﻿184 Rejjorts and Proceedings — 



approximate numbers seem to be sufficiently exact for every useful purpose. The 

 conclusions arrived at are as follows : — 



(1) The displacement of the earth's axis of figure from the axis of rotation that 

 would be effected by the elevations and depressions suggested in the question above 

 referred to, would be less than 10' of angle. 



(2) A displacement of as much as 20"^ could he effected by the elevations and 

 depressions of the kind suggested only if their heights and depths exceeded by many 

 times the height of the highest mountains. 



(3) Under no circumstances could a displacement of 20° be effected by a transfer of 

 matter of less amount than about a sixth part of the whole equatorial bulge. 



(4) Even if a transfer of this quantity of matter were to take place, it need not 

 produce any effect, or only a small effect, on the position of the axis of figure, e.g. if 

 it took place in a way resembling that suggested in the question, it would produce a 

 displacement amounting to but a small part of 20°. 



(5) If, however, we suppose a deviation of the axis of figure from the axis of 

 rotation amounting to as much as 20° to have been by any means brought about, the 

 effect would be to cause a sort of tidal motion in the ocean, the greatest height of 

 which woiild tend to be about twice the depth of the ocean. The author suggests as 

 probable that the effect of this tendency would be to cause the ocean to sweep over 

 the continents in much the same way that a rising tide sweeps over a low bank on a 

 level shore. 



(6) The notion that a large deviation of the earth's axis of figure from its axis of 

 revolution maybe effected by elevations and accompanying depressions is at first sight 

 an inviting way of bringing polar lands into lower latitudes, and thereby accounting 

 for the more genial climate that is believed to have once prevailed in such countries as 

 Greenland. The investigation by which the above results have been obtained seems 

 to show that the desired explanation is not to be sought in the direction indicated by 

 Mr. Evans's question. Whether there is any other agency by which a gradual 

 displacement of the pole geographically could be effected is a question of far Avider 

 scope than that discussed in the present paper, and one which the author does not 

 profess to determine.' 



Discussion. — Mr. Evans was willing to admit that in his Address he had some- 

 what overstated the amount of change in the position of the polar axis which was 

 likely to result from the supposed belts of elevation and depression. When, however, 

 be was told that the displacement would not exceed ten miles, notwithstanding his 

 implicit faith in mathematics, there arose an inward feeling of disbelief as to the 

 conditions of the problem having been accurately stated in order to obtain such a 

 result. It seemed to him that the author had treated the globe as an absolutely solid 

 spheroid instead of a terraqueous globe, with the proportions of land and sea upon its 

 surface as at present existing, which were important elements in the case. 



The depth of the ocean in equatorial and polar regions ought surely to be taken 

 into account, as it was quite possible to conceive of the irregularly-shaped solid 

 portion of the globe projecting in places through a spheroidal coating of water, so as 

 to form large tracts of land, and yet on the average forming a sphere. Such a sphere, 

 fi-om disturbances of its equilibrium, he believed would be much more liable to 

 changes in its axis than a spheroid, and the nearer a spheroid approached a sphere, 

 the more sensible it Avould become to such disturbances. 



He had never intended to suggest that the hypothetical belt was to be suddenly 

 elevated so as to produce the enormous tidal movements of which the author spoke. 

 On the contrary, he believed that all such disturbances of equilibrium were gradual, 

 and that the axis of rotation and that of figure were never at any great distance from 

 each other. There was one portion of the paper which he found diflicult to com- 

 prehend. He could not conceive why so enormous a protuberance as 125 miles over 

 a belt 20° in width should be necessary in order to displace the polar axis by 20°, 

 when the present equatorial protuberance was only about one-tenth of that height. 



Moreover, the probability is that the earth, instead of being a rigid solid, is to_a 

 certain extent viscous or plastic, and that such should be the case seems quite in 

 accordance with geological facts. If the globe were a viscous body, with a solid crust 



1 The flr8t drausht of the paper, of which the above is an aoconnt, was drawn up last August, 

 and was shortly after sent to Mr. Evans. It was written independently of the wider view of the 

 subject taken by Sir \V. Thomson in his Address delivered at the last Meeting of the British 

 Association, and by Mr. G. Uarwin in his paper, of which an abstract has been published in 

 No. 175 of the Proceedings of the Koyal Society. 



