XXVI 



Introduction. 



Table 10. — Value of the Bifilar Magnetometer Scale Divisions. 



Scale 



Theodolite 



Scale 



Theodolite 



Value of 50 



Division. 



Reading. 



Division. 



Reading. 



Scale Divisions. 



110 



5 27 40 



160 



4 31 30 



/ // 



56 10 



120 



5 16 25 



170 



4 20 15 



56 10 



130 



5 5 10 



180 



4 9 



56 10 



140 



4 53 55 



190 



3 57 45 



56 10 



150 



4 42 45 



200 



3 46 35 



56 10 



160 



4 31 30 



210 



3 35 20 



56 10 



Mean value of 1 Scale Division l'-1233. 



30. June 23. 1841. A number of observations were made, following tli(^ 

 directions of the Report of tbe Royal Society, which were rendered valueless by 

 various accidental causes, but chiefly from an extreme idea of the extent of accurac}' 

 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 



= 0'0005194 inch, —.^-kt^^^^tfta — = 175 was the number of divisions through which 

 ' 00005194 ° 



the micrometer head was turned back, in order to equalize the intervals above and 



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 



F 



angles to the meridian. From eq. (1) ^ = sin w = 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 Y u — Gv, whence u + v~ (^ + l\ v 



V = (118-89) r-1233 = 33'. 



U-^V =:-.;q x33'= 67' 



The arms of the torsion circle were turned 1° 1'. Torsion circle reading 270° 1' 

 Scale reading with weight attached, 149. 

 magnet . . 152. 



