320 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
the ivory scale was always made use of instead of the horizontal limb of the instru- 
ment ; and the advantage gained by this was rapidity of observation. 
Jones’s, or No. Ill Declinometer, was received at the termination of the Survey; 
it was liable to derangement from the diflSculty of clamping the limb securely ; in 
other respects the instrument was a very great improvement upon No. II. Declino- 
meter ; the ivory scale had a greater range ; the deflecting bar at right angles to the 
instrument was a strong brass scale, supporting a moveable stirrup on its upper edge, 
on which the deflecting magnet was placed, so that it could be moved backwards 
and forwards by sliding the stirrup along the scale : the great advantage of this 
arrangement consisted in the deflecting magnet never being touched, nor affected by 
the heat of the hand. 
Six deflecting needles were used with the three declinometers, D 5, D 6, A 7, A 8, 
A 9 and A 10. D 6 was lost at Padang in Sumatra. 
The Portable or No. IV. Declinometer, was used both for variation and for hori- 
zontal intensity ; so that after altitudes of the sun and the magnetic axis of the col- 
limator magnet had been observed, the instrument became available for observations 
of deflection and vibration; C 15 a collimator magnet, being suspended, and C7 
the deflecting needle. The angles of deflection were read off on the glass scale of 
the suspended magnet, the divisions of which were very coarsely cut. 
Explanation of Table G. 
The observations are contained in Table G. from page cxl to page cliii. The re- 
sults are given in eleven columns. The first contains the date ; the second, the 
station ; the third and fourth, the suspended and deflecting magnets ; the fifth and 
sixth, the distances at which the magnets were placed, with the corresponding angles 
of deflection ; the seventh, the observed time of 300 vibrations ; the eighth, the De- 
clinometer ; O standing for the Observatory, I for the Induction, J for Jones’s, 
and P for the Portable Declinometer ; the ninth and tenth, the values of m and X, 
m being the moment of free magnetism of the deflecting bar, and X the horizontal 
component of the earth’s magnetic force ; and the eleventh, the mean value of X. 
Mode of Observation in determining the Horizontal Component of the Force. 
The position of the deflected magnet was read off without the deflecting magnet, 
both at the commencement and at the end of the observations, and the change 
which had occurred in the declination in the interval was thus shown without the 
necessity of a subsidiary instrument. The change of declination observed was spread 
over the observations, on the supposition that it had been uniform. The deflecting 
magnet was placed on the brass scale to the west of the declinometer, and moved 
successively to four or eight distances, with the marked end of the needle to the west 
and to the east. The needle was then placed to the east of the deflected magnet, 
and a similar operation performed. This concluded the experiments of deflection. 
