INTRODUCTION. XXXV 
plane higher than the other; the north 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 Sms, of London, adjusted the agate planes of the balance 
magnetometer. They do not, however, seem to be perfect planes, but have a slight 
degree of convexity in the centre. Mr Srus also inserted spider’s threads for 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 ecard in the first four observations, and of the same silk 
as the suspension thread on the other 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 ¢, by the formula 
The value of 2 for the balance magnet is deduced from that of = for the de- 
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.. 
i Semi-Are. Time of one Vibration. 
Number Number bs 
f of — 
Dei. fej: Vibra- || , 3 ° Corrected for |Vorrected for 
Hionk: Hana ommencing.| Ending. Observed. Monson Torsion and 
Are. 
° r ° , s. s. Ss. 
July 6.1841 28 28 11-920 11.950? 11-950 ? 
Aug. 3.1841 16 144 11-980 12010 12-000 ? 
Sept. 4. 1841 12 128 11-930 11-962 11-952? 
Jan. 11. 1842 6 150 11-969 11-998 11-986 ? 
Aug. 24. 1843 28 120 8 0 2 0 11965 12012 12.008 
Sept. 4. 1843 40 190 2 30 3°40 11-978 12.026 12-017 
Jan. 27.1844 16 70 9 30 3 30 11-959 12.007 11-999 
