THE INTERIOR OF THE EARTH. 235 



along the surface of the earth. Mihie soon improved these curves 

 by adding observations of a large number of recorded shocks.^ The 

 curves of the first and second " preliminary tremors," as Milne called 

 the first two groups of waves, are curved, indicating that the velocity 

 of transmission increases with the distance from the origin ; a conclu- 

 sion which had already been drawn from earlier, but less accurate, 

 observations. Milne attempted to explain this by assuming that the 

 path of the seismic disturbance lay along the chord and not along the 

 earth's surface ; this practically shortens the distance to the observing 

 stations, and if the curves are plotted, with distances measured 

 along the chord, the curvature is considerably diminished ; but later 

 and more accurate observations show that even under this assump- 

 tion the velocity still increases with the distance. The conclusion 

 is unavoidable that as the path of the disturbance sinks deeper into 

 the earth the velocity increases. The interior of the earth then is not 

 a homogeneous but a refractive medium, and the path of the dis- 

 turbance can not be straight but must be curved with the concavity 

 turned upward. This condition had been described by A. Schmidt 

 as early as 1888.- Seismologists now believe that the three groups 

 discovered by Oldham are respectively the longitudinal, the trans- 

 verse, and the surface waves. The transmission curve of the latter 

 is a straight line indicating that the waves are transmitted with 

 uniform velocity along the surface of the earth. They have affected 

 seismographs after having passed completely around the earth. It 

 can not be said that the evidence, that the first two groups are re- 

 spectively longitudinal and transverse, is complete; but it is suffi- 

 cient, in connection with theory, to make seismologists fairly con- 

 fident that the conclusion is correct; and the passage of transverse 

 waves through the earth to great depths is proof that, to those 

 depths, the earth is solid; for transverse waves can not exsit in a 

 liquid. Further, since the velocity of transmission depends on the 

 ratio of the elasticity to the density of the medium, and since both 

 the longitudinal and transverse waves increase in velocity with the 

 depth below the surface, both the elasticity of volume and the elas- 

 ticity of figure of the earth, not only increase, but increase more 

 rapidly than the density as we penetrate below the surface. The 

 earth therefore is not only rigid, but its rigidity increases toward its 

 center; though seismological evidence does not yet prove that this 

 characteristic extends to the very center itself. 



The next step was to determine the path of the waves in the earth 

 and their velocity at different depths; the data for these determi- 

 nations were the times of arrival of the earthquake waves at various 



1 Rep. of Com. on Seismol. Investig., B. A. A. S., 1902, p. 7. 



2 " Wellenbewegung und Erdbeben," Jahreshefte fiir Vaterlands Naturkunde in Wiirt- 

 temberg. 1888, p. 248. 



