Penny Blood: Serial Sensation and Working-Class Entertainment — page 187: what you’re looking at

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

170 AMOUNT OF COMPRESSION. [CH. XIII. We have then to seek for the cause of this compression, which has effected the continental areas. If we have given a sphere of a certain radius, whose outer crust 1s solid and rests upon a fluid substratum, compression of the crust may arise from contraction of the volume of the sphere, either through cooling, or by the extravasation of some portion of the matter which was originally beneath the crust. In these cases the compression of the upper surface of the crust would be equal to the contraction of the sphere; that is to say, if the radius of the sphere before contraction was r+er, and after contraction became 7, e being the coefficient of contraction, then the mean coefficient of compression along any line drawn upon the sphere would also bee. If the length of the line after com- pression was /, then before compression 1t must have been / + el. But there may be another cause for the compression which has elevated continental areas. The solid crust may have expanded horizontally. It 1s quite possible that these two causes of compression may have coexisted. The sphere may. have contracted, and the crust expanded simultaneously; in which case the contraction of the volume of the sphere will be no measure of the compression of the crust. We have already made it apparent, that the cooling of the earth considered as a solid sphere cannot account for the inequali- ties, and therefore neither sufficiently for the compression of the crust. And if we regard the crust as resting on a liquid sub- stratum, it does not appear how its contraction through mere cooling can have been so much greater than in the case of a solid globe, as to account for what the latter will not explain. We have therefore to examine whether other hypotheses will more satisfactorily explain the phenomena. But it is obvious that our first step, before we can reason at all about the com- pression under the condition of a fluid substratum, must be to estimate its amount afresh: for the grounds upon which that must be done are very different from those upon which we formerly estimated the compression in the case of .a solid globe’. : 1 See chap. v., p. 55. COLA @ NOOKS ACO _— $$»? —~—aiie..