THE GEOLOGIST. 
DECEMBER 18G3. 
PLANETAEY CEBITS. 
By the Editor. 
Evert day's experience confirms more and more the opinion tliat 
the central heat doctrine lias less foundation than formerly it was 
supposed to possess. Its great supporters have gradually increased 
the necessary thickness of the solid crust in proportion to the 
internal supposed fluid core from forty to eiglit hundred miles at 
least : rather a wide difference in itself, but not perhaps so very great 
in respect to the absolute diameter of the earth, to which such a 
relationship would be about in proportion to the thickness of a 
sheet of cartridge-paper round a 12-inch globe. AVe know nothing, 
however, so perfectly a nou-conductor that so thin would resist the 
heat of the internal molten mass. Moreover, upon the alleged 
increase of temperature with depth in coal and other mines, much 
doubt has been thrown by the subsequently ascertained facts that 
in many instances the higher temperatures have disappeared after 
the mines had ceased to be worked. The necessity, if the interior 
were fluid, for internal tides below the supposed solid crust, also 
militates against the existence of a fluid core, because we can detect 
no such tides at the surface of our earth; and if they existed, it 
is difficult to conceive the rigidity and strength of so thin a crust to 
be equal to restraining them entirely ; and if the crust were in the 
least degree yielding or elastic, we must have evidence of such 
tides in the heavings of the surface. Besides this, with a shifting 
axis of rotation, as our earth undoubtedly has, and for other reasons, 
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