292 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
merit and observed. At Piilo Peesang, Carimon, Pulo Booaya, Sarawak, Batavia, 
Poolo Bay, Padang and Sambooanga, Nos. I. and II. were employed ; at Moulmein, 
Nos. II. and III. ; at Keemah, the Cocos, Pulo Binding, Penang, Nicobar, and Madras, 
Nos. I. II. and III.; and at Singapore, in November and December 1848, not only 
were the three above-mentioned observed, but two in addition, viz. No. IV., a small 
cylindrical magnet 3 inches in length, in use for deflection in a separate building, 
and No. V., the large 15-inch bar in the observatory. 
Declinometer No. I. — Declinometer No. I. is the Portable Declination Magnetometer 
described in Captain Riddell’s work on Magnetic Instruments ; the magnet is a 
solid cylinder 3 inches in length and suspended by a filament of untwisted silk ; the 
former enclosed in a copper box, the latter in a glass tube. A scale being fixed above 
the telescope and reflected by a mirror attached to the stirrup in which the magnet is 
placed, the changes in the position of the magnet are shown by the mirror reflecting 
different portions of the ivory scale, and are read off by the telescope attached to the 
instrument. The mirror is immediately below the magnet and parallel to its longer 
axis ; the scale faced the east, and increasing readings denote a movement of the 
north pole east. The ratio of the torsion force to the magnetic directive force, 
H 
or the value of p, varied from *000207 to *000278. The value of one scale division 
in arc value is a'=3437’75X where /=y^th of an inch=*01 and r=17T9 
inches. 
DecUnometerl^o. II. — Declinometer No. II. is the Induction Inclinometer described 
at page 12 of Riddell’s work. The magnet is of the same length as No. I., but 
hollow. The adjustments are precisely similar. The mirror is parallel to and imme- 
diately below the longer axis of the magnet. The value of one scale division is 
exactly the same as for No. I., viz. l' of arc. The coefficient of torsion varied from 
*000139 to *00025, and is therefore not worth taking into account, except for large 
angles of deflection. 
Declinometer No. III. — Declinometer No. III., or Jones’s Declinometer, is pre- 
cisely similar in principle to the two others ; the magnet being a solid cylinder 
3 inches in length. The mirror, instead of being parallel to the longer axis of the 
magnet, is at right angles to it, so that the scale, instead of facing the east, being to 
the south of the magnet, faced the north. Increasing scale readings denote a move- 
ment of the north pole east. The value of one scale division is l'*0036; the coeffi- 
cient of torsion *00031. 
Contents of Abstracts of Table A. referring to the Declinometers. 
In the abstracts of Table A. are given, from page i to page xi inclusive, the oscil- 
lation, or more properly the variation of the declination* at the different stations in 
* In order to obviate confusion as much as possible, I have substituted in Table A. for hourly variation the 
word oscillation, which is meant to express the extreme range or swing of the needle from its most western 
to its most eastern position : the term hourly variation has not been used in Table A., as it might be con- 
