THE GEOLOGIST. 



DECEMBEB 1863. 



PLANETAEY OBBITS. 



Bt the Editor. 



Eveet day's experience confirms more and more the opinion that 

 the central heat doctrine has 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 eight 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. We know nothing, 

 however, so perfectly a non-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, w T ith a shifting 

 axis of rotation, as our earth undoubtedly has, and for other reasons, 



VOL. YT. 3 L 



