292 REID— CONSTITUTION OF THE [April 24, 



measured along the chord, the curvature is considerably diminished ; 

 but later and more accurate observations show that even under this 

 assumption the velocity still increases with the distance. The con- 

 clusion 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 disturbance cannot 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 longi- 

 tudinal, the transverse 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 cannot be said that the evidence, that the 

 first two groups are respectively longitudinal and transverse, is com- 

 plete ; but it is sufficient, in connection with theory, to make seismolo- 

 gists fairly confident 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 cannot exist 

 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 

 elasticity 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 towards 

 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 

 determinations were the times of arrival of the earthquake waves at 

 various distances from the origin ; these times are collected in the 

 transmission curves. At first sight this seems an insoluble problem ; 



4 " Wellenbewegung und Erdbeben," Jahreshefte fiir Vaterlands Natiir- 

 kunde in Wiirttemberg, 1888, p. 248. 



