1909.] SEE— THE PAST HISTORY OF THE EARTH. 125 



This simple expression shows that a change of a given percentage 

 in M produces a contrary change half as large in t. In other words, 

 if the sun's mass be increased by one per cent., the length of the 

 year will thereby be decreased by two per cent. Thus in the lapse 

 of ages the augmentation of the sun's mass may have shortened 

 the periods of the planets very materially; and this would slightly 

 decrease their mean distances, as in the case of the resisting 

 medium. Nevertheless, a gradual change in the sun's mass would 

 not affect the eccentricity as it does the major axis. 



Accordingly the small size and round form of the planetary 

 orbits must be explained mainly by the secular effects of the resist- 

 ing medium formerly pervading our system. And as the earth has 

 been formed by accretion, and not at all by detachment from the 

 sun, as supposed by Laplace, it follows that the matter of the globe 

 is essentially of the same character throughout. For we have else- 

 where shown that friction and resistance to motion in the body of 

 our globe would prevent the heavier elements from separating 

 from the lighter ones. So that the old theories which ascribe an 

 iron nucleus to the earth must be given up as unjustifiable and mis- 

 leading. And the increase of density, rigidity, and temperature 

 towards the center is due principally to the pressure of the super- 

 incumbent matter upon the layers confined within. It is this pres- 

 sure which gives the globe its great effective rigidity. If the pres- 

 sure were relieved, the imprisoned matter, which now behaves as 

 solid, would expand as vapor, owing to the high temperature still 

 existing within the globe. 



U. S. Naval Observatory, 

 Mare Island, California, 

 April 5, 1909. 



