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by n single spider thread whose torsional rigidity was found to bo abont ~z of 

 that of a single silk fibre. The upper end of this thread is fixed to the atom of 

 a {an-shnpe<l horn damper. The top part of the magnetometer case is a brass 

 tube and can bo slide up or down by loosening a jam-nut. The upper end of 

 this tube is plane and has in its center a triangular hole through which tho 

 stem of the damper passes. The damper can be geometrically fixed by moans 

 of a small screw pressing it up against the corner of the triangular hole. As 

 a protection from air-currents a small glass cap is fitted to the brass tube. Tho 

 lower part of the case is also a brass tube, which is furnished with four glasa 

 windows, two square and two rectangular. Tho mirror hangs with its face 

 jiarallel to the two square ones while the rectangular (jues are just large enough to 

 allow the mirror to bo viewed through them edge-on. One of tho two square 

 windows is a thin convex lens and tho other is plane, so that the magnetometer can 

 bo made to suit either the Ijimp or the telescopo method. Directly below tho 

 mirror, there is a small brass vice whoso jaws are linetl with cluimois skin and 

 which is worked by a screw from outside the case. To take off the initial 

 torsion of tlic suspending fibre, the top part of the magnetometer is slippe«! 

 down until the mirror can be caught by tho vice, and the whole case is invertc<l 

 and re-set on the tripod stand. The small screw which bears against the atom 

 of the damper is unscrewed, and the top part of the magnetometer is »lipped 

 further along, so that the damf)er hangs free with the spider thread passing 

 through the triangular holo. When the damper comes to rest tho operations 

 are gone through in the reversed order, and the magnetometer is thus 8uspende<l 

 free from initial torsion. Tho magnetometer can be transported safely by 

 having the mirror magnet clampe<l in the vice. 



To work with this instrument, the theodolite stand is set in tho 

 astronomical meridian by any of the ordinary processes. The telescope is 

 dismounted without disturbing the base of the instrument, tho magnetometer is 

 placed in its center, and the coil is mounted on tho Y's. Tho magnetometer is 

 adjustetl to the central position by means of four screws working horizontally in 

 the circular socket on which tho base of the magnetometer case rests. It ia 

 brought into tho north and south axis by sightiug tho edge of tho mirror 

 through tho slits in the ends of the coil, and to the east and west axis by sighting 

 the face of the mirror through one of tho hollow pivots and making clearanco 

 equal all round. A scale is now place<! at a proper position with reference to 

 the magnetometer, and tho reflected imago of a scale division which coincides 

 with the wire of the telescopo is observed. A current from a Daniell cell, which 

 is running steadily through a higli resistance <jf an ordinary resistance-box, is 

 shunte<l through tho declinometer coil by inserting the terminal twin-plug of 

 the leading wires into the plug hole of the high resistance. Tho current ia 

 made so as to prcxluce in the center of tho coil a field which has the same 

 direction as that due to the earth. This is easily determined by observing the 

 rate of vibration of the reflected image. The coil is now turned by mcaos of a 



