444 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1908. 



poles. We are also enabled to calculate the compressibility of the 

 earth's strata by pressure. As a definite inference therefrom, we 

 learn that the increase in density toward the earth's interior can not 

 be explained by the pressure of the overlying strata. What I was at 

 first able to present to you merely as a conjecture, namely, that the 

 differences in density in the earth indicate differences in material, 

 thus becomes a definite and precise outcome of observations. 



But how are we to explain why the rigidity increases so rapidly to- 

 ward the interior, and in the core is even four times as great as the 

 rigidity of steel, seeing that we are to suppose the mantle to be com- 

 posed of the known rocks, and the core mainly of iron ? The answer 

 is very simple : Evidently we have here the effect of the pressure of 

 the earth's strata, which forces the molecules closer and closer to- 

 gether the farther we penetrate into the interior of the earth. As 

 proved by calculation, the pressure at the surface of the metal core 

 amounts already to half a million atmospheres and increases toward 

 the center of the earth to about 3,000,000 atmospheres. Under such 

 circumstances, it may readily be conceived that there must be an 

 increase in rigidity. The fact that earthquake phenomena furnish 

 information concerning the behavior of matter under such high pres- 

 sure is of the greatest importance for physics, for by our human 

 appliances in laboratories we are enabled to produce a pressure of at 

 most a few thousand atmospheres. 



You will probably expect me to say something regarding the tem- 

 perature in the interior of the earth. I can not say much. That the 

 temj^erature is very high is proved by the rapid rise observed wher- 

 ever men have penetrated into the interior — in mines, in railway tun- 

 nels, in drill holes. It is also proved by the hot springs and, above 

 all, by the volcanoes. Near the earth's surface, where we are able to 

 make direct observations, we find an increase of temperature of about 

 2 to 4 degrees centigrade for every 100 meters. Considerations based 

 on these observations suggest that in the far interior of the earth the 

 temperature must surely attain some thousands of degrees centigrade. 

 That the material of the earth nevertheless does not become liquid or 

 even gaseous at such high temperatures, but is proved to be very 

 rigid, must be attributed to the extreme pressure, which packs the 

 molecules together and robs them of their mobility. Keeping this 

 in mind while trying to ascertain the physical behavior of bodies 

 with increase of temperature, we may infer that the temperature in 

 the interior of the earth must certainly remain below 9,000 degrees ; 

 in all probability it does not even reach 4,000 degrees. 



Let us look back over everything that has been discussed to-day. 

 As the verdict of science, we learn that the earth has become, as it 

 were, transparent to our view. In particular the earthquake waves 



