THE EARTH'S INTERIOR, ITS NATURE AND 

 COMPOSITION ' 



By Leason H. Adams 

 Geophysical Laboratory, Carnegie Institution of Washington 



[With 2 plates] 



Two events in the last few years have made possible a noteworthy 

 advance in our knowledge of the earth's interior, and have for the 

 first time extended our ideas on this subject definitely beyond those 

 of the early Greek philosophers. The two aids to progress were, first, 

 the precise measurement of the elastic properties of rocks and, second, 

 an improved technique in seismology which permitted the accumu- 

 lation of reliable data on the speeds of waves from near and from 

 distant earthquakes. Other researches in geophysics have played an 

 important, although secondary role, and have promoted a steady 

 improvement in our knowledge of the earth as a whole. It is the 

 object of this communication to summarize briefly the present notions 

 concerning the earth's interior and the steps by which the information 

 has been obtained. 



ORIGIN OF THE EARTH 



A discussion of the interior of the earth should properly start with 

 a consideration of its origin and its place in the universe. Geophysics 

 begins with cosmogony. Our earth is a spherical body about 8,000 

 miles in diameter floating in nearly empty space. Its nearest neigh- 

 bor, the moon, is a quarter of a million miles away. Together, they 

 revolve around the sun at a distance of some 90 million miles. The 

 other planets of the solar system chcle around the same sun, which, 

 although by far the largest object in the system, is merely a star like 

 countless others that dot the sky, and is, as stars go, a rather small 

 and insignificant one. Separated from the sun by enormous distances 

 are the other stars of our galaxy, which has a disklike form and an 

 extent of at least 50,000 light years, and is merely one of the innumer- 

 able spiral nebulae scattered irregularly through space at an average 

 distance of perhaps 1,000,000 light years. 



On so vast a scale, our earth, a tiny planet accompanying a small 

 star, seems to dwindle into insignificance, but it is after all the place 

 where we dwell and have our being, and for us it has the importance 



' An address delivered at the Administration Building, Carnegie Institution of Washington, October 27, 

 1336. Reprinted by permission from the Scientific Monthly, vol. 44, pp. 199-209, March 1937. 



255 



