PLANETARY EVOLUTION 



ward the common centre have their molar mo- 

 tions constantly opposed by friction upon each 

 other, and most of the motion thus arrested is 

 converted into heat. If this heat is lost by radi- 

 ation 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 dur- 

 ing 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 imme- 

 morial been losing, implies the transformation 

 of an enormous quantity of molar motion of 

 contraction. It implies that from time imme- 

 morial 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 inevi- 

 table corollary from the companion laws thatcon- 



*S3 



