56 C. R. VAN HISE 



present the centrifugal force at the equator would be sixteen 

 times greater than now. This being the case, it is evident that 

 the effectiveness of gravity in producing interior pressures in the 

 earth must have been less than at present. If the pressures 

 were less, other things being equal, the earth would have less 

 density than at present, and thus by a steady increase in the 

 effectiveness of gravity during the time of decreasing rotation, 

 we have a cause for contraction of the earth. 



After reaching this qualitative conclusion, I asked Professor 

 Slichter to handle the problem quantitatively. He finds when 

 the period was five and one-half hours, on the hypothesis of 

 homogeneity, that the pressure at the center of the earth was 

 1,688,000 atmospheres, instead of 1,772,000 atmospheres, or 

 4.8 per cent. less. Following Laplace's hypothesis that the 

 earth is heterogeneous, and increases from a density of 2.7 at 

 the surface to 10.74 at the center, and supposing that the hetero- 

 geneous oblate spheroid had an eccentricity of .4, the same as 

 the homogeneous spheroid which has a five and one-half hour 

 period, he finds that the pressures at the center would be 2,920,- 

 000, instead of about 3,000,000 atmospheres, or 2 x / 2 per cent, 

 less than now. Further, as suggested by Professor Slichter, if 

 it be supposed that during the geological history of the earth 

 there has been a steady change from homogeneity in the direc- 

 tion of heterogeneity, the pressures at the centers of the spheroid, 

 instead of increasing by the small amounts given, might have 

 increased a much larger amount, depending upon the amount 

 of differentiation (see Fig. 2, p. 72). The extreme case would 

 be a change of pressure from those at the center of the homo- 

 geneous oblate spheroid when the period was five and one-half 

 hours, that is, 1,688,000 atmospheres, to the present pressures 

 of the heterogeneous spheroid, 3,000,000 atmospheres. In this 

 case the pressures would have been 43.7 per cent, less than at 

 present. It is not supposed that any such change of pressure 

 has occurred during geological time, but the truth probably lies 

 somewhere between this amount and the minimum, 2*^ percent., 

 and probably much nearer the latter amount than the former. 



