Declinometer. xv 



12. The following are all the observations for the elimination of torsion made 

 in 1844, together with all the occasions on which the magnet was touched or removed 

 from its box. When the mean position of rest for the north end of the brass bar 

 was found to the east of the magnetic north, the torsion existing is considered posi- 

 tive, and the effect of + 10° of torsion = — 0'*84 (from the previous observation for the 



TT 



value of — •) I n the first case below, the north end of the brass bar was found to 

 F 



rest 2° east of the magnetic north ; and the brass bar was thereafter made to coin- 

 cide with the magnetic meridian. 



Jan. l d 3 h — 4 h . Torsion removed, + 2°. Jan. 12 d 2 h — 3 h . Torsion removed, - 11°. 



Jan. 26 d 21 h — 27 d 7 h . The magnet with the short scale was used in the decli- 

 nometer box. 



Jan. 27 d 2 h — 3\ Torsion removed, 0°. Feb. 13 d 2 h — 3 h . Torsion removed, + 2°. 



Feb. 17 d h — 7 h . Short scale magnet in the declinometer box and deflection 

 bar vibrated. 



Feb. 23 d 3 h — 4 h . Torsion removed, + 11°. March 22 d 23 h — h . Torsion re- 

 moved, 0°. Afterwards, the short scale magnet was placed in the box, and the de- 

 flection bar vibrated. 



March 27 d 4 h — 5 h . Torsion removed, + 3°. The suspension thread was found 

 to have stretched a little, it was now wound up two-tenths of an inch, and the 

 torsion again removed as follows : — March 27 d 4 h — 5 h . Torsion removed, + 9^°. 

 April 5 d 4 h — 5 h . Torsion removed, - 31°. May 28 d h — l h . Torsion removed, 0°. 

 May 28 d P — 29 d 9 h . Short scale magnet used in the declinometer box, and after 

 29 d 9 h , the deflecting bar was vibrated, for the intensity observation, in the decli- 

 nometer box ; afterwards, the torsion was removed as follows : — 



May 29 d 23 h — -0 h . Torsion removed, + 81°. Aug. 4 d 22 h — 23 h . Torsion removed, 

 + 1°. Aug. 4 d 23 h — 5 d 6 h . Short scale magnet used in the declinometer box, and 

 the deflection bar vibrated. 



Oct. 6 d 19 h — 20 h . Torsion removed,— 10-|°. This observation was made rather 

 hurriedly, as the hour of observation was approaching : it was conceived that the 

 true amount of torsion might have been less. Oct. 6 d — 7 d . Experiments were made 

 to determine the effect of the copper ring on the declination magnet.* Oct 15 d 4 h . 

 Other observations were made for the effect of the copper ring, after which the use 

 of the copper ring was discontinued. Oct. 15 d 23 h — 16 d 4 h . Observations were made 

 to determine the zero point of the declination scale ; the amount of torsion was then 

 determined as follows : — 



Oct. 16 d 4 h — 5 h . Torsion removed, + 9-|°. The amount of torsion found is nearly 

 the same in amount as that found Oct. 6 d , but opposite in sign. Nov. 4 d — 6 d . The 



* See Introduction, 1843, pp. xvii. and xviii. 



