NOTES. H 



crust above it," see Mallet, Meeting of Brit. Assoc. 1850, p. 20. Poisson also, 

 with whom I more than once conversed on the hypothesis of subterranean tides 

 caused by the moon and sun, regarded the impulse, which he did not deny, as 

 inconsiderable, "as in the open sea the effect hardly amounts to 15 inches." On 

 the other hand, Ampere said, " Ceux qui admettent la liquidity du noyau inte- 

 rieur de la terre paraissent ne pas avoir songe' assez a 1'action qu'exercerait la 

 lune sur cette enorme masse liquide, action d'ou re'sulteraient des mare'es ana- 

 logues k celles de nos mers, mais bien autrement terribles, tant par leur e'tendue 

 que par la densite' du liquide. II est difficile de concevoir, comment 1'enveloppe 

 de la terre pourrait resister, e"taut incessamment battue par une espece de belief 

 hydraulique (?) de 1400 lieues de longueur." (Ampere, The'orie de la Terre 

 in the Revue des deux Mondes, juillet, 1833, p. 148.) If the interior of the 

 Earth is fluid, as in general we cannot doubt is the case, the particles still re- 

 maining movable, notwithstanding the enormous pressure, then there will be in 

 the interior the same conditions which, on the surface, produce the tides of the 

 ocean, and the tide-causing force will become weaker in approaching the centre, 

 as the difference of distance, from every two opposite points, regarded relatively 

 to the attracting heavenly bodies, will become less the greater the depth below 

 the surface, and the force depends solely on the difference of the distances. If 

 the solid crust opposes resistance, the interior will at these places only exert a 

 pressure against the crust, but (as my astronomical friend, Dr. Brunnow, expresses 

 it) there will be no more tide than there would be if the ocean had an icy covering 

 which could not be burst open. The thickness of this solid unmolten crust has 

 been calculated from the melting point of different kinds of rock, and from the 

 increase of heat from the surface of the Earth downwards. I have already 

 (Kosmos, Bd. i. S. 27 and 48; English edition, p. 27 and Note 13) adduced 

 reasons in support of the conjecture that at about 201 geographical miles 

 below the surface the heat would attain the melting point of granite. Elie de 

 Beaumont (Geology, edited by Vogt, 1846, vol. i. p. 32) had made the very 

 similar estimate of 45,000 metres for the thickness of the solid crust. The in- 

 genious experiments of Bischof on the melting of different minerals, so important 

 to the progress of geology, would also lead to the assignment of a thickness of 

 between 115,000 and 128,000 French feet, or a mean of about 2l geographical 

 miles, for the thickness of the unmolten strata. See Bischof, Warmelehre des 

 Innern unsers Erdkorpers, S. 286 and 271. I am therefore the more surprised 

 to find that, assuming a definite limit between the solid and the molten materials 

 (not a gradual transition), Mr. Hopkins derives from the principles of his " Specu- 

 lative Geology " the result, that " the thickness of the solid shell cannot be less 

 than about one fourth or one fifth (?) of the radius of its external surface." 



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