PHYSICAL ASTRONOMY. 321 



sent form, by the wearing down of the more pro- 

 minent parts, and their subsequent consolidation 

 in the form of horizontal strata, the axis of the 

 Earth's rotation may have been very different 

 from what it now is ; it may have gone through a 

 long series of changes, and may have carried the 

 equator, and the accumulation of waters which 

 accompanied it, over regions from which they are 

 now far distant. 



Many facts in the natural history of the Earth, and 

 of the mineral kingdom, give countenance to these 

 suppositions ; and if it be true that the more an- 

 cient strata have been set on edge, and that conti- 

 nents have been raised up by the action of an ex- 

 pansive force in the interior of the Earth, such ac- 

 tion may have materially assisted in changing the 

 position of the Earth's axis. 



Obliquity of the Ecliptic. 



3%5. The position of the ecliptic is subject to 

 change by the action of the planets ; each of 

 them producing a retrograde motion in the in- 

 tersection of the plane of the ecliptic with the 

 plane of its own orbit. This does not affect the 

 inclination of these two planes, nor does it af- 

 fect the plane of the equator, but it neverthe- 

 VOL. II. X less 



