336 MAGNETIC METHOD [Chap. 8 



orientation is much greater on the horizontal than on the vertical field 

 balance. As it is not customary to apply a correction, the instrument 

 must be set up accurately before readings are taken; the graduated 

 circle on the tripod head furnishes the required accuracy. 



A tilt of the instrument in any direction may be resolved into a com- 

 ponent tN in the N-S and another i^ in the E-W direction. The influence 

 of the former is of an optical nature, and the change in reading 



As = ±2f(r(nN - no), (8-30) 



where no is the reading of the level bubble (mean of the two ends) in the 

 horizontal position, Wn the new reading if the instrument is tilted toward 

 north, and a the scale- value of the level. For an old-style telescope, 

 2{<T ~ 0.2. As it is not customary to apply a correction for incomplete 

 leveHng, the instrument should be leveled within one-half a division in 

 the N-S direction; at right angles thereto, not so much care is necessary. 

 In comparisoji with the vertical balance, greater care in leveling is neces- 

 sary in the plane of oscillation of the system, since two operating positions 

 to cancel the effect are not available. 



As in the vertical instrument, auxiliary magnets are used for determina- 

 tions of scale value and for increasing the range, and they are applied in 

 the second principal Gauss position. If the scale has disappeared toward 

 the south (readings too great), the auxiliary magnet is used with its north 

 pole toward north. The correction is positive, since the effect of the 

 magnet is negative, and vice versa. The correction in scale divisions 



As = =F ; 



Kl'^-s^') 



Variations in vertical intensity affect the scale value and must be allowed 

 for. As the horizontal balance is generally used in conjunction with the 

 vertical instrument, the vertical intensity is known. From eq. (8-28e), 



Ae = ^. (8-32a) 



The vertical intensity effect is positive for deflections to one side and 

 negative for deflections to the other side of 20. Therefore, the correction 



AH = Ae(s - 20) = ^ (s - 20). (8-326) 



2i 



As differences in H are figured from the base station, differences in Z 

 should be taken the same way, provided that the scale-value determina- 



