OUR PRESENT KNOWLEDGE OF ISOSTASY 429 



extent. At the same time, it is equally improbable that the com- 

 pensation should be extended horizontally and uniformly out to any 

 definite distance from a station. It would seem to be more probable 

 that the compensation is distributed regionally, with the greater 

 amount of the compensation directly under the topographic 

 feature, and that it diminishes in amount with the distance from 

 the feature. 



A test was made to show whether local distribution or regional 

 distribution was the more probable. The distribution in each case 

 in the regional method was uniform. The method employed was 

 to take the average elevation of the topography within a certain 

 distance of the station. In one case the distance was 18.8 km., in 

 another 58 . 8, in a third case 166 . 7 km. from the station. With the 

 average elevation within these areas, a computation was made of 

 the effect of the compensation, which was supposed to be uni- 

 formly distributed out to the limit of the area and also uniformly 

 distributed from the surface to the depth of compensation. 



The method of distributing the compensation horizontally 

 necessarily leads to some error, for, as a matter of fact, the com- 

 pensation of each topographic feature should be distributed 

 regionally with respect to that particular feature. But such com- 

 putations would be extremely laborious and it is believed that the 

 results would not be materially different from those which were 

 obtained. That erroneous results might be obtained for a single 

 station is readily perceived when we consider that the station may 

 be on a plain or plateau, say 167 km. in radius, and that just 

 outside of this area there are massive mountain masses. According 

 to the theory of regional distribution of the compensation, the 

 compensation of the mountain masses should be extended under the 

 plains ; it should therefore have an effect on the computed gravity at 

 the station. It would tend to make the computed value of gravity 

 at the station smaller than it would otherwise be. On the other 

 hand, we might have a station in a mountain mass of, say, 167 km. 

 in radius, with plains surrounding the mass. In this case all the 

 compensation of the mountain mass would be used in the computa- 

 tion of the corrections to gravity. Some of the compensation 

 should be distributed for a distance of 167 km. out into the plains. 



