OUR PRESENT KNOWLEDGE OF ISOSTASY 437 



within which all of the compensation should be concentrated in order 

 to have its attractive effect equal to the effect of the compensation 

 uniformly distributed from the surface of the earth to a depth of 

 113.7 km. In other words, if all of the compensation were con- 

 tracted to the disk, at the particular depth, it would have the same 

 effect as the uniform distribution. 



If the compensation is distributed regionally to a distance of 

 10 km. from a station the disk within which all of the compensation 

 is supposed to be concentrated must be placed 21.3 km. below the 

 station. With regional compensation distributed to a distance of 

 20 km., 60 km., or 100 km. from the station, the depth of the disk 

 becomes respectively 28 . 6 km., 41.2 km., or 45 . 5 km. If the com- 

 pensation is started at sea-level instead of at the surface of the 

 ground, .each of the depths given above should be increased by 

 about 1 km. 



These depths are of particular significance, for they represent 

 what may be called the effective center of the compensation on the 

 basis of uniform distribution with respect to depth and with a 

 depth of 1 13. 7 km. This depth, as shown by certain tests, gives 

 practically as good results as what may be called the most probable 

 depth of 96 km. It is significant that there can be a variation in 

 the depth of as much as 18 km. without materially affecting the 

 anomalies. 



It is reasonably certain that the effective depth as given above 

 would be practically the same for all of the intermediate depths 

 used in the computations to show which was the most probable 

 depth. We may conclude, therefore, that the figures given above 

 actually represent the effective center of the compensation, regard- 

 less of the method of distribution of the compensation. If, for 

 instance, the compensation were considered to be confined to a 

 zone about 20 km. in thickness, the center of that zone would have 

 to be between 30 and 50 km. below sea4evel. If the compensation 

 is distributed according to the Chamberlin method, 1 the greater 

 portion of the compensation would necessarily have to come within 

 100 km. of the surface, but there would be part of it at some 

 distance below that depth. 



1 See The Figure of the Earth and Isostasy from Measurements in the United States, 

 p. 160. 



