PRESENT PROBLEMS OF GEOPHYSICS 509 



the spectroscope, though the facts thus revealed were unknown to 

 Kant and Laplace. It is also compatible with and accounts for the 

 heterogeneity in the composition of the earth manifested in the 

 actual asymmetric distribution of oceans, mountain ranges, and ano- 

 malies of 'gravitational force, as well as in the curiously local occur- 

 rence of certain ores (such as those of tin and mercury) and in the 

 predominance of certain alkalies among the rocks over wide areas. 



This heterogeneity, however, is of a small order of magnitude. The 

 general dependence of gravity on latitude, the nearly spheroidal 

 shape of the earth, and other phenomena show that the distribution 

 of density is nearly symmetrical, while the divergence of the spheroid 

 from the figure characteristic of a fluid of the same mean density 

 and mass as the earth demonstrates that the interior layers of equal 

 density are oblate. These and similar facts are consistent with and 

 are strong evidence for the hypothesis that the globe has been fused 

 at least to a considerable depth from the growing surface of the 

 gathering nebulous mass. Nevertheless, Houghton, and more 

 recently Professor Chamberlin, have supposed that the accretion of 

 nebulous matter was so slow that the heat of impact did not suffice 

 to produce fusion. The hypothesis of superficial fusion is not incom- 

 patible with the minor heterogeneity pointed out above; for the 

 laws of diffusion in viscous fluids give proof that sensibly perfect 

 homogeneity could not be produced even in 50,000,000 years through- 

 out a body of liquid originally heterogeneous and possessing a tenth 

 of the mass of the earth. On the other hand, there is no known ground 

 other than mere convenience for supposing an original homogeneity 

 either of the nebula or of the earth. 



The problem of the distribution of density in the earth is one of the 

 most important in all geophysics. It is as significant for geodesy and 

 terrestrial magnetism as for geology. That Laplace's empirical law 

 represents it approximately is generally acknowledged, but it 

 appears substantially certain that this is merely an approximation 

 without theoretical value. Only extended researches, however, can 

 replace it by one better founded. 



The solidity of the earth is now very generally accepted, though 

 Descartes's hypothesis of its fluidity, invented to satisfy his erroneous 

 theory of vortices, died hard. Lord Kelvin showed from tidal phe- 

 nomena that the effective rigidity of the earth is about that of a con- 

 tinuous globe of steel. Professor Newcomb pointed out that the 

 Chandlerian nutation leads to the same conclusion and an almost 

 identical value of the modulus of rigidity, and Professor George H. 

 Darwin demonstrated that, if the earth is a viscous liquid, its vis- 

 cosity must be some 20,000 times as great as that of hard brittle 

 pitch near the freezing-point of water. From the point of view of 

 modern physical chemistry, and in consideration of Professor Ar- 



