C^' 



^: 



204 



BATE OF HEAT INCEEASING WITH DEPTH. [Cn. XXXII. 



earth to be constituted as above stated ; i. e. fluid internally, 

 and enveloped by a solid shell ; and lie finds that the amount 

 v^ill not agree with the observed motion, unless the crust of 

 the earth be of a certain thickness. In calculating the exact 

 amount, some ambiguity arises in consequence of our ignorance 

 of the effect of pressure in promoting the solidification of 

 matter at high temperatures. The hypothesis least favourable 

 for a great thickness is found to be that which assumes the 

 pressure to produce no effect on the process of solidification. 

 Even on this extreme assumption, the thickness of the solid 

 crust must be nearly four hundred miles^ and this would lead 

 to the remarkable result that the proportion of the solid to 

 the fluid part would be as 49 to 51, or, to speak in round 

 numbers, there would be nearly as much solid as fluid matter 

 in the globe. The conclusion, however, which Mr. Hopkins 

 announces as that to which his researches have finally con- 

 ducted him, is thus expressed : '^ Upon the whole, then, we 

 may venture to assert that the minimum thickness of the 

 crust of the globe, which can be deemed consistent Avith the 

 observed amount of precession, cannot be less than one- fourth 



or one-fifth of the earth's radius ; ^ that is, from 800 to 1,000 

 miles.*^ 



It will be remarked, that this is a minimum^ and any still 

 greater amount would be quite consistent with the actual 

 jjhenomena ; the calculations not being opposed to the sup- 

 position of the general solidity of the entire globe. Nor do 



miles 



Bclude us from imagining that gr( 

 matter mav be distributed throne 



motion of rotation may be communi 



disturbing forces of the sun and moon. 



/ 



increasing with depth. — The hypothesis of 



internal fluidity calls for the more attentive consideration 



■& 



ments 



portion as we descend. Observations have been made, not 



mines 



Phil. Trans. 1839, and Ecsenrclies Phenomena and Theory of Voleanos, 

 in Physical Geology, 1st, 2nd, and 3rd Eeport Brit. Assoc. 1847. 

 series, London, 1839—1842 : also on 



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