OUR PRESENT KNOWLEDGE OF ISOSTASY 423 



of unit cross-section which extends from the depth of compensation 

 to the sea-level surface has the same weight. 



One of the main objects of the investigations made in the Coast 

 and Geodetic Survey is to determine to what extent isostasy is 

 proved to exist. Other objects are to determine, if possible, the 

 method of distribution of the compensation, horizontally and 

 vertically, with respect to each topographic feature, and to discover, 

 if possible, the cause or causes of the gravity anomaly which cannot 

 be accounted for by the topography and by the isostatic compen- 

 sation of the topography. By topography is meant the material 

 above sea-level on the continents and islands and the deficiency of 

 density in the matter between the ocean surface and the bottom of 

 the oceans. 



Professor Hayford made the following statement in his second 

 publication on the figure of the earth and isostasy. "One may 

 properly characterize the isostatic compensation as departing, on 

 an average, less than one-tenth from completeness or perfection. 

 The average elevation of the United States above sea-level being 

 about 2,500 feet, this average departure of less than one- tenth 

 part from complete compensation corresponds to excesses or 

 deficiencies of mass represented by a stratum only 250 feet thick, 

 on an average." 1 



Professor Hayford based his conclusion upon the fact that the 

 mean residual or deflection of the vertical, after the isostatic correc- 

 tion was applied, was 2.91 seconds of arc. After only the topo- 

 graphic deflection was applied, the residual was 30.37 seconds. 

 It is seen, therefore, that the application of the effect of the isostatic 

 compensation reduced the average residual from 30.37 to 2.91 

 seconds. 



In making the corrections for the effect of the isostatic com- 

 pensation, Professor Hayford assumed that the compensation was 

 directly under the topographic feature and that it was distributed 

 uniformly to the depth of compensation. This depth of compensa- 

 tion was an unknown quantity, to be determined from the available 

 data. The depth derived by him is 122 km. 



1 Supplementary Investigation in igog of the Figure of the Earth and Isostasy, 

 special publication (not numbered) of the Coast and Geodetic Survey. 



