Declinometer. xiii 



From the adopted values of the long and short scale divisions, the coefficient 

 for reducing the divisions of the short scale to the same value as the divisions of the 

 long scale = 1'115 ; the reciprocal = 0*897. 



Scale readings at the magnetic axes of the declinometer magnets, the copper 

 ring or damper being in its place. 



The reading for the magnet with the long scale at the magnetic axis = 25744 

 short scale = 147-11* 



The copper ring used for checking the vibrations of the magnet was removed 

 October 15 d 4 h — 5 h , 1844, as it was found to have an effect upon the magnet, varying 

 with its position. 



The adopted zeros of the declinometer scales are as follow : — 



Reading of the long scale at the magnetic axis, ..... 257-14 

 Correction for the effect of the bifilar and balance magnets, . . . +016 

 Correction for the effect of the copper ring, Jan. l d — Oct. 6 d 23 h , 1844, — 1-00 

 Oct. 7 d h — Oct. 15 d 4 h , 1844, - l-70f 



Jan. l d —Oct. 6 d 23 h , 1844. Adopted zero for the long scale, . 256-30 



Oct. 7 d h — Oct. 15 d 4M844 255-60 



Oct. 15 d 5 h , 1844, and afterwards 257-30 



Reading of the short scale at the magnetic axis 147-11 



Correction for the effect of the bifilar and balance magnets, . . . +0-14 



Correction for the effect of the copper ring, Jan. l d — Oct. 7 d , 1844, . — 0-90| 



Jan. l d — Oct. 7 d , 1844. Adopted zero for the short scale, . . . 146-35 



Oct. 7 d , 1844, and afterwards 147-25 



9. The determinations for the effects of the different magnets upon each other 

 are given in the previous volumes. The plate-glass in the declinometer boxes was 

 found to have no effect upon the reading of the magnet : the effect of the copper 

 ring is allowed for above. The determinations of the corrections are given in the 

 volume for 1843. 



10. Correction for the torsion force of the suspension thread. 



The errors due to the torsion force of the suspension thread are produced, first, 

 by the magnet moving out of the plane of detorsion ; secondly, by the variation of 

 this plane (due generally to the varying humidity of the atmosphere). The error 

 due to the former, even in the most marked cases, is less than the probable error of 



* For details, see Introduction to the Observations for 1843, pp. xv. and xvi. 

 f Ibid., pp. xvii. and xxi. 



MAG. AND MET. OBS. 1844. d 



