Physics and Mathematics to Geology. 



345 



Tresca's experiments and the state of a body subjected to 

 nearly uniform pressure all round. 



Mr. Blytt apparently does not stand alone in believing 

 Sir W. Thomson to hold that the solid earth is incapable 

 of altering its form as the rotation alters, and that it possesses 

 the same eccentricity as when it solidified. Professor Darwin 

 in 'Nature,' vol. xxxiv. (188(3), pp. 420-3, seems also to put 

 this interpretation upon a passage he quotes from § 830 of 

 Thomson and Tait's ' Natural Philosophy.' Supposing this 

 interpretation correct, Professor Darwin's opinion that Sir 

 W. Thomson does not allow " a sufficient margin for un- 

 certainties " expresses only a part of the objections I should 

 entertain. I find it difficult, however, to believe that Sir W. 

 Thomson, who elsewhere gives data for the eccentricity pro- 

 duced by rotation in solid spheres of steel, can actually 

 suppose no change at all in the eccentricity to follow an 

 alteration in the angular velocity. Still it must be confessed 

 that though the passage contains the statement, " It must 

 necessarily remain uncertain whether the earth would from 

 time to time adjust itself completely to a figure of equilibrium 

 adapted to the rotation," its most natural interpretation is that 

 given by Professor Darwin. I need hardly say that the con- 

 clusion that the earth, however solid, would retain a constant 

 eccentricity while the rate of rotation varied, seems to me 

 directly opposed to the conclusions to which the elastic solid 

 theory leads. 



Professor Darwin himself, in his paper in i Nature/ refers 

 to Tresca's experiments and thinks it probable there would 

 be from time to time a flow of material as the angular velocity 

 altered. One of the " uncertainties " he refers to is the possi- 

 bility that, in accordance with Dr. Croll's* views, a greater 

 rapidity of denudation in equatorial than in polar regions may 

 have reduced the eccentricity markedly below the value it pos- 

 sessed when the earth solidified. He does not seem, however, 

 to refer to the considerable change of eccentricity that might 

 occur in a solid through mere variation of elastic strain. 



As regards the present state of the earth's interior there 

 are, according to Greikie's ' Text-book,' p. 49, only three theories 

 which merit serious consideration, viz. : — 



(1) That there is a solid crust and a molten interior. 



(2) That with the exception of local vesicular spaces the 



earth is perfectly solid. 



(3) That the earth consists of a solid crust and nucleus 



with an intervening liquid layer. 



Climate and Time ' (1885), p. 336. 



