PHYSICS AND MATHEMATICS TO GEOLOGY. 14% 
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, accord- 
ing to Geikie’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 per- 
fectly solid. 
(3) That the earth consists of a solid crust and nucleus with an in- 
tervening liquid layer. 
According to the Text-book, the theory of a thin crust containing 
liquid or viscous matter is exposed to *‘ weighty and indeed insuper- 
able objections,” p. 18, and ‘is now abandoned by most geologists,” p. 
43. 
According to Dr. Croll * the * general opinion among geologists ” is 
that the earth ‘‘ consists of a fluid interior surrounded by a thick and 
rigid [really solid] crust.” 
Prof. Prestwich? believes that ‘the crust rests on a yielding sub- 
stratum, and that of no great thickness.” In fact he advocates the 
third of the above-mentioned theories, and believes 380 miles to be 
probably in excess of the crust’s thickness. Most writers on the sub- 
ject appear to have subsidiary theories of their own. 
Whether the assurance that the question is beyond the reach of ex- 
periment accounts for the multitude of theories and the confidence 
with which they are proposed, is a question for philosophers not mathe- 
maticians to consider, but it seems @ prior? a possible explanation of 
such a declaration of faith as that of Mr. W. B. Taylor: { “ The liquid- 
ity of our globe, and the relative thinness of its encrusted envelope— 
as attested by all legitimate geological induction—will be assumed 
without misgiving or hesitancy, and the supposed matematical argu- 
ments for its solidity ignored as essentially fallacious and wholly in- 
conclusive.” 
Of course if the geological evidence were conclusive, it would be 
mere waste of time further to consider the matter, but the evidence 
that satisfies Mr, Taylor does not seem to carry conviction to all geolo- 
gists even in America. Mr. G. Becker,§ for instance, who appears to 
have same practical experience, says: ‘* For a considerable number of 
years | have constantly had the theory of the earth’s solidity in mind 
while making field observations on upheaval and subsidence, with the 
result that to my thinking, the phenomena are capable of much more 
satisfactory explanation on a solid globe than on an encrusted fluid 
one.” 
Climate and Time, p. 395. 
Geology, vol. 1, p. 540. 
American Journal of Science (1885), vol. XXX, p. 250. 
§ American Journal of Science (1890), vol. XXXIX, pp. 351, 352. 
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