Introduction. 



XXXV 



plane higher than the other ; tlie r.orth agate plane was also inclined to the west, 

 and the south agate plane slightly inclined to the east. 



In the second series, July 27. 1841, each reading is the mean of two observa- 

 tions, for each of which the needle was lifted. After this observation, the horizontal 

 screw was moved till the readings became about — 150*0 ; the north pole of the 

 magnet being; turned east. 



The correction for deviation of the line joining the crosses from the magnetic 

 axis has not been applied. 



Aug. 30. 1841 . Mr Simms, of London, adjusted the agate planes of the balance 

 magnetometer. They do not, however, seem to be perfect piane.^, but have a slight 

 degree of convexity in the centre. Mr Simms also inserted spider's threads ibr the 

 bisection crosses, instead of the silver wire; the reading micrometers were adjusted 

 at the same time. 



41. Vibration of the balance magnet in a horizontal plane. 



The magnet was attached to the suspension thread of the declinometer by 

 means of a light stirrup of card in the first four observations, and of the same silk 

 as the suspension thread on the o;her occasions. A small piece of wire in the first 

 four observations, and of paper afterwards, was gummed to the extremity of the 

 needle, and the times of passing the wire of the fixed telescope were noted. The 

 corrected time T' is deduced from the observed time of vibration t, by the formula 



T' = / ( 



1 + 



2 1^^" 



a a'\ 



IE) 



XT XT 



The value of rrr^ for the balance magnet is deduced from that of ^, for the de- 



J r '-' a 



clination magnet, in multiplying the latter by the coefficient.. 



Force of Declination Magnet 

 2 Force of Balance Magnet. 



Table 15. — ^Values of T', the Time of Vibration of the Balance Needle in a Horizontal Plane. 



Date. 



Number 



of 

 Estima- 

 tions. 



Number 



of 

 Vibra- 

 tions. 



Semi-Arc. 



Time of one N'ibration. 1 



ommenciag. 



Ending. 



Observed. 



Corrected for 

 Torsion. 



Corrected for 



Torsion and 



Arc. 



July 6. 1841 

 Aug. 3. 1841 

 Sept. 4. 1841 

 Jan. 11.1842 

 Aug. 24. 1843 

 Sept. 4. 1843 

 Jan. 27. 1844 



28 

 16 

 12 

 6 

 28 

 40 

 16 



28 

 144 

 128 

 150 

 120 

 190 



70 



O f 



8 

 2 30 



9 30 



2 



3 

 3 30 



s. 



11-920 

 11-980 

 11-930 

 11-969 

 11 965 

 11-978 

 11-959 



s. 



11 950? 

 12010 

 11-962 

 11-998 

 12012 



12 026 

 12 007 



s. 



11-950? 



12000 ^ 



11-952? 



J 1-98^1 ? 



12008 



12017 



11-999 



