Royal Society. 387 



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 heen 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 difference 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. 



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

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

 could not exist, if, as in the manner here shown, 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 

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

 be as well assured of the stability of our system, with respect to the 

 motion of rotation of its several members, as we are already respect- 

 ing their motion of translation. 



In a postscript, referring to a third cause of disturbance in the place 

 of the earth's axis of rotation, suggested in a letter from Sir John 

 Lubbock, namely, the effects of local elevation and depressions at 

 the earth's surface, the author states ; if, with Humboldt, we re- 

 gard the numbers expressing the mean heights of the several conti- 

 nents as indicators of the plutonic forces by which they have been 

 upheaved, we shall readily see that these forces are of an inferior 

 order to those affecting the general forms and structure of the earth. 

 If the second class of forces acted so as not to influence in any way 

 the stability of the earth's axis of rotation, the former class might, 

 under certain conditions, produce a sensible change in the position 

 of the axis. But when the tendency of the second class of forces is 

 to increase the stability of the earth's axis, it would not be easy to 

 show the possibility of such conditions as to render the operation of 

 the other forces, not only effective in counteracting that tendency, 

 2C2 



