Rocks and the Velocity of Seismic Waves. 67 



a certain relation between these two physical constants seems 

 to exist. Comparing the elastic constants of cainozoic and 

 archaean rocks, we find that with the increase of density 

 from 2 to 3, the modulus of elasticity has increased more 

 than ten times in certain specimens. Thus it would not be 

 a wild conjecture to put E = b' x 10 13 when the density is 3'5. 

 As the mean density of the earth is little over 5*5, we shall 

 come across a stratum of the density above cited not very far 

 from the surface. These considerations give support to the 

 view above stated that there is a stratum of maximum velocity 

 of propagation 



From the minute study of the records of distant earth- 

 quakes, Prof. Omori found that in the intermediate position 

 between the first tremor and the principal shock, there is 

 almost always a slight abrupt change in the seismograph 

 record. The velocity of propagation for this kind of dis- 

 turbance is about 7 kilometres per second." If we suppose 

 that the interior of the earth consists of an isotropic substance, 

 and that the longitudinal wave travelling through it gives rise 

 to the first tremor, and the transversal to the second, we can 

 easily find the Poisson ratio of the hypothetical interior. 

 Obviously 



"\ + 2/a_ 13 



and the ratio of lateral contraction to longitudinal extension 



<r = —~ — . = 0-298. 

 2 (X + fi) 



This value is nearly the same as that generally attributed to 

 iron, glass, and other isotropic substances. In his paper on the 

 Application of Physics and Mathematics to Seismology (Phil. 

 Mag. 1897), Dr. Chree supposes that the velocity of 12'5 kilo- 

 metres belongs to the longitudinal wave, that of 2*5 to the 

 transversal, and adduces proof that the substance in the 

 interior of the earth offers enormous resistance to compression. 

 On the above supposition, Young's modulus turns out to be 

 100 xlO 10 c.G.s. units, which is less than that observed on 

 chlorite schist. It seems quite unlikely that the elastic con- 

 stants of the deeply-seated material should be less than those 

 within a few thousand feet of the earth's crust. In addition 

 to this, we have to remark that the disturbance whose velocitv 

 is greater than 10 kilometres is very small compared to that 

 of the principal shock, which is generally 3 kilometres ; and 

 we have no ground to suppose that the effect of the trans- 

 versal wave is greater than that of the longitudinal. On the 

 hypothesis that the interior of the earth consists of homo- 



F 2 



V' : 



