Chap. 8] MAGNETIC METHOD 343 



in a position at right angles to the inclination by a counterweight at the 

 end of a nonmagnetic bar fastened symmetrically to the magnet. The 

 angle between the bar and the magnetic axis can be changed. The posi- 

 tion of the counterweight changes the "latitude" adjustment, while the 

 angle of the arm with the magnet controls essentially the sensitivity of 

 the instrument. In the construction of the instrument, the centers o 

 gravity of magnet and counterarm are made as nearly coincident with the 

 axis of rotation as possible. During transportation, the swinging assembly 

 is lifted from the bearings and pressed against two confining brackets. 

 Readings in degrees are taken on a graduated circle. Zero of this gradua- 

 tion is up at the highest point of the circle. A trigger is provided to re- 

 lease the magnet from this position. The amphtude of swing from the 



T 

 Fig. 8-28. Action of Hotchkiss superdip. 



vertical position of the north pole is proportional to twice the deflection 

 angle measured statically as described below. A thermometer is mounted 

 inside the instrument case. For dip determinations, the instrument is 

 oriented in the meridian, the counter weight unscrewed, the counterarm 

 placed parallel to the magnetic axis, and a number of readings taken. In 

 making total-intensity observations, the counterarm is set at an angle f 

 determined by the desired sensitivity and the counterweight is so adjusted 

 that the rest position of the system will be at right angles to the direction 

 of incHnation. Since it is too laborious to determine both inclination and 

 total intensity (which, moreover, disturbs the constancy of horizontal bal- 

 ance essential for a relative instrument), total intensity alone is generally 

 observed. 



The following theory holds for static deflections ip from a rest position 

 normal to the direction of dip when the counterarm is adjusted at the 

 angle f. 



