312 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
and azimuth instrument, made by Robinson, and sent out on the first establishment of 
the observatories, and a chronometer (824), Arnold and Dent, which had been so 
long in use, that it was not to be trusted for a fixed rate. The first instrument set 
up was the portable declinometer ; this instrument was tedious to adjust, in conse- 
quence of the suspension thread, to support the collimator magnet, being stronger 
than there was occasion for so as to obviate the necessity of frequent renewal; the 
stirrup with the brass weight attached to the suspension thread, was allowed to swing 
for more than a couple of hours ; the opportunity was taken during the interval of 
determining the dip with four needles. At 8^ a.m. the dip observations being completed, 
the stirrup of the portable declinometer, by frequent small twists of the torsion circle 
having been brought to rest in the magnetic meridian, was finally in adjustm£nt. 
The brass weight for taking the torsion out of the threads was removed, and the col- 
limator magnet placed in the stirrup. The altitude and azimuth instrument was 
adjusted in rear of the collimator magnet, and in the direction of its axis, but always 
to the south of it, at a distance of about 4 feet. By 9 a.m., this instrument being also 
in adjustment, the copper damper was placed carefully in the oaken box containing 
the collimator magnet, by which the latter in a few minutes was brought to rest. 
During this short interval three or five altitudes of the sun were taken with the 
sextant and artificial horizon, and the corresponding times with the chronometer. 
At 9|a.m., the collimator magnet being at rest, altitudes of the sun were observed 
with the altitude and azimuth for magnetic declination or variation; these being 
finished, observations for horizontal intensity, both of deflection and vibration, were 
proceeded with and generally finished at 1 1^ a.m. At noon circum meridional altitudes 
of the sun were observed for latitude, and in the afternoon, if fine, equal altitudes 
were taken to confirm the morning sights. The tent was then taken down, the instru- 
ments packed up, and generally sent off* in the 
evening, if practicable, to the next station. 
The following is a rough diagram (not drawn 
to scale) of the relative positions of the instru- 
ments in the tent : — 
a. The dip circle, removed at 8^ a.m. 
h. Declinometer-box, with deflecting arms,/’,/’, ^ 
for the determination of the absolute horizontal 
intensity. 
c. Collimator magnet, in declinometer-box, sus- 
pended by three or four fibres of untwisted silk. 
d. Telescope of altitude and azimuth. 
e. Position of table on which the chronometer 
was placed. 
Dimensions of the tent 12x 14 feet. 
