Chap. 7] 



GRAVITATIONAL METHODS 



139 



The diagram must obviously be drawn at the reduced scale of 1 :p, which 

 means that the scale must be considered in the calculation. The elements 

 are so calculated that their effect remains the same regardless of distance 

 and direction. In this case it is convenient to make rm — rm+i = 4/7r 

 and (pn — (Pn+i = 7r/40, so that the resultant constant for each mass element 

 becomes 0.1. If the numerical value is substituted for the gravitational 

 constant, the effect of each mass element as shown in Fig. 7-38 is 



A^ 



6.6710 ^-S-p-il - cos ^) microgals. 



(7-38e) 



It is seen that the spacing of the concentric circles and of the angles is the 



same and that the effect is independent of azimuth. In application to 



/I 



Fig. 7-38. Diagram (horizontal quadrant) for calculating terrain effect on 

 gravity (after Jung). If \[/ (terrain angle) = tan"^ /i/r, the effect of each field is 

 — 6.67-10~'-5-p(l — cos \J/) microgals, where l:p is the scale to which the diagram is 

 drawn, and 6 is densit3^ 



terrain calculation, the diagram of Fig. 7-38 is first completed for the three 

 remaining quadrants. The surrounding terrain is surveyed by rod and 

 alidade. When the above diagram is used, the elevations are most con- 

 veniently taken in the form of terrain angles. Lines of equal terrain 

 angle are then drawn on transparent paper and superimposed on the 

 diagram, and the number of elements is counted between two adjacent 

 contour lines. The function 1 — cos ^ is taken from the diagram in Fig. 



