292 



G. R. PUTNAM CONDITION OF THE EARTH'S CRUST 



This attraction was computed as that of an indefinitely extended hori- 

 zontal plate of average surface density and of thickness equal to the ele- 

 vation of the station, with a correction added, where necessary, for the 

 departure of the surface about the station from a plain. Another term 

 in this reduction corrected for elevation above sealeve!. 



2. The free air (elevation only) reduction simply omitted the attrac- 

 tion term in the Bouguer formula. This was done on the theory that the 

 material above sealevel was balanced by a deficiency below. In the terms 

 of compensation, however, omitting the attraction term is equivalent to 



For station above the average level 



Jerey 



1. Bouguer reduction : no 

 compensation. 



i!. Free air reduction ; 

 compensation for station 

 elevation, showing o r e r 

 compensation. 



.!. Average elevation re- 

 duction ( 1895 ) : compensa- 

 tion for average elevation. 



For station below the average level 



i 



B 



1. Bouguer reduction 

 compensation. 



2. Free air reduction : 3. Average elevation re- 

 compensation for station duction (1895) : compensa- 

 Hevation. showing under tion for average elevation, 

 compensation. 



Figure 1. — Diagram illustrating three Methods, used in 1895, of Reduction of Gravity 

 Observations to Sealer el, for tivo stations, located one above and one below the 

 average Elevation. 



In these examples the average elevation is the same for the two stations. The vertical 

 scale is exaggerated to emphasize the compensation effects. If drawn the same as the 

 horizontal scale, a hetter idea would he given of the attraction correction. The compen- 

 sation (shown by horizontal lines) is. for convenience, placed immediately helow sea- 

 level, and of depths equal to the station elevation and the average elevation respectively. 



introducing a compensation term of equal amount and opposite sign : in 

 other words, a compensation correction equivalent, to the attraction of an 

 indefinitely extended horizontal plate whose thickness is the elevation of 

 the station and of average surface density, or the equivalent of this. This 

 method resulted in excessive compensation for stations above the average 

 level and deficient compensation for stations below the average level (as 

 shown in the diagram), and large corresponding anomalies resulted in 

 mountainous regions. For a station at the summit of a peak, for exam- 

 ple, the surrounding region was compensated as though it were all at the 

 elevation of tin 1 peak, evidently an over compensation for the region in 



