Page:Proceedings of the Royal Society of London Vol 4.djvu/108

Rh hypothesis that its entire mass was originally in a fluid state ; an hypothesis which was at first founded on astronomical considera- tions, and is now corroborated by the discoveries of modern geology, exhibiting the apparent injection from below of large masses of un- stratified rocks, through the fissures of sedimentary strata. As- suming that this state of fluidity was the eff^ect of heat, we are led to consider the steps of transition by which the earth has passed into its present state of solidity, and apparently permanent temperature. After adverting to the analytical investigations of Fourier and Poisson on this subject, the author proceeds to inquire into the results of the laws of refrigeration of heated bodies, which may be conceived to operate in the present case; namely, refrigeration by circulation, which obtains when the fluidity is perfect, and that by conduction, when the particles of the mass, by the diminution of fluidity, no longer retain that mobility among one another which is requisite for their circulation. Thus while, in either case, the superficial parts of the earth would rapidly cool and solidify by the radiation of their heat into sidereal space, forming a crust of small thickness compared with the whole radius of the globe, the internal mass may be in one or other of the three following conditions : — First, it may consist of matter still in a state of fusion, of which both the tem- perature and the fluidity are greatest at the centre, but which has been brought, by the long-continued process of circulation, into a state no longer admitting of this process, and capable, therefore, of cooling only by conduction. Secondly, the earth may consist of an external shell, of a central nucleus, rendered solid by the enor- mous pressure to which it is subjected, and of an intermediate stratum of matter in a state of fusion. The thickness of the shell, as well as the radius of the solid nucleus, may possibly be small compared with the radius of the earth. The fluidity of the inter- vening mass must necessarily be here, also, considerably more im- perfect than that which would just admit of cooling by circulation. Thirdly, the earth may be solid from the surface to the centre.

The author then shows that the direct investigation of the manner in which the earth has been cooled, assuming its original fluidity from heat, cannot determine the actual condition of its central parts, not from any imperfection in the analytical process, but from the want of the experimental determination of certain values, which it is extremely diflScult, if not impossible, accurately to obtain. It has occurred to the author that a more indirect test of the truth of the hypothesis of the central fluidity of the earth might be found in the delicate but well-defined phenomena of precession and nutation. The investigation of the problems thus suggested is reserved by the author for the subject of a future memoir.

At the Anniversary Meeting of the Royal Society, Mr. Galloway,