ASTRONOMICAL CAUSES. 415 



the laws of nature is evident from the fact, that 

 the obliquity of the ecliptic has been constantly 

 diminishing for the last 4000 years. 



It has been often asserted by writers on astro- 

 nomy, that great changes in the planetary incli- 

 nations would be inconsistent with the stability of 

 thesystem, as if all the operations of the universe 

 were not maintained by unceasing changes and 

 revolutions. But as Mr. Whewell observes, 

 " the cause of perturbation has the whole extent 

 of time to work in, " there is reason to believe, 

 that in the progress of long astronomical cycles, 

 every part of the earth has successively been 

 exposed to a vertical sun, and that such changes 

 are absolutely essential to the conservation of 

 the system in its existing state. For if the equa- 

 torial protuberance of the planets be owing to 

 the greater amount of light which falls upon the 

 tropics than upon the higher latitudes, it is ob- 

 vious that they would in time lose their present 

 spherical forms, by which their rotary motions 

 would be deranged, unless prevented by a gra- 

 dual transposition of the equator to higher lati- 

 tudes, or even to the polar circles. And that 

 such a transposition has actually taken place, is 

 strongly corroborated by the high and uniform 

 temperature which prevailed throughout the 

 middle and higher latitudes of the northern he- 

 misphere when it was the abode of tropical plants 

 and animals. 



