II.. 



CONSTITUTION OF THE INTERIOR OF THE EARTH 



AS INDICATED BY SEISMOLOGICAL 



INVESTIGATIONS. 



By harry fielding REID. 

 {Read April 24, 191 5.) 



In 1883 Milne predicted that earthquake disturbances would be 

 registered by seismographs at great distances from their origin, a 

 prediction first verified when the earthquake of April 18, 1889, 

 whose origin lay off the coast of Japan, affected the horizontal 

 pendulum which von Rebeur-Paschwitz had set up at Potsdam to 

 study the attraction of the moon. Milne was so convinced of the 

 correctness of his idea and of the importance of the results to be 

 obtained that in 1893 ^^ established an observatory on the Isle of 

 Wight to record earthquakes from distant regions ; and he also suc- 

 ceeded in having instruments of similar model set up at observatories 

 very widely scattered in various parts of the world. 



Wertheim in 1851 showed that a disturbance in the interior of 

 an elastic solid would break up into two groups of waves, longi- 

 tudinal and transversal, which would be propagated at different 

 rates, and as their velocities are so great that they cannot be sepa- 

 rated from each other in the laboratory he suggested with rare 

 insight that their separation might first be noticed in connection 

 with the propagation of earthquake disturbances.^ A few years 

 later Lord Rayleigh showed that a third kind of wave could be 

 propagated along the surface of the earth. ^ Seismologists naturally 

 looked for indications of these three groups of waves in their 



1 " Sur la propagation du movement dans les corps solides et liquides," 

 Ann. de Chimie et Phys., 1851, Vol. XXL, p. 19. 



2 " On Waves Propagated along the Plane Surface of an Elastic Solid," 

 Proc. London Math. Sac, 1855, Vols. XLVIL, L. 



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