172 Evans — A Possible Cause of Climatal Changes. 



assumption the doctrine of the persistence of the direction of the poles 

 seems almost unassailable. 



Assuming, however, that the solid portion of the globe consists of a 

 comparatively thin but rigid crust, with a fluid nucleus of incan- 

 descent mineral matter within, and that this crust from various 

 causes is liable to changes disturbing its equilibrium, such disturb- 

 ances may lead, if not to a change in the position of the general 

 axis of the globe, yet at all events to a change in the relative 

 positions of the solid crust and fluid nucleus, and in consequence to 

 a change in the axis of rotation so far as the former is concerned. 



The author admitted that the existence of a central mass of matter 

 fluid by heat, although accepted by most geologists, had been called 

 in question by some, and among them a few of great eminence. 



The gradual increase of temperature observed all over the world 

 in descending beneath the surface of the earth in mines, and deep 

 borings, the existence of hot springs, and the traces of volcanic 

 action, either extinct or still in operation, strongly support the 

 hypothesis of central heat. 



The fact of the increment of heat at different depths from the 

 surface seems so well established, it would appear that at a certain 

 point such a degree of heat must be attained as would reduce all 

 mineral matter with which we are acquainted to a state of fusion. 

 Beyond this point probably no great variation in temperature takes 

 place. Those who regard the globe as a solid, or nearly solid mass, 

 consider that many volcanic phenomena may be accounted for upon 

 the chemical theory. But the existence of local subterranean seas of 

 fluid matter would hardly account for the increase of heat at great 

 depths in places remote from volcanic centres, nor for the rapid 

 transmission of earthquake-shocks ; whilst the enormous amount of 

 upheaval and subsidence of the sedimentary strata seem inconsistent 

 with the general solidity of the globe, or any very great thickness of 

 its crust. 



The authoi*illustrated his theory by a diagram of a sphere as the 

 simplest form in which to consider the relations to each other of a 

 solid crust and a fluid nucleus in rotation together. Let the crust of 

 the sphere be composed of solid material of uniform thickness and 

 density, and the interior filled with fluid matter, over which the 

 solid shell can freely move, and the whole be in uniform rotation 

 upon an axis, the hollow sphere being in perfect equilibrium, its 

 axis, and that of its fluid contents, would perpetually coincide. If, 

 however, the equilibrium of the shell or crust be destroyed by the 

 addition of a mass of extraneous matter, midway between the Pole 

 and the Equator, the centrifugal force of the mass of matter so 

 added would gradually draw over the shell towards the Equator ; 

 thus, though the whole sphere continued to revolve around its 

 original axis, yet the position of the Pole of the hollow shell would 

 be changed by 45° and the whole surface would have shifted 

 from its original position to the same extent. The axis of the 

 hollow sphere and of its fluid contents would again coincide, and 

 would continue to do so until a fresh disturbance took place. If, 



