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Fig. 1.s s=Surface; a b=depth along radius; a x b=curve of contraction. Fig. 2.c b=curve of radial descent. Fig. 3.d d=level of no strain. and then decreases again more rapidly to zero at the surface. This is shown in diagram, Fig. 1. In this figure the curve represents the relative rate of contraction whether radial or circumferential of the several layers. We use it, however, only to represent the latter. For in considering the radial contraction, it is not the relative rate of the several layers that immediately concerns us, but their rate of radial descent. Now this is a summation series and therefore increases to the very surface, but at different rates of increase. The law of increase of radial descent as we come toward the surface is shown in diagram, Fig. 2^[1] in which the rate of increase is greatest at seventy miles, just where the curve changes from concavity to convexity. If now we superpose these two diagrams the depth a at which the two curves,