PRESENT KNOWLEDGE OF THE EARTH WIECHERT. 443 



We will now turn to the point in which we are specially interested. 

 TVhile the behavior of the earth's strata changes at a uniform rate 

 down to a depth of about 1,500 kilometers, a sudden jump occurs at 

 that depth. What does this mean? Evidently we have to infer 

 that at a depth of 1,500 kilometers there is a sudden change in the 

 condition of the earth's strata. Recall now, if you please, what I 

 said in the beginning of my address concerning the conclusions which 

 we are authorized to draw concerning the masses in the interior. We 

 found that in all probability the earth contains a metal core embedded 

 in a rocky mantle some 1,300 to 1,600 kilometers thick. You will 

 admit that the evidence furnished by earthquake study agrees with 

 this conclusion in a remarkable manner. We are justified in assuming 

 that the j)lace of the jump in the propagation of earthquake waves 

 is precisely the passage from the rock mantle to the metal core. Thus 

 vanishes the uncertainty which affected our conclusions concerning 

 the composition of the earth out of core and mantle, so long as we 

 were able merely to appeal to observations on gravitation and on the 

 flattening of the earth. With greatly strengthened confidence we 

 may now affirm that the earth consists of a metal core enveloped in a 

 rock mantle. Previously we were only able to estimate the thickness 

 of the rock mantle in a crude way as approximately 1,300 to 1,600 

 kilometers. The earthquake phenomena now tell us that the thick- 

 ness amounts to about 1,500 kilometers. The residuum of uncertainty 

 can hardly exceed 100 kilometers. Similarly, our previous inference 

 regarding the density of the metal core now attains increased cer- 

 taint}?^, and the conclusion that the material of the metal core is 

 mainly iron gains a new support. 



I have not j'et given you any indication of the velocity of propa- 

 gation of earthquake waves. I will now state that the first pre- 

 cursors traverse near the surface about 7 kilometers in a second, and 

 in the metal core about 13 kilometers in a second. In the case of the 

 second precursors the corresponding figures are about 4 kilometers at 

 the surface and 7 kilometers in the metal core. The principal waves, 

 which move along the surface of the earth, traverse about 3.4 kilo- 

 meters in a second. 



The mathematical theory of elasticity enables us to infer the 

 elastic properties from the velocity of the elastic waves. Thus the 

 earthquake observations of the present day also place it within our 

 power to determine the elasticity of the earth's strata with accuracy 

 down to half the distance from the center. We find that as we de- 

 scend, the rigidity of steel, that is to say, its elastic resistance to 

 changes of form, is exceeded after a few hundred kilometers. The 

 metal core shows a rigidity four times as great as that of steel. All 

 this agrees perfectly with what we learned concerning the rigidity of 

 the earth as a whole, from the tides and from the oscillations of the 



