136 TRANSACTIONS. 1906-7. 



triangulation of the United States, and has found 71 miles as the 

 most probable value for the depth of compensation, that is, the 

 depth at which the compensation of the excess of matter at the 

 surface (continents) by defect of density below, and of surface 

 defect of matter (oceans) by excess of matter below is complete. 

 At and below this depth the condition as to stress of any element 

 of mass is isostatic, that is, any element of mass is subject to 

 equal pressure from all directions as if it w^ere a portion of a perfect 

 fluid. From this it appears that the behaviour of the magma, 

 situate beyond 71 miles, is that of a liquid. 



As earthquakes are so intimately bound up with stresses, 

 we quote Hayford: "In terms of stresses it is safe to say that 

 these geodetic observations prove that the actual stresses in and 

 about the United States have been so reduced b};- isostatic ad- 

 justment that they are less than one-tenth as great as they would 

 be if the continent were maintained in its elevated position, and 

 the ocean floor maintained in its depressed position, by the rigidity 

 of the earth. It is certain that for the United States and adjacent 

 regions, including oceans, the isostatic compensation is more 

 than two-thirds complete, perhaps much more. " Hayford's 

 result is one of the most brilliant mathematical deductions in 

 geodesy and geophysics in recent years. 



Several causes have been adduced which contribute or ma}^ 

 contribute to the production of earthquakes. In investigations 

 one is sometimes led to the discovery of widely different phenomena, 

 which, however, synchronize with each other, thereby raising the 

 ciuestion whether one is dependent upon the other, or whether 

 both phenomena depend ujDon a common cause. Such a case is 

 the possibility of a connection between latitude variations and 

 earthquakes. On this point Professor Omori, one of the foremost 

 of seismologists, says: "From an examination of the mean 

 monthly values of the latitude of Tokio, I have found that all the 

 destructive earthquakes of recent years in Japan occurred exactly 

 or very nearly when the latitude was at a maximum or minimum. " 



Verily our solid earth is only so in a Pickwickian sense. 

 The surface slides bodily over the figure of revolution, our excur- 

 sions in latitude being about 26 feet. On solid rock we make 

 marks as reference points, unalterable as we thi ik, for levels and 

 other, measures, — the earth heaves but a sigh and our basal points 

 lose their value. In despair almost Ave exclaim "Is their nothing 

 stable on this earth? " 



