294 Mr. 0. Fisher — Changes of Latitude on the Earth's 8urjace. 



and afterwards to solidify, in tlie superficial portions of the mass : 

 whereas, if the effect of the augmentation of pressure predominate 

 over that of temperature, this transition from perfect to imperfect 

 fluidity, and consequent solidity, will commence at the centre." 



Assuming the latter to be the case, when the mass should have 

 arrived at that stage of cooling that "a solid nucleus had been 

 formed, surrounded by an external portion of which the fluidity 

 would vary continuously from the solidity of the nucleus to the 

 fluidity of the surface, where, at the instant we are speaking of, it 

 would be just such as not to admit of circulation ; . . . a change 

 would take place in the process of solidification which it is important 

 to remark. The superficial parts of the mass must in all cases cool 

 the most rapidly, and now (in consequence of the imperfect fluidity) 

 being no longer able to descend, a crust will be formed on the 

 surface, from which the process of solidification will proceed far 

 more rapidly downwards, than upwards on the solid nucleus." And 

 in a note to the Eeport,^ he writes thus decidedly : " Supposing the 

 earth once to have been fluid, it must be now, or have been at some 

 antecedent epoch, in that state in which a solid exterior rests on an 

 imperfectly fluid and incandescent mass beneath. It is important 

 to know that this state of the earth, assuming its original fluidity, 

 is one through which it must necessarily have passed in the course 

 of its refrigeration, whatever might be the process of its solidifi- 

 cation." 



These remarks of Mr. Hopkins deserve serious consideration, 

 and are not lightly to be set aside. They are entirely independent 

 of his subsequent calculations by which he considered he had proved 

 (and was until very lately indeed considered by the greatest mathe- 

 maticians to have proved) ^ that this crust was not at the present 

 time a thin one, but had grown to the thickness of at least not far 

 short of a thousand miles ; even if its downward growth had not 

 already met the upward growth of the solid central nucleus. 



The reasoning by which Mr. Hopkins concluded that a crust 

 would form (and he clearly supposed it would be supported also), 

 seems to be assailable only on the supposition that upon solidifi- 

 cation a sudden considerable contraction, and consequent increase of 

 density, would occur, which would enable the fragments to sink 

 in a fluid, too viscous to admit of the sinking of portions cooled to 

 the verge of solidification. 



It is practically seen that a crust can be formed and supported 

 upon liquid lava by what is known to happen in the crater of 

 Kilauea. This is of an elliptical form, three miles in its longer 

 diameter : consequently the attachment to the cliff-like sides can 

 have little effect in supporting the crust over so large an area. 

 Moreover, this has been observed to rise and sink with variations 

 in the level of the lava, showing that it really rests upon the fluid 

 support. "The floor of the pit is described as usually presenting 

 a crust over a vast pool of lava, which is from time time broken 

 through by a fresh upboiling of the incandescent mass beneath. 

 ' Page 48. ^ Vide supra, p. 292. 



