Chap. 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



13 



curvature of the equipotential surfaces (see Fig. 2-5). Hence, the de- 

 flection is proportional to the deviation of the equipotential surface from 

 the spherical shape and the deviation of the beam from the direction of 

 minimum curvature. 



In the second type of beam the two weights are attached to its ends at 

 different levels (Fig. 2-46). In addition to the above "horizontal direct- 



(a) 



(6) 



(c) (d) 



Fig. 2-4. Eotvos torsion balance, (a) Beam of the first kind; (6) beam of the 

 second kind; (c) tilt beam; (d) Askania double tilt-beam balance {American Askania 

 Corp.). 



ing forces," this beam is affected by the difference in direction of the 

 gravity forces on the upper and lower weights, which increases with the 

 convergence of the equipotential surfaces passing through the weights, 

 that is, with the rate of change, or ''gradient," of gravity (see Fig. 2-6). 

 The torsion balance is so sensitive that one may detect convergences of 

 equipotential surfaces of the order of 1/100,000 of an arc-second, which 

 corresponds to a horizontal variation of gravity of 10~' galsVcm. 



' See footnote on p. 10. 



