficient at first, 

 it eventually became so, as the earth grew ; and so the planet 

 slowly acquired an atmosphere and, later, oceans. The 

 method by which the atmospheric gases were supplied is 

 simple, and doubtless continued in a measure down to the 



