XXV1 INTRODUCTION. 
TABLE 10.—Value of the Bifilar Magnetometer Scale Divisions. 
Scale Theodolite Scale Theodolite Value of 50 
Division. Reading. Division. Reading. Scale Divisions. 
ayn 
4 20 
9 
3 57 
3 46 & 
3 35 
5 
B) 
5 
4 
4 
4: 
Mean value of 1 Scale Division 1/1233. 
30. June 23. 1841. A number of observations were made, following the 
directions of the Report of the Royal Society, which were rendered valueless by 
various accidental causes, but chiefly from an extreme idea of the extent of accuracy 
to be expected in the adjustments, and probably from the changes in the horizontal 
force being considerable at the time. The collimator was also sometimes moved 
accidentally during the adjustment. 
A silver wire having been prepared, and the extremities attached to the suspen- 
sion roller, the grooved wheel, No. 8, whose diameter = 0-409 inch, was inserted, and 
the stirrup with a brass weight appended. The zero of the micrometer head of the 
bifilar screw being equivalent to an interval of half an inch, and each of its divisions 
=. : 0-500 — 0-409 
= 0-0005194 inch, —)o9o5i94 — 
the micrometer head was turned back, in order to equalize the intervals above and 
= 175 was the number of divisions through which 
below. 
The weight being now removed, and the magnet inserted, it was found necessary 
to turn the torsion circle through 159°, to deflect the magnet to a position at right 
angles to the meridian. From eq. (1) = = sin v= cos 159°=0-93. 
This wheel was retained. 
The magnet was nearly in the magnetic meridian when the torsion circle read 
268° 54’; the scale reading was then 118-0. 
The magnet was removed and the brass weight suspended, when the scale read 
89:0. Approximately equation (1) becomes F w = Gv, whence w + ¢ = (5 + 1) v 
v = (118—89) 11233 = 33’. 
1:93 33'= 67" 
ae 
The arms of the torsion circle were turned 1° 7’. Torsion circle reading 270° 1’ 
Seale reading with weight attached, 149. 
magnet . . 152. 
U+tv= 
