OUR PRESENT KNOWLEDGE OF ISOSTASY 425 



as any other simple method of distributing the isostatic compensa- 

 tion. It was realized by Professor Hay ford and by the writer of 

 this paper that uniform distribution to the depth of compensation 

 could not be true at all places; yet it was thought that the depar- 

 tures from normal densities in the lithosphere due to the compensa- 

 tion were heterogeneous and that for a large area the effect would 

 therefore be practically that of uniform deviation from normal. 



What follows in this paper is largely based upon the results of 

 the most recent investigation of gravity and isostasy (Special 

 Publication No. 40). 



In this investigation there were used 219 stations in the United 

 States, 42 in Canada, 73 in India, and 40 other stations, principally 

 in Europe — 374 in all. 



There were certain phases of the investigation which had to be 

 confined to stations in the United States, owing to lack of data for 

 the other stations. These consisted of tests to show whether local 

 or regional distribution of compensation horizontally is the more 

 probable, and also of tests to derive the most probable value of 

 the depth of compensation based upon uniform distribution of the 

 compensation vertically from the surface of the ground to the 

 depth of compensation. 



Three hundred and fifty-eight stations were used for the deter- 

 mination of the most probable values for the constants in the gravity 

 formula which gives the value of gravity, y , at sea-level for any 

 latitude, cf>. This formula is 



70 = 978.039 (1+0.005294 sin 2 $—0.000007 sin 2 2 <£)• 



It is a remarkable fact that this formula agrees almost exactly with 

 the formula derived in 191 2 from only 122 stations in the United 

 States alone. That formula was 



Yo = 978-°38 (1+0.005304 sin 2 $—0.000007 sin 2 2 <£)• 



This agreement between the derived formulas is a clear indication 

 that isostasy is present to practically the same degree in other 

 countries as it is in the United States. 



