Nature of the Ear tJis Interior. 169 



for the advance of this inquiry, M. Delannay, now Director of the 

 Observatory of Paris, an authority equally eminent as a mathema- 

 tician and astronomer, was induced to undertake the reconsideration 

 of this problem ; a labour which has not only resulted in his having 

 altogether reversed the above decision by demonstrating the com- 

 plete fallacy of the premises upon which so much elaborate reasoning 

 had been based, but which proved conclusively by experiment that 

 a sphere filled with liquid matter would, under circumstances such as 

 are present in the case of the earth, behave in precisely the same 

 manner as an entirely solid one, and consequently that the fact of 

 the earth being either solid or liquid in its interior could neither 

 have any influence whatsoever upon the rates of precession and 

 nutation, nor be of any use as a means of deciding as to the real or 

 approximate thickness of the earth's crust. 



It may be remarked, however, that the conclusions arrived at by 

 Mr. Hopkins, even when supported by Sir William Thomson and 

 Archdeacon Pratt, were not universally acquiesced in ; the cele- 

 brated German physicist Helmholtz for example, amongst others, 

 was not satisfied as to their correctness ; and in direct opposition to 

 the deduction of Sir William Thomson that the earth's crust must be 

 some 1000 miles in thickness, we have the conclusions of Mr. 

 Henessy, whose calculations show that it cannot be more than 600 

 miles or less than 18 miles in thickness. We may conclude, therefore, 

 that all the objections as yet advanced from an astronomical point 

 of view against the theory of the fluid condition of the inteiior of 

 our planet have been invalidated or explained away. 



The only other argument in favour of internal solidity is one 

 which bases itself upon the law announced, from purely theoretical 

 considerations, by Professor Thomson in 1849, that the fusing 

 points of bodies become more elevated when subjected to pressure, 

 or in other words, that under the influence of pressure, bodies will 

 require more heat to melt them or keep them in the molten state. 



Starting from this, Bunsen argued that the earth could not be other 

 than solid to the core, since, according to him, the enormous pressure 

 accumulated at its centre would render its internal substance so 

 infusible that it could not possibly remain in a molten state. To a 

 certain extent this law was corroborated by the experimental re- 

 searches of Bunsen and Hopkins made upon some of the very easily 

 fusible substances, such as wax, spermaceti, parrafine, and sulphur ; 

 but later experiments did not appear to confirm it in the case of 

 metallic substances, nor did it appear to hold true in other than the 

 more compressible bodies. 



In the case of the earth, therefore, the conclusions of Bunsen can- 

 not be warranted or accepted, since we have to deal with materials 

 to which this law has not as yet been even proved to apply ; still, 

 assuming, as seems most probable, that the materials composing the 

 earth's mass do become to a certain extent more and more infusible, 

 according as they approach nearer to its centre, it must on the other 

 hand be remembered that this effect would be at the same time more 

 or less neutralized by the expansion which these substances would 

 undergo from the action of the earth's internal heat, since incontro- 



