THE COMPOSITION OF THE EARTH'S INTERIOR 



By L. H. Adams and E. D. Williamson 

 Geophysical Laboratory, Carnegie Institution of Washington 



Curiosity is one of the dominant characteristics of man. From 

 early childhood he seems impelled to examine every object within 

 his reach and when possible to open it up to see what makes the 

 wheels go 'round. But the urge which led Pandora so deeply 

 into trouble is, in another aspect, called science, and provides the 

 mainspring for the procurement and classification of facts. Scien- 

 tific investigation is merely a manifestation of curiosity concerning 

 those things which we see or know about but do not understand. 



It is only natural that the internal constitution of the globe 

 upon which we live should excite our curiosity. Although the 

 diameter of the earth is 8,000 miles, the deepest borings have pene- 

 trated down to a depth of little more than 1 mile ; it is as if a sphere 

 the size of an orange were inhabited by diminutive beings who 

 had explored their globe only at the surface and to a depth of 

 one- fourth the thickness of the paper on which these words are 

 written. The inaccessibility of the earth's interior, and the appar- 

 ently insuperable difficulties which are presented, only serve to 

 sharpen our zeal for finding out something about it. Nowadays we 

 are becoming more accustomed to investigate things which can 

 not be seen, and in this paper it is hoped to show what can be 

 learned of the earth's interior, even though it be beyond the reach 

 of direct observation. 



The principal sources of information concerning the interior of 

 the earth are as follows: (1) The constant of gravitation, from 

 which the total mass and average density of the earth are deter- 

 mined; (2) the constant of precession and other astronomic and 

 geodetic data from which the moments of inertia of the earth may 

 be calculated, the moment of inertia allowing important inferences 

 to be drawn concerning the density distribution within the earth; 

 (3) the known flattening of the earth, as determined from the 

 data of geodesy, with which any assumed distribution of mate- 



1 The substance of this paper appeared, under the title "Density distribution in the 

 earth," in the Journal of the Washington Academy of Sciences (vol^ 13, 413-428, 1923) 

 just before Mr. Williamson's death. The paper is now being republished, with slight 

 rearrangement and with minor additions. 



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