178 Stability of the Earth's Axis of Rotation. 



rotation: — 1. The necessary displacement of the earth's interior 

 strata, arising from chemical and physical actions during the process 

 of solidification. 2. The friction of the resisting medium in which 

 the earth is supposed to move. 



With reference to the first of these disturbing causes, the author 

 states, that in his Researches in Terrestrial Physics (Philosophical 

 Transactions, 1851, Part 2), he has been led to conclusions which 

 may assist in clearing up the question. From an inquiry into the 

 process of the earth's solidification, which appears to him most in ac- 

 cordance with mechanical and physical laws, he has deduced results re- 

 specting the earth's structure, which throws some light on the changes 

 which may take place in the relation between its principal moments 

 of inertia, which relation is capable of being expressed by means of 

 a function which depends on the arrangement of the earth's interior 

 strata. 



He then states that he has found strong confirmation of his pecu- 

 liar views respecting the theory of the earth's figure, in the experi- 

 ments of Professor Bischof of Bonn, on the contraction of granite 

 and other rocks in passing from the fluid to the solid crystalline state. 

 From the results of these experiments, he has been led to assign a 

 new form to the function, expressing the relation of the earth's 

 principal moments of inertia. Referring to his paper for the mathe- 

 matical processes by which he arrived at this result, he states that, 

 from the theory he has ventured to adopt, it follows that, as solidifi- 

 cation advances, the strata of equal pressure in the fluid spheroidal 

 nucleus of the earth, acquire increased ellipticity, and each stratum 

 of equal density, successively added to the inner surface of the solid 

 crust, is more oblate than the solid strata previously found. 



From these considerations alone, he remarks, it is evident that the 

 difference between the greatest and least moment of inertia of the 

 earth would progressively increase during the process of solidifica- 

 tion. It follows, therefore, that if the earth's axis of rotation were 

 at any time stable, it would continue so for ever. But, from the 

 laws of fluid equilibrium, the axis must have been stable at the epoch 

 of the first formation of the earth's crust; consequently, it continued 

 undisturbed as the thickness of the crust increased during the se- 

 veral geological formations. Thus it appears that the displacement 

 of the earth's interior strata, instead of having a tendency to change 

 its axis of rotation, tends to increase the stability of that axis. 



With reference to inequalities arising from the friction of a re- 

 sisting medium at the earth's surface, the author observes that they 

 could not exist, if, as in the manner here shewn, the axis of rota- 

 tion coincided from the origin with the axis of figure. 



In conclusion, he remarks, that if we could assume for the planets 

 a similarity of physical constitution to that of the earth, the theorem 

 as to the difference of the greatest and least moments of inertia of 

 tho earth would be applicable to all the planets ; and thus we should 



