608 HARMON LEWIS 



FOR OCEAN AREAS (h BEING A NEGATIVE QUANTITY) 



i. If Wa>Wb thenM>i 



2. If Wa = Wb then M= i 



3. If W A -W B = aSh then if =0 



4. If Wa — Wb< ahh then M< o 



When M=o, there would be no isostatic compensation. The 

 condition that M is negative is equivalent to a distribution of den- 

 sity so related to the surface that the material under any surface 

 is heavier than the material under a lower surface. There would 

 be no isostatic compensation in this case. In view of the facts 

 brought out by Hayford the fourth case is, however, very improb- 

 able. 



"Over-compensation" is such an isostatic compensation that 

 M>i. 



"Complete compensation" is such an isostatic compensation 

 that M=i. 



" Under-compensation " is such an isostatic compensation that 

 o<M<i. 



Isostatic compensation is considered the more complete, the 

 closer M approaches to 1 . 



SECTION II. THE GEODETIC WORK OF JOHN F. HAYFORD 

 BRIEF DESCRIPTION OF HIS WORK AND METHODS 



On certain assumptions as to the size and shape of the earth and 

 as to the position of a base station on this ideal earth, Hayford, 

 by triangulation and geodetic observations, measured the prime 

 vertical and meridian components of the deflection of the plumb 

 bob from the true vertical at several hundred stations scattered 

 over the United States. He then calculated the deflections which 

 all the topographic features within a radius of 2,564 miles of each 

 station should produce if the density of the earth were the same at 

 similar depths. He found that these calculated deflections, which 

 he called the "topographic deflections," were universally larger 

 than the "observed deflections." The only explanation of such 

 widespread observations is that there is some sort of isostatic 

 compensation of the surface excesses and defects of mass. Recog- 



