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



earth at the Glacial Period is constantly receiving fresh corroboration ; 

 and various theories have been proposed which account for this acces- 

 sion of cold in a more or less satisfactory manner. 



Variations in the distribution of land and water, changes in the 

 direction of the Gulf-stream, the greater or less eccentricity of the 

 earth's orbit, the passage. of the Solar System through a cold region 

 in space, fluctuations in the amount of heat radiated by the sun, 

 alternations of heat and cold in the northern and southern hemi- 

 spheres, as consequent upon the precession of the equinoxes, and even 

 changes in the position of the centre of gravity of the earth, and con- 

 sequent displacements of the polar axis, have all been adduced as 

 causes calculated to produce the effects observed ; and the reasoning 

 founded on each of these data is no doubt familiar to all. 



The possibility of any material change in the axis of rotation of 

 the earth has been so distinctly denied by Laplace* and all succeeding 

 astronomers, that any theory involving such a change, however 

 tempting as affording a solution of certain difficulties, has been re- 

 jected by nearly all geologists as untenable. 



Sir Henry James f, however, writing to the c Athenaeum ' news- 

 paper in 1860, stated that he had long since arrived at the conclusion 

 that there was no possible explanation of some of the geological phe- 

 nomena testifying to the climate at certain spots having greatly varied 

 at different periods, without the supposition of constant changes in 

 the position of the axis of the earth's rotation. He then, assuming 

 as an admitted fact that the earth is at present a fluid mass with a 

 hardened crust, showed that slaty cleavage, dislocations, and undu- 

 lations in the various strata are results which might be expected from 

 the crust of the earth having to assume a new external form, if caused 

 to revolve on a new axis, and advanced the theory that the elevation 

 of mountain-chains of larger extent than at present known produced 

 these changes in the position of the poles. 



The subject was discussed in further letters from Sir Henry James, 

 the Astronomer Royal, Professors Beete Jukes and Hennessy, and 

 others ; but throughout the discussion the principal question at issue 

 seems to have been whether any elevation of a mountain-mass could 

 sensibly affect the position of the axis of rotation of the globe as a 

 whole ; and the general verdict was in the negative. 



At an earlier period (1848) the late Sir John Lubbock, in a short 

 but conclusive paper in the * Quarterly Journal of the Geological 

 Society ' J pointed out what would have been the effect had the axis 

 of rotation of the earth not originally corresponded with the axis of 

 figure, and also mentioned some considerations which appear to have 

 been absent from Laplace's calculations. 



Sir John Lubbock, however, in common with other astronomers, 

 appears to have regarded the earth as consisting of a solid nucleus 

 with a body of water distributed over a portion of its surface ; and 

 there can be but little doubt that, on this assumption of the solidity 

 of the earth, the usually received doctrines as to the general per- 

 sistence of the direction of the poles are almost unassailable. 



* Mecanique Celeste, vol. v. p. 14. t Athenasum, Aug. 25, 1860, &c. 



+ Vol. v. p. 5. 



