Evans — A Possible Cause of Climatal Changes. 173 



instead of the addition of fresh matter, a portion of the shell be 

 removed, a movement the reverse of that caused by the addition 

 of matter would result, and that portion of the shell so excavated 

 would eventually find its way to the Pole. In order more clearly 

 to exhibit these effects the author had prepared a very ingenious 

 model, suggested by Mr. Francis Galton, F.E.S., consisting of a 

 wheel, having its axis in a frame, which is itself made to revolve 

 with rapidity, in such a manner that the axis of its rotation passes 

 through one of the diameters of the wheel, or what would be the 

 axis of the sphere of which it a section. A series of screws, with 

 heavy heads, are inserted around its rim ; by screwing any of these 

 in or out, the addition or abstraction of matter at any part of the 

 sphere can be represented. With the screws on the wheel evenly 

 balanced, a slight alteration in the adjiistment of any of them, 

 except at the Poles or the Equator, immediately tells upon the 

 position of what may be called the Polar axis. [A number of ex- 

 periments which were made with this model fully testified the con- 

 stancy of this rule]. 



Assuming that the earth, instead of being a spheroid, was a perfect 

 sphere, consisting of a hardened crust of moderate thickness, sup- 

 ported on a fluid nucleus, over which the crust could travel freely in 

 any direction, but both impressed with the same original motion and 

 revolving upon the same axis ; assuming, moreover, that the surface 

 had certain projecting portions, represented in nature by continents 

 and islands, rising above the level of the sea, it is evident that so 

 long as these remained unchanged, the position of the crust to the 

 fluid nucleus would remain unaltered also. But the further up- 

 heaval or the wearing away or depression of any land-surface would 

 result in a corresponding change in the axis of rotation, as seen in 

 the experiments with the model. 



The author contended that if all this be true of a sphere, it will 

 also be true in a modified degTce of a spheroid so slightly oblate as 

 our globe. In the case of a spheroid every portion of its internal 

 structure would, however, be more or less disturbed in proportion to 

 the thickness and rigidity of the crust. The author next alluded to 

 the ingenious speculations upon this subject by Mr. Hopkins, and 

 suggested that if once the possibility of a change in the axis of the 

 earth's crust be admitted, the value of the data upon which his calcu 

 lations are based will be materially aff'ected. 



Taking the increase of heat as 1° Pahr. for every 55 or 60 feet in 

 descent, a temperature of 2,400° Fahr. would be reached at a depth 

 of about 25 miles, sufficient to keep in fusion such rocks as 

 Basalt, Greenstone, and Porphyry ; and such a thickness appears 

 much more consistent with the fluctuations in level and the internal 

 contortions and fracture of the crust everywhere to be observed. 

 With a crust 50 miles in thickness instead of 25 miles, and the 

 present elevations of land, it seems probable that all the foregoiag 

 experiments would hold good. Without undervaluing other causes 

 of climatal changes, Mr. Evans thought that possibly his hypothesis 

 might, if accepted, satisfactorily explain the extreme variations from 



