THE INTERIOR OP THE EARTH. 239 



results than the horizontal pendulum. He measured the deflection 

 of the vertical under the influence of the moon by what was prac- 

 tically a water level 500 feet long, sunk G feet in the earth.^ Michel- 

 son's results for the east-west rigidity do not differ greatly from 

 those of Orloff; but his north-south rigidity is somewhat less than 

 Orloff's. Michelson's experiments also show that the viscosity of 

 the earth must be as great as that of steel. These experiments are 

 of great interest; they should be repeated at various places, and 

 especially at places symmetrically situated with respect to the great 

 oceans, and on midoceanic islands, in order to determine how far 

 they are affected by the oceanic tides. 



We can say in conclusion that the transmission of transverse 

 earthquake waves shows that the earth is solid, at least to a great 

 depth below the surface; and that experiments on the deflection of 

 the vertical show that it is quite as rigid and as viscous as steel. 

 There are still difficulties in the interpretation of the observations, 

 but their elucidation can not alter the general character of the 

 conclusions. 



III. 



THE EARTH FROM THE GEOPHYSICAL STANDPOINT. 

 By John F. Hayfokd. 



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 compari- 

 son 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 semidiameter 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. 



1 " Preliminary Results of Measurements of the Rigidity of the Earth," The Astro- 

 physical Journal, 1914, vol. 39, p. 97. 



