ch. v.] PLANETARY EVOLUTION. 359 



lost by radiation as fast as it is thus generated, the contrac- 

 tion of the mass will go on unceasingly. It is in this way 

 that physicists now account for the internal heat of the sun 

 and the planets. A diminution of the sun's diameter by 

 the amount of twenty miles could not be detected by the 

 finest existing instruments ; yet the arrest of motion implied 

 in this slight contraction would generate enough heat to 

 maintain the present prodigious supply during fifty centuries. 

 And in similar wise the internal heat of the earth during a 

 given moment or epoch must be chiefly due to that very 

 contraction which the radiation of its heat during the 

 preceding moment or epoch has entailed. 



The generation of all this heat, therefore, which sun and 

 planets have from time immemorial been losing, implies the 

 transformation of an enormous quantity of molar motion of 

 contraction. It implies that from time immemorial the 

 various members of our planetary system have all been 

 decreasing in volume and increasing in density ; so that the 

 farther back in time we go, the larger and less solid must we 

 suppose them to have been. This is an inevitable corollary 

 from the companion laws that contracting bodies evolve heat, 

 and that radiating bodies contract. 



Obviously, therefore, if we were to go back far enough, we 

 should find the earth filling the moon's orbit, 1 so that the 

 matter now composing the moon would then have formed a 

 part of the equatorial zone of the earth. At a period still 

 more remote, the earth itself must have formed a tiny portion 

 of the equatorial zone of the sun, which then filled the 

 earth's orbit. At a still earlier date, the entire solar system 

 must have consisted simply of the sun, which, more than 



1 It is not presumed, however, that the moon's orbit was originally so large 

 us at present. For by its tidal action upon our oceans the moun exerts a drag 

 apon the earth's rotation, and the motion thus lost by the earth is added to 

 the moon's tangential momentum, thus increasing the dimensions of its orbit. 

 A precisely similar (jualiiication is needed for the two next-succeeding state- 

 ments in the text. 



