xiv INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 anv 1846. 
The adopted zeros of the declinometer scales are as follow :— 
Reading of the long scale at the magnetic axis, . é 4 . 257:14* 
Correction for the effect of the bifilar and balance SN 3 : s + 0:16 
Adopted zero for the long scale in 1845 and 1846, 6 ; - . 257-30 
Reading of the short scale at the magnetic axis, . ; j ‘ : 147-11* 
Correction for the effect of the bifilar and balance magnets, . : ; + 0-14 
Adopted zero for the long scale in 1845 and 1846, : : ; ; 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 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 
the observations, and it is altogether inappreciable, compared with the error due to 
the second. 
If the plane of detorsion be that of the magnetic meridian, and the magnet be 
deflected through an are w by turning the arms of the torsion circle w, the torsion 
is w—u, and the equation of equilibrium is 
m Xu=(w—u) H 
where m is the magnetic moment of the bar, X is the horizontal component of 
the earth’s magnetic force, and H is the torsion force for an are equal to radius, 
whence, ifm X = F, 
Paras - 
is the quantity by which the deviations of the magnet from the plane of detorsion 
should be multiplied to obtain the decrements due to torsion. If» be the observed 
deviation, (1 + ©) = the true deviation. The following observation for the value 
of ® for the suspension thread placed in the instrument, June 22, 1843, and for the 
long scale magnet will serve for the observations in 1845 and 1846 :— 
Dec. 26% 23%, 1844. Arc-! w = { +3005 ave") w= {is9 “eq; mean value of ® = 0-00140. 
* For details, see Introduction to the Observations for 1843, pp. xv. and xvi. 
