Sxrr. X. AXIS OP ROTATION INVARIABLE. 71 



and extensive archipelago throughout the greater part 

 of the northern hemisphere. Mr. Lyell is therefore of 

 opinion that the climate of these islands must have 

 been sufficiently mild in consequence of the surrounding 

 ocean to clothe them with tropical plants, and render 

 them a fit abode for the huge animals whose fossil 

 remains are so often found. That the arborescent ferns 

 and the palms of these regions, carried by streams to 

 the bottom of the ocean, were imbedded in the strata 

 which were by degrees heaved up by the subterranean 

 fires during a long succession of ages, till the greater 

 part of the northern hemisphere became dry land as it 

 now is, and that the consequence has been a continual 

 decrease of temperature. 



It is evident from the marine shells found on the tops 

 of the highest mountains and in almost every part of 

 the globe, that immense continents have been elevated 

 above the ocean, which must have ingulfed others. 

 Such a catastrophe would be occasioned by a variation 

 in the position of the axis of rotation on the surface of 

 the earth ; for the seas tending to a new equator would 

 leave some portions of the globe and overwhelm others. 

 Now, it is found by the laws of mechanics that in every 

 body, be its form or density what it may, there are at 

 least three axes at right angles to each other, round 

 any one of which, if the solid begins to rotate, it will 

 continue to revolve forever, provided it be not disturbed 

 by a foreign cause, but that the rotation about any 

 other axis will only be for an instant, and consequently 

 the poles or extremities of the instantaneous axis of 

 rotation would perpetually change their position on the 

 surface of the body. In an ellipsoid of revolution the 

 polar diameter and every diameter in the plane of the 

 equator are the only permanent axes of rotation (N. 

 142). Hence if the ellipsoid were to begin to revolve 

 about any diameter between the pole and the equator, 

 the motion would be so unstable that the axis of rota- 

 tion and the position of the poles would change every 

 instant. Therefore as the earth does not differ much 

 from this figure, if it did not turn round one of its prin- 

 cipal axes, the position of the poles would change daily ; 

 the equator, which is 90 distant, would undergo cor- 



