8 W. Bowie — Relation of Isostasy to 



by the two older methods of correcting gravity observa- 

 tions. One of these is the Bouguer method, based on the 

 theory that the earth is rigid and that the topographic 

 features are actual excesses or deficiencies of mass sup- 

 ported by the earth. The other is the free-air method 

 of reduction in which no account whatever is taken of the 

 topography and the isostatic compensation. It is the 

 same as if we assumed that the topographic features were 

 compensated for at zero depth. Of course this is an 

 artificial method but it was adopted in order to reduce 

 some of the anomalies by the Bouguer method. 



Anomalies greatly reduced hy considering isostasy. 



The investigations of the figure of the earth and isos- 

 tasy showed that the topographic features were compen- 

 sated to such an extent that the residuals or the 

 unexplained deflections of the vertical were only one-tenth 

 of what they would have been if the earth had been rigid. 

 Therefore, we may assume that the columns from the 

 surface down to the depth of compensation are compen- 

 sated within approximately ten per cent of the topog- 

 raphy at the top of the column, in some cases land above 

 sea-level and in others deficiency of matter in the oceans. 



An attempt was made to show what reduction has been 

 made in the gravity anomalies as a result of applying the 

 isostatic theory. Two methods were employed. One 

 considered the effect of the topography for the whole 

 world and the other considered the effect of the topog- 

 raphy out to a distance of approximately 100 miles from 

 the stations. By the first method, it is found that the 

 isostatic gravity anomalies are only 17 per cent of what 

 they would be with a rigid earth, ^j the second method, 

 the average isostatic gravity anomaly was found to be 

 only about 13 per cent of what would be the average 

 anomaly on the rigid earth. 



In the first test, all stations of the country were used, 

 whether at high elevations or at low ones, but the data 

 for the effect of the topography on the opposite side of 

 the earth from the station were not as reliable as we 

 should have liked them to be. This is due to the fact that 

 the data for the effect of topography and compensation 

 for the very distant areas were not computed separately 

 but in combination. In the second test, only stations 

 having an elevation greater than 2000 feet were used, and 



