504 JOSEPH BARRELL 



order of magnitude, or the direction of the truth, is all which may 

 be now ascertainable. 



In conclusion, it is seen that the hypotheses outlined by Arrhe- 

 nius imply a thinness of the crystalline lithosphere and a crust al 

 weakness wholly at variance with the conclusions regarding strength 

 which have been reached in this investigation. They imply a 

 difference in nature of the earth's interior from that given by the 

 more direct lines of evidence, as shown by the body resistance of 

 the earth to vibratory distortions of both short and long periods. 

 Because of these many difficulties, this group of hypotheses, 

 adopted by Arrhenius, has already been largely discarded, though 

 they still find considerable acceptance, more especially by workers 

 in related fields of science. But the measures of Kthospheric 

 depth and strength which appear to be given by geodesy add their 

 testimony to the cumulative evidence against these views. 



THE EVIDENCE OF TIDES ON RIGIDITY AND STRENGTH 



The tidal distortion of the soHd earth measured by means of 

 the horizontal pendulum has shown that its rigidity is of the order 

 of magnitude of steel. But the recent measurements by Michelson 

 and others, employing a long horizontal pipe partly filled with 

 water, showed clearly that the earth's rigidity is even greater than 

 that of steel' This higher value is in agreement with the induc- 

 tions from the observations on the variations of latitude. But 

 these measurements give the rigidity of the earth as a whole, not 

 the distribution of rigidity. The resistance to tidal deformation 

 is furthermore compHcated by the influence of gravity and increas- 

 ing density with greater depth. Even if the earth were a liquid 

 globe it would resist tidal distortion to one-third the degree of the 

 resistance of a globe of steel, and if the hquid sphere were denser 

 inside, this ratio would be further decreased.^ Notwithstanding 

 this factor, however, it is clear that the earth as a whole is more 

 rigid than steel. As the outer part is known to be less rigid than 

 steel, it follows that the rigidity of much of the interior must be 



" "Preliminary Results of Measurements of the Rigidity of the Earth," Jotir. 

 Geo/., XXII (1914), 118. 



" A. E. H. Love, Elasticity, p. 306. 



