THE NATURE AND INTERPRETATION OF GEODETIC EVIDENCE. 27 



attraction of the plateau is obviously larger in amount than that of 

 the compensation, and the net effect will be a downward pull at 

 S, which will increase the amount of the local measure of gravita- 

 tion at that station. Conversely in the small column of rock situated 

 so that the centre of compensation lies at C", on the line S M, 

 and of the attraction of the plateau at A", the effect of the com- 

 pensation is obviously in excess of the attraction of the mass above 

 sea level, and the net effect at S will be a diminution of the local 

 measure of gravity. Somewhere between these two points must 

 come a limiting distance, where the effect of the attraction of the 

 mass above sea level is exactly balanced by that of the compensa- 

 tion and the net effect at S reduced to zero ; at lesser distances the 

 effect of the elevated mass will exceed that of the compensation, 

 and the net effect will be an increase in the local measure of gravity, 

 but to a less extent than if there were no compensation, and at 

 greater distances the effect will be reversed, and the net effect be a 

 diminution of the force of gravity at S. 



The distance from the station at which this reversal takes place 

 depends in part on the height of the station and the surrounding 

 topography above sea level, and partly on the depth and nature 

 of the compensation. For the particular hypothesis of compensa- 

 tion used by Messrs. Hayford and Bowie the distance is about five 

 or six miles ordinarily, but in the case of stations of great altitude 

 may reach nearly twelve miles. An idea of the nature and amount 

 of the effect of the direct attraction of an elevated mass and its 

 compensation may be got from Table 3 (on next page), which shows 

 the effect of the attraction of a circular plateau, of varying heights 

 and dimensions, at a point in the centre of its upper surface, the 

 values being expressed in dynes and calculated from the Hayford 

 and Bowie tables. 



Here we see that the effect of the mass of a circular plateau of 

 1,000 feet in height, contained within a radius of 1*4 miles, amounts 

 to '031 dynes, and that no appreciable increase results from an enlarge- 

 ment of the plateau to a radius of 100 miles, the moie distant masses 

 being so nearly on a level with the station that the vertical com- 

 ponent of their attraction is negligible. If, however, we take the 

 effect of compensation into consideration a reduction in the net 

 effect becomes apparent beyond five miles from the station. For 

 greater heights there is a continuous increase in the effect of the 

 visible mass up to the limits considered in the table, but beyond a 



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