The Evolution of the Sciences 



solid crust; therefore the fact that the origin of 

 similar disturbances has in no case been traced 

 to the deeper regions of the globe supplies a new 

 and powerful argument in favour of internal 

 fluidity. 



But we have better proofs than these argu- 

 ments, because we are able to appeal directly to 

 experiment, and to utilise the earthquake to 

 auscultate the globe in its depths. Some of the 

 oscillations recorded by seismographs are pro- 

 pagated at velocities between three and four 

 miles per second, and are probably transmitted 

 through the solid crust of the globe. Experi- 

 ments with explosives, carried out in Japan, 

 have shown the velocity of propagation of waves 

 through the earth's surface to be about two 

 miles per second; through granite the velocity 

 exceeds three miles. But besides these the 

 instruments record other and much more rapid 

 waves (about eight miles per second). These 

 tremors, which can cross the earth from end to 

 end in less than half an hour, must pass through 

 mediums entirely different from those forming 

 the crust. We must add, however, that physics 

 is not at present in a position to inform 



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