304 CELESTIAL DYNAMICS. 



tion, powerful enough to melt worlds ; it teaches us to con- 

 sider the molten state of a planet as the result of the mechan- 

 ical union of cosmical masses, and thus derive the radiation 

 of the sun and the heat in the bowels of the earth from a 

 common origin. 



The rotatory effect of the earth also may be readily ex- 

 plained by the collision of its constituent parts ; and we must 

 accordingly subtract the vis viva of the axial rotation from 

 the whole effect of the collision and mechanical combination, 

 in order to obtain the quantity of heat generated. The rota- 

 tory effect, however, is only a small quantity in comparison 

 with the interior heat of the earth. It amounts to about 

 4400 X T kilogrammetres, T being the weight of the earth in 

 kilogrammes, which is equivalent to 12 XT units of heat, if 

 we assume the density of the earth to be uniform throughout. 



If we imagine the moon in the course of time, either in 

 consequence of the action of a resisting medium or from 

 some other cause, to unite herself with our earth, two princi- 

 pal effects are to be discerned. A result of the collision 

 would be, that the whole mass of the moon and the cold crust 

 of the earth would be raised some thousands of degrees in 

 temperature, and consequently the surface of the earth would 

 be converted into a fiery ocean. At the same time the velo- 

 city of the earth's axial rotation would be somewhat acceler- 

 ated, and the position of its axis with regard to the heavens, 

 and to its own surface, slightly altered. If the earth had 

 been a cold body without axial rotation, the process of its 

 combining with the moon would have imparted to it both 

 heat and rotation. 



It is probable that such processes of combination between 

 different parts of our globe may have repeatedly happened 

 before the earth attained its present magnitude, and that lux- 

 uriant vegetation may have at different times been buried un- 

 der the fiery debris resulting from the conflict of these masses. 



