Chap. 8] 



MAGNETIC METHOD 



325 



of revolution causes a difference in the E and W readings which cancels 

 in the average of the readings. If the case is inclined, if the lid with the 

 telescope is tilted, or if the instrument is so placed on the tripod head that 

 an E-W tilt is produced, a change in reading results, which does not cancel 

 in the mean of the two positions. Therefore, care must be taken to set 

 up the instrument on the tripod head correctly leveled in the E-W direc- 

 tion. The effect of a meridional inclination of the axis of rotation is the 

 same for the E and W positions, does not cancel in the mean, requires a 



American Aakania Corp. 



Fig. 55-196. Views of magnetic systems o to c of Fig. 8-19a; d is compensated hori- 

 zontal magnetometer system. 



correction or very accurate leveling in the N-S direction. Conversely, a 

 N-S inclination of the case, of the bearings, or of the axis of revolution of 

 the system cancels in the mean of the two positions, and the dijfference in 

 readings is Sw — Se = 22H/c. It is seen that the tripod axis must be 

 carefully leveled in the direction of the magnetic meridian. 



For increasing the range of the magnetometer, auxiliary magnets are 

 used in the extension tube under the tripod head (in first Gauss position). 

 If the scale has disappeared to the left (toward low readings), the magnet 



