540 Royal Society : — Mr. J. Evans on a possible Cause of 



Those who wish to see the arguments for and against the theory of 

 there being a fluid nucleus within the earth's crust, will find them 

 well and fairly stated in Naumann's 'Lehrbuch der Geognosie'*. 

 My object is, not to discuss that question, but to point out what, 

 assuming the theory to be true, would be some of the effects result- 

 ing from such a condition of things, more especially as affecting cli- 

 matal changes. The agreement or disagreement between these hy- 

 pothetical results and observed facts may ultimately assist in testing 

 the truth of the assumption. 



The simplest form in which we can conceive of the relations to 

 each other of a solid crust and a fluid nucleus in rotation together 

 is that of a sphere. 



Let ACBD be a hollow sphere composed of solid materials 

 and of perfectly uniform thickness and density, and let it be filled 

 with the fluid matter E, over which the solid shell can freely move, 

 and let the whole be in uniform rotation about an axis F G, the line 

 C D representing the equator. It is evident that in such a case, the 



hollow sphere being in perfect equilibrium, its axis and that of its 

 fluid contents would perpetually coincide. If, however, the equili- 

 brium of the shell or crust be destroyed, as, for instance, by the 

 addition of a mass of extraneous matter at H, midway between 

 the pole and the equator, not only would the position of the axis of 

 rotation be slightly affected by the alteration in the position of 

 the centre of gravity of the now irregular sphere, but the centrifu- 

 gal force of the excess of matter at H would gradually draw over 

 the shell towards D until, by sliding over the nucleus, it attained its 

 greatest possible distance from the centre of revolution by arriving 

 at the equator. The resultant effect would be that though the 

 whole sphere continued to revolve around an axis as nearly as pos- 

 sible in the line F G, yet the position of the pole of the hollow 

 shell would have been changed by 45°, as by the passage of H to 



* 2nd edit., 1858, vol. i. p. 36. 



