234 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1916. 



and sedimentation. A gravitational adjustment by the wedging up 

 and down and laterally of such sectors is thus offered tentatively as 

 a working competitor to theories of adjustment by fluidal or quasi 

 fluidal undertow. The necessary brevity of this statement leaves 

 this new hypothesis little more than a crude suggestion that gravi- 

 tative adjustment (=isostasy) may perhaps take place as fully as 

 the case requires in a highly rigid elastic earth, affected by vertical 

 schistosity and an adaptability in wedging action, without resort to 

 flowage or even quasi flowage. 



II. 



CONSTITUTION OF THE INTERIOR OP THE EARTH, AS INDICATED 

 BY SEISMOLOGICAL INVESTIGATIONS. 



By Harry Fielding Reid. 



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 he 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, longitu- 

 dinal and transversal, which would be propagated at different rates, 

 and as their velocities are so great that they can not be separated 

 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 seismograms, 

 but it was not until 1900 that Oldham succeeded in showing definitely 

 that the seismograms of a number of Milne instruments gave clear 

 evidence of the existence of three groups of waves. Oldham also 

 published a diagram, which was an extension of Seebach's so-called 

 " hodograph," showing the relation between the time of transmission 

 of each group and the distance from the earthquake origin, measured 



1 " Sur la propagation du movement dans les corps solides et liquides," Ann. de Chimie 

 et Phys., 1851, vol. 21, p. 19. 



2 " On Waves PropaRated Along the riane Surface of an Elastic Solid," Proc. London 

 Math. Soc, 1855, vols. 47, 50. 



