Penny Blood: Serial Sensation and Working-Class Entertainment — page 287: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

270 SUMMARY. (CH. XXII. fluid substratum of no great thickness compared to the radius, upon which a crust of lesser density floats in a state of equili- brium? This is the supposition as to the condition of the earth which appears on the whole to satisfy best the requirements both of geology and of physics, as we have attempted to show in the preceding chapters. That the above arrangement of the materials of the earth is compatible with the laws of cooling of such a body, has been proved by Mr Hopkins to be the case. The nucleus owes its solidity to the enormous pressure of the superincumbent matter while the crust owes its solidity to having become cool. The fluid substratum beneath it is not under sufficient pressure to be rendered solid, and is sufficiently hot to be fluid. As to the degree of its fluidity, we have no certain knowledge. It is pro- bably more viscous in its lower portions through pressure, and likewise passes into a viscous state in its upper parts through cooling, until it joins the crust. But how fluid the most fluid intermediate parts may be we cannot determine. III. It has been stated that the mean density of the whole earth is known, and also the density of surface rock. But the actual densities of the successive layers, and their corresponding forms and thicknesses, are not known. There must be certain relations among these attributes in order to satisfy the ascer- tained facts regarding the earth’s shape, and its mean and surface densities. But these requirements may be satisfied in more ways than one. Laplace suggested a law of density, suit- able for purposes of calculation, which satisfies these require- ments very closely; and that law has been generally assumed, as representing the true state of the case, 1n- investigations on the figure of the earth. It makes no assumption as to the actual cause of the variation of density with the depth, al- though it 1s assumed to be such as might be caused by a certain relation between density and pressure. But it is much more likely that the increase in density towards the centre of the earth arises from the heavier materials gravitating thither. Waltershausen has formed a theory on the ground that the earth is a hot globe, of which a considerable portion is fluid, COMIC OOOKS.EO | inn