192 



GRAVITATIONAL METHODS 



[Chap. 7 



scale distance of 2.5 m. Accuracy of 1 Eotvos (1/3000 part of the normal 

 gradient) would require that the apparent change in weight be determined 

 to 1-10 mg. In that case it would be necessary to observe the vertical 

 gradient in different azimuths, since gravity varies not only in vertical 



rib^ 



Fig. 7-64. Schmerwitz balance for determination of vertical gravity gradients. 



but also in horizontal direction. Thus, the action of all three gravity 

 components on the beam is given by 



/ dm\{gx cos a + ^„ sin a) -z + gz{^ cos a + y sin a)]. 



C. Instrument Types, Instrument Constants 



1. Types. Although numerous types of torsion balances have been 

 proposed and designed, comparatively few have attained commercial sig- 



