394 SEE— THE NEW THEORY OF EARTHQUAKES [November 15, 



these movements correspond to quantities represented in arc by 

 o".30 and o".i5, which are on the very Hmits of our most accurate 

 astronomical measurements. The apparently sharp turns in the pole 

 path, therefore, may not be real, but merely a delusion arising from 

 some unknov^n cause. If, hov^ever, these sharp turns are real, 

 this fact cannot be certainly established till the polar motion has 

 been repeated many times, and the data now available do not seem 

 sufficient for this purpose. 



Moreover, even if the sharp turns are real and it could be proved 

 that the great earthquakes gathered about them, in regard to which 

 there is still much doubt, our interpretation of the results would be 

 open to much uncertainty. For it could not be inferred that large 

 masses of matter are in motion within the earth, and that the dis- 

 placement of these masses causes a sudden change in the movement 

 of the pole. We have seen that the matter of the earth's interior 

 is kept rigid by pressure, and that no movement deep down in the 

 globe ever takes place. This is shown by the fact that a shock 

 originating deep down would be felt with moderately uniform in- 

 tensity over a large area, which is disproved by observations showing 

 that all great earthquakes are quite superficial. If no shocks orig- 

 inate at great depth, the only other possible movement would be 

 just beneath the crust ; and it is quite impossible that currents could 

 move for great distances just beneath the surface without disturbing 

 the whole intervening region of the globe. 



Much as has been done on this difficult subject by the greatest 

 mathematicians, it is not yet known what causes operate to displace 

 the pole from the mean axis of figure, and maintain the displacement 

 in spite of friction and viscosity; but recurring seasonal effects, to- 

 gether with the imperfect rigidity of the earth, have been generally 

 accepted as the chief causes. Is it not, therefore, probable that 

 whatever displaces the pole and thus maintains its revolution about 

 the mean pole, according to the Eulerian Theory as modified by 

 viscosity and imperfect rigidity, may suffer variations through com- 

 binations of storms, and other atmospheric and tidal agencies ? And 

 that these combinations of varying stresses operate to bring on earth- 

 quakes where instability of the crust already exists, from the accu- 

 mulation of subterranean steam pressure? 



