CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 293 
the Eastern Archipelago ; pages ii and iii contain the mean oscillation of the declina- 
tion, the range extending from the most westerly declination assumed as zero, to the 
most eastern position of the magnet. As increasing readings denote a movement of 
the north pole of the magnet eastward, the lowest number will of course be the most 
western position or zero, the highest number being the most eastern position. As 
these tables contain the mean oscillation for several months, the oscillations at those 
stations where observations have been taken for more than one month are given in 
pages iv to vii inclusive. 
In addition to the three Portable Declinometers, No. IV., a small 3-inch cylinder 
used as the deflected magnet in the determination of the absolute horizontal intensity, 
and No. V., the 15-inch observatory bar, were observed at Singapore. The oscilla- 
tions of these two instruments are given at pages iv and v, reduced to minutes of 
angular space, and the mean results are included in the Table of Declinometer 
No. III. at pages ii and iii. It occurred to me constantly whilst at the observatory 
to note, that not only did the large magnet differ from the smaller magnets in its time 
of changing, but the range or oscillation was not the same ; the time of change dif- 
fered but slightly it is true ; but it was not identically the same*. I determined there- 
fore to use on the Survey never less than two magnets ; one a solid, the other a hollow 
cylinder, for the changes of declination; and on my return to Singapore in 1848, I 
took nearly simultaneous observations of as many declination magnets as could 
be observed together. 
On inspection of the Tables of the mean oscillation of the declination at pages 
ii and iii, I was struck with the dissimilarity of the times of occurrence of minimum 
westerly declination at the different survey stations ; and as the observations were 
taken at all seasons of the year, it appeared to be exceedingly desirable to have some 
fixed observatory to refer to, to ascertain what changes might be due to geographical 
position and what changes to the time of the year. I have, therefore, at pages 
viii, ix, X and xi, given the mean hourly oscillation of the magnetic declination at 
the Singapore Observatory, for each season being the mean of three years ; the mean 
of each of the months ; and finally, the mean of each of the three years, showing the 
founded with the angle a, or the angle which the magnetic meridian forms with the true meridian, and which 
is usually called the variation of the needle ; but to prevent mistakes, for the angle a, the term magnetic decli- 
nation has been substituted, in accordance with the method now generally adopted. 
* I attributed at the time this dilFerence of range, and the turning points not being exactly simultaneous, 
to the method of suspension by filaments of silk, and the constant change to which the force of torsion was 
liable from the hygrometric properties of the silk. The large observatory 15-inch bar was suspended by 
eighteen fibres of untwisted silk ; and when a fresh adjustment became necessary from the threads breaking, a 
brass bar was swung, in order to take the torsion out of the new suspension thread : after getting the brass bar 
accurately into the magnetic meridian, it would remain in the same position for several hours ; but, if adjusted 
during the day, it would be found, after the lapse of a night, forming some angle with the magnetic meridian ; 
thus, from some cause, hygrometric or otherwise, the plane of detorsion was no longer in the same position in 
which it had previously been in adjustment. 
