Penny Blood: Serial Sensation and Working-Class Entertainment — page 169: what you’re looking at

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

152 THE REVELATIONS OF [CH. XII. chapter, the partial rigidity of the crust will render this assumption in strictness incorrect, yet the deviations from accuracy consequent upon it cannot amount to anything con- siderable when compared to the whole thickness; and this will be especially true in the neighbourhood of the chains, near which the centre of gravity of a disturbed tract will lie; and which will therefore be least depressed or elevated by any slight tilting action. If this be a true statement of the case, then the existence of these roots ought to be revealed in another and entirely distinct manner, namely by the phenomena of underground temperature. Where the roots of the mountains are in contact _ with the molten fluid, they must be slowly and gradually becoming fused. At other localities, where there is no such protuberance, the bottom of the mean crust must either remain sensibly at a permanent temperature, which will be that of solidification (the same as of fusion), or else it will be slowly and gradually freezing, and growing thicker. At all localities we shall therefore have, maintained at the bottom of the solid crust, sensibly the melting temperature of the materials of which the crust 1s there composed, modified perhaps by the difference of pressure at their different depths, which we need not for the present purpose consider. Now it is evident that, if the two surfaces of a plate of any substance are maintained at two different constant tempera- tures, the mean rate of increase of temperature within the plate will depend solely upon its thickness irrespectively of its conductivity. The mean rate therefore of increase of tempera- ture within the crust, at any locality where we may suppose these conditions approximately fulfilled, will depend upon the difference of the temperatures of its upper and under sur- faces at that locality, and on its thickness, and not upon the conductivity of the material, nor upon the absolute tempera- tures, For although contortions in the bedding, or difference of composition, may affect the rate as between one level and another at that locality, still they will not affect the mean rate. Nor yet need such geological accidents be considered in com- paring the mean rate at one place with that at another. The COMIC OOOKS.EO inn