THE GEODETIC EVIDENCE OF ISOSTASY 29 



Above this depth, on the other hand, each element of mass is 

 subject in general to different pressures in different directions, 

 to stresses which tend to distort it and to move it. 



The idea implied in this definition of the phrase "depth of 

 compensation," that the isostatic compensation is complete 

 within some depth much less than the radius of the earth, is not 

 ordinarily expressed in the literature of the subject, but it is an 

 idea which it is difficult to dodge if the subject is studied care- 

 fully from any point of view. The data to be discussed to-night 

 indicate that all the isostatic compensation occurs within a thin 

 surface layer of the earth, extending down J^ or possibly ^V of 

 the depth from the surface to the center. 



The geodetic evidence which may be used to test whether or not 

 the condition called isostasy exists, consists of determinations of 

 gravity and of determinations of deflections of the vertical. 



It is to the evidence furnished by the latter that I wish to call 

 your attention to-night. Within the limits of the United States 

 and connected by continuous triangulation, which has all been 

 reduced to one datum, 507 astronomic determinations have been 

 made; 265 of latitude, 79 of longitude, and 163 of azimuth. 

 These furnish that component of the deflection of the vertical 

 which lies in the meridian at 265 stations, and the prime vertical 

 component at 232 stations. These astronomic stations are scat- 

 tered from Maine to southern California, in portions of 33 states. 

 This triangulation and the astronomic determinations connected 

 with it are furnished to the world by the Coast and Geodetic 

 Survey and the Lake Survey and constitute a magnificient 

 contribution by the United States toward the determination of 

 the figure and size of the earth. 



In deriving the figure and size of the earth from observed 

 deflections of the vertical the usual practice has been to ignore 

 the topography around each station, except that occasionally 

 observed deflections have been rejected because they were in or 

 near a mountainous region. The effect of a possible systematic 

 distribution of density in each horizontal stratum of the earth 

 has also been ignored. 



The topographic irregularities are visible and known. The 

 systematic distribution of density below the surface is invisible 



