226 In Hopkins’s hypothesis he assumed the crust to be infinitely rigid and unyielding, which is not true of any material substance. Sir William Thomson has recently returned to the problem, in the light of his own researches in vortex-motion. He now finds that, while the argu- ment against a thin crust and vast liquid interior is still invincible, the phenomena. of precession and notation do not decisively settle the question of internal fluidity, though the ealar semi-annual and lunar fortnightly nutations absolutely disprove the existence of a thin rigid shell full of liquid. If the inner surface of the crust or shell were rigorously spherical, the interior mass of supposed liquid could experience no precessional or nutational influ- ence, except in so far as, if heterogeneous in composition, it might suffer from external attraction due to non-sphericity of its surfaces of equal density. But “ a very slight devia- tion of the inner surface of the shell from perfect sphericity would suffice, in virtue of the quasi-rigidity due to vortex- motion, to hold back the shell from taking sensibly more precession than it would give to the liquid, and to cause the liquid (homogeneous or heterogeneous) and the shell to have sensibly the same precessional motion as if the whole con- stituted one rigid body.“ The assumption of a comparatively thin crust requires that the crust shall have such perfect rigidity as is possessed by no known substance. The tide-producing force of the moon and sun exerts such a strain upon the substance of the globe that it seems in the highest degree improbable that the planet could maintain its shape as it does unless the supposed crust were at least 2000 or 2500 miles in thickness? That the solid mass of the earth must yield to this strain is certain, though the amount of deformation is so slight as to have hitherto escaped all attempts to detect it. Had the rigidity been even that of glass or of steel, the deformation would probably have been by this time detected, and the actual phenomena of precession and nutation, as well as of the tides, would then have been very sensibly diminished.” The conclusion is thus reached that the mass of the earth “ is on the whole more rigid certainly than a continuous solid globe of glass of the same diameter.”4 (b.) Argument from the tides.-The phenomena of the oceanic tides are only explicable on the theory that the earth is either solid to the centre, or possesses so thick a crust (2500 miles or more) as to give to the planet practical solidity. Sir William Thomson remarks that, “ were the crust of continuous steel, and 500 kilometres thick, it would yield very nearly as much as if it were india-rubber to the deforming influences of centrifugal force, and of the sun's and moon's attractions.” It would yield, indeed, so freely to these attractions “ that it would simply carry the waters of the ocean up and down with it, and there would be no sensible tidal rise and fall of water relatively to land.”"’ Mr George H. Darwin has iecently investigated mathematically the bodily tides of viscous and semi-elastic spheroids, and the character of the ocean tides on a yielding nucleus.“ His results tend to increase the force of Sir William Thomson’s argument, since they show that “ no very considerable portion of the interior of the earth can even distantly approach the fluid condition,” the effective rigidity of the- whole globe being very great. (r.-.) Argument from relative densities of melted and solid rock.—The two preceding arguments must be considered decisive against the hypothesis of a thin shell or crust covering a nucleus of molten matter. " It has been further urged, however, as an objection to this hypothesis, that cold 1 Sir W. Thomson, Brit. Assoc. 12:519., 1876, Sections, p. 5. 9 Thomson, Proc. Roy. Soc., April, 1862. 3 Thomson, loc. cit. 4 Thomson, Trans. Roy. Soc. Edin., xxiii. 157. -" Thomson, Brit. Assoc. Rep., 1876, Sections. 1). 7. 5 Proc. Roy. Soc., No. 188, 1878. GEOLOGY [11, GEOGNOSY. solid rock is necessarily more dense than. hot melted rock, and that even if a thin crust were formed over the central molten globe it would immediately break up and the frag- ments would sink towards the centre.7 l'ndoubtedly this would happen were the material of the earth’s mass of the same density throughout. But, as has been already pointed out, the specific gravity of the interior is at least twice as much as that of the visible parts of the crust. If this difference be due, not merely to the effect of pressure, but to the presence in the interior of intensely heated metallic substances, we cannot suppose that solidified portions of such rocks as granite and the various lavas could ever have sunk into the centre of the earth, so as to build up there the honey—co1nbed cavernous mass which might have served as a nucleus in the ultimate solidification of the whole planet. From the considerations above advanced we have seen that the earth's central mass may be plausibly conjectured to be metallic. Into this dense central mass the comparatively light crust could not sink, though its earliest formed portions would no doubt descend until they reached a stratum with specific gravity agreeing with their own, or until they were again melted.5 3. The ingenious suggestion of Mr Fisher, already cited (ante, p. 217), in favour of the existence of a possible fluid or viscous substratum between the flexible outer shell and an inner rigid nucleus, is made with the view of reconciling the requirements of physics with those facts in geology which seem to demand the existence of a mobile mass of intensely hot matter at no great depth beneath the surface. Whether it does so must be let t for physicists to decide. But, on geo~ logical grounds, it may be questioned whether such a fluid substratum is needed. We must bear in mind that the land of the globe, regarding the geological structure of which alone we know anything, covers but a small part of the whole surface of the planet; that the existing continents seem from earliest times to have specially suffered from the reaction between the heated interior and the cooled exterior, forming, as it were, lines of relief from the strain of coni- pression ; and that along such lines, if the substance of the interior be everywhere just about the melting point, relief from pressure by corrugation would cause liquefaction of the matter so relieved, and its ascent towards the surface ; so that evidences of volcanic action on the terrestrial ridges might be expected to occur, and to be referable to all ages. Mr Fisher assumes the contraction of rock in cooling to be 000007 linear for one degree F ahr. ; and he argues that, as this amount would not account for the observed contraction in the crust, we must have recourse to some additional explanation, such as the escape of steam and vapours from volcanic orifices. The validity of the asser- tion that the amount of horizontal compression of the superficial strata is greater than the cooling of a solid earth can account for may be questioned. The violently contorted rocks indicative of great horizontal compression occur chiefly along the crests of the great terrestrial ridges where the maximum effects of corrugation were to be looked for. To the argument from climate it may be replied on the other hand, with great plausibility, that secular changes may be accounted for by the effect of the variations in the eccentricity of the earth's orbit combined with the pre- cession of the equinoxes, as already described. (6.) Age of tlze Earth and Jlleasures of Geological T v'me.—-—- The age of our planet is a problem which may be attacked either from the geological or physical side. 1 . The geological argument rests chiefly upon the observed rates at which geological changes are being effected at the 7 This objection has been repeatedly urged by Sir William Thomson. See Trans. Roy. Soc. Edin., xxiii. 157; and Br-it. Assoc. Rcp., 1870, Sections, p. 7.
3 See D. Forbes, Geo]. }[ag., vol. iv. p. 435.