CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 299 
Adjustment of the Bifilar at the Cocos. 
The bifilar magnetometer was adjusted on the 27th of August, 1848. Inthe unifilar 
suspension the scale read 100'2 ; the bifilar suspension was then substituted. 
Horizontal circle. 
Torsion circle. 
Scale. 
Magnet direct . . . 
o ! 
50 27 
233 55 
99*3 
Magnet direct . . . 
. 140 27 
293 55 
100*0 
Magnet direct . . . 
50 27 
233 55 
96*5 
Magnet reversed . . 
. 230 27 
233 55 
85*5 
Magnet direct . . . 
. 140 17 
293 55 
95*3 
The angle v or angle of torsion =60°. Thermometer 82°‘6. 
Value of di, or of one Scale Division in terms of Radius. 
The variations of the horizontal intensity are obtained by multiplying the differ- 
ences of the scale readings, or of their mean values (corrected for changes of tem- 
perature) by a constant coeflScient 
h=ia . cot V, 
where a is the value of one scale division in terms of radius ; as the horizontal circle 
by means of verniers read off to single minutes, I ascertained the value of a 
directly. 
The first series gave the value of a' . =1*45 
The second series gave the value of a! =1’38 
The third series gave the value of a' . =1*43 
The fourth series gave the value of a! =1*44 
Mean . . 1*433 
The other method consisted in knowing the length, of one division of the scale, and of 
the radius, 
where a'=^X3437''75, 
of an inch, 
r= 17*19 inches. 
, 1 3437*75 100 . 
®““70^ 34*38 “7o'“^’^^* 
1'’43 has been the angular value assumed, and the value of a in terms of radius is 
*0002909 X l'*43 = *000415987, 
and A:=a . cot t?= *0002402 the value of one scale division in terms of the force. The 
scale readings or mean values are corrected for temperature according to the formula 
/=/'-(<' -4 
2 Q 2 
