25714* 



+ 016 



257-30 



14711* 



+ 0-14 



xiv Inteoduction to the Makerstoun Observations, 1845 and 1846. 



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



Reading of the long scale at the magnetic axis, 



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



Adopted zero for the long scale in 1845 and 1846, 



Reading of the short scale at the magnetic axis, . 

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



Adopted zero for the long scale in 1845 and 1846, • . . . 147-25 



9. The determinations for the eflFects of the diflFerent magnets npon 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 arc v, by turning the arms of the torsion circle w, the torsion 

 is w — u, and the ei^uation of equilibrium is 



m X M = (w — m) 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 arc equal to radius, 

 whence, if m X = F, 



u H 



= <i> 



w — u F 



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 n be the observed 

 deviation, (1 + <i') w = 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= ['^^^qc; arc-^ u={ T^',^^ ; mean value of $ = 0-00140. 

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



