EARTH S INTERIOR ADAMS AND WILLIAMSON 



257 



It is truly remarkable that Laplace's equation, which can be re- 

 garded merely as an empirical relation, should yield values of the 

 density so close to those obtained in what we believe is a more ra- 

 tional way. The fact that Laplace's law leads to the right moment 

 of inertia, of course, indicates that the densities obtained by it may 

 be correct to at least a rough approximation, and accounts for its 

 general use among geophysicists. The assumption involved in its 

 derivation is, however, by no means warranted. 



Figure 4 is intended to illustrate the segregation of iron toward 

 the center and the fringe of pallasite surrounding the iron core. 



Fig. 4. — Diagram intended to suggest the segregation of metallic iron toward the center, 

 and the zone of pallasite (mixture of iron and silicates) surrounding the central core 



The depth of the surface layer — 60 km. — is shown to scale by the 

 thickness of the outer circular line. 



COMPRESSIBILITY, RIGIDITY, AND BULK MODULUS 



By combining the earthquake velocities given in Figure 1 with 

 the densities given in Figure 3, the elastic constants of the material 

 within the earth at various depths may readily be obtained. 88 The 

 values of the rigidity and bulk modulus have been calculated and are 

 shown graphically in Figure 5. It is very interesting that these 

 curves are comparatively smooth; they show no pronounced irregu- 

 larities or sudden changes of direction, such as are shown by the 



33 The equations used are: R=pvb'. K^pW—iva'P)- 



