III. 



THE EARTH FROM THE GEOPHYSICAL STANDPOINT. 



By JOHN F. HAYFORD. 



{Read April 24, 19 15.) 



This is a broad topic on which much intensive thinking has been 

 done by many men. It is impossible to treat it adequately or com- 

 prehensively in the short time available. 



In this address an attempt will be made to so concentrate atten- 

 tion on a certain few points as to tend to clarify existing ideas and 

 to correlate them. An attempt will also be made to help in locating 

 the lines of least resistance to future progress in the study of the 

 earth. 



The size of the earth, as well as its shape, is now known with 

 such a high degree of accuracy that the errors are negligible in 

 comparison with the errors in other parts of our knowledge of the 

 earth. The probable error of the equatorial radius is less than 

 1/300000 part, and of the polar semi-diameter is about the same. 



The three physical constants of the earth, and of its different 

 parts, on which you are now asked to concentrate your attention 

 are the density, the modulus of elasticity, and the strength. 



It is important to know as much as possible about the density. 

 The more one knows about the density in all parts of the earth the 

 more surely and safely one may proceed in learning other things 

 about the earth. 



The modulus of elasticity at each point in the earth controls the 

 behavior of the earth under relatively small applied forces. 



The strength of the earth, at each point, as measured by the 

 stress-difference at that point necessary to produce either slow con- 

 tinuous change of shape or rupture, decides the behavior of the 

 earth under the greater forces applied to it. 



As to density we know that the earth's surface density is about 



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