298 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
the accuracy of the adjustment is tested by turning the telescope 180° in azimuth, 
so as to reverse the direction of the magnet, or bring its north end to the south. The 
telescope is again turned 90° at right angles to the magnetic meridian, so that in- 
creasing readings denote a decrease of force ; the vernier of the torsion circle is 
turned till the central division of the scale is again on the vertical wire. The magnet 
is then perpendicular to the magnetic meridian, and the instrument is in adjustment. 
Thus much was necessary for understanding the following adjustments; those at 
the Cocos and at Batavia only are given ; the adjustments at the rest of the stations 
being precisely similar, since the same angle of torsion z;=60° was selected for every 
station. 
Adjustment of the Bifilar at Batavia. 
The bifilar was adjusted in the afternoon of the 8th of November 1846 at 3 p.m. 
With the single suspension thread, the scale read 100; the horizontal circle 82° 23'. 
The bifilar apparatus was then carefully substituted. 
Horizontal circle. 
Torsion circle. 
Scale. 
Magnet direct . . 
o / 
... 82 23 
53 50 
100 
Magnet direct. . 
... 172 23 
113 50 
100 
Magnet direct. . 
... 82 23 
53 50 
99 
Magnet reversed . 
. . . 262 19 
53 50 
99 
Magnet direct . . 
... 172 23 
113 50 
92-3 
Before taking down the instrument a fresh series was taken on the 7th of August 
1847 : premising that the reading 82° 23' on the circle is direct, or the north end of 
the magnet to the north, and 262° 23' is reverse, or the north end of the needle to the 
south ; the following is the readjustment : — 
Horizontal circle. 
Torsion circle. 
Scale. 
Magnet direct .... 
0 / 
. 172 23 
113 50 
132-8 
Magnet direct. . . . 
82 22 
53 52 
95-0 
Magnet reversed . . . 
. 262 20 
53 52 
73*0 
Magnet direct. . . . 
82 30 
53 40 
85-0 
Magnet reversed • . . 
. 262 50 
53 40 
78-0 
Magnet direct .... 
. 82 50 
53 40 
79-0 
Magnet direct .... 
. 172 50 
113 40 
121-0 
Magnet direct. . . . 
. 172 50 
113 30 
115-0 
Magnet reversed . . . 
. 262 40 
53 45 
110-0 
Magnet direct. . . . 
82 40 
53 45 
99-5 
Magnet direct . . . . 
. 172 20 
113 50 
110-0 
This small change of numbers is satisfactory, considering that the portable 
had been up for more than eight months. 
