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2. A Letter to Sir John W. Lubbock, Bart., F.R.S. &c., On 

 the Stability of the Earth's Axis of Rotation." By Henry Hennessy, 

 Esq., M.R.I. A. &c. Communicated by Sir John Lubbock. Re- 

 ceived November 20, 1851. 



The author refers to a communication to the Geological Society 

 by Sir John Lubbock, in which he appeals, in support of the possi- 

 bility of a change in the earth's axis, to the influence of two disturb- 

 ing causes, which appear to have almost entirely escaped the notice 

 of Laplace and Poisson in their investigations on the stability of the 

 earth's axis of 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 

 accordance with mechanical and physical laws, he has deduced re- 

 sults respecting the earth's structure which throw some light on 

 the changes which may take place in the relation between its prin- 

 cipal 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 pe- 

 culiar views respecting the theory of the earth's figure, in the expe- 

 riments 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 

 mathematical processes by which he arrived at this result, he states 

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

 solidification 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 

 formed. 



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

 the diff'erence between the greatest and least moment of inertia of 

 the earth would progressively increase during the process of solidifi- 

 cation. 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 several geological formations. Thus it appears that the displace- 

 ment 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. 



