'Baron Fourier, after making a curious series of experiments on the cooling of incandescent bodies, considers it to be proved mathematically, that the actual distribution of heat in the earth's envelope is precisely that which would have taken place if the globe had been formed in a medium of a very high temperature, and had afterwards been constantly cooled.'
Sir Charles replied to this in the same edition that, if the earth were a fluid mass, a circulation would exist between centre and circumference, and solidification of the latter could not commence until the whole had been reduced to about the temperature of incipient fusion. We fail to see that this is an answer to Baron Fourier. What necessity is there for supposing that the solidification of the crust commenced before the matter of the globe had been reduced throughout to about the temperature of incipient fusion? The water in a pond must be reduced to about the temperature of incipient freezing before ice can form on the surface, but this does not prevent the formation of a sheet of ice on the top.
In the article in _Nature_, from which we have already quoted, Mr. David Forbes mentions that M. De Launay, Director of the Observatory at Paris, 'an authority equally eminent as a mathematician and an astronomer,' having carefully considered Mr. Hopkins's problem, decided that its data were incorrect, and that it could shed no light whatever on the question whether the globe is liquid or solid. There is some doubt, however, as to the import of M. De Launay's statement.
We may be the more disposed to wonder at the decision with which Sir Charles Lyell pronounces upon this subject in his latest edition, by the fact that, since the publication of the previous edition, he has modified, to a very serious extent, his conception of the evidence on which the theory which he adopts is based. In the ninth edition of the 'Principles' he laid so much stress on Sir Humphry Davy's hypothesis of an un-oxidized metallic nucleus of the globe, liable to be oxidized at any point of its periphery by the percolation of water, and thus to evolve heat sufficient to melt the adjacent rocks, that Hugh Miller, in contending against Sir Charles, selected this as an essential part of the argument. In his tenth edition Sir Charles does not even mention Sir Humphry Davy's theory. The star under the influence of which the tenth edition was prepared was that of Mr. Darwin. No brighter star may be above the geological horizon, and Sir Charles may have done well to own its influence, but we submit that opinions which undergo important modification within a few years ought hardly to be promulgated as marking the limit between the era of darkness and the era of light in geological discovery.