CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 297 
curves during similar periods^ we find only two which differ materially from the 
Singapore curves at similar periods of the year; one of the stations, Moulmein, being 
considerably to the north of the line of minimum intensity, the other, Madras (not 
differing so much), in its immediate vicinity. Sambooaiiga and Nicobar are indeed 
likewise close upon the line of no dip or minimum intensity, yet they agree generally 
with the Singapore curves. It is to be observed, that as we proceed north and ap- 
proach the line of minimum intensity, the similarity to the Singapore curves becomes 
more faint ; this may be seen on inspection of the Plates, where the most southerly 
stations, the Padang, Batavia and Cocos curves, agree exceedingly well with those at 
Singapore at similar periods*. 
Bifilar Magnetometer. 
The bifilar magnetometer is described at page 1 1 of Riddell’s work ; the apparatus 
is not very dissimilar to the unifilar ; it consists of a tripod base with a fixed circular 
plate 5 inches in diameter, graduated so as to be read off by verniers to single minutes, 
carrying an upper revolving plate and two projecting arms for supporting the reading 
telescope and counterpoise weight ; the magnet and bifilar suspension are inclosed, 
the former in a copper box, the latter in a suspension tube, 10 inches in length, carry- 
ing a circular plate, torsion circle and right and left hand screw-cylinder at its upper 
extremity, and the circle divided so as to be read off by verniers to 5' ; the magnet is 
suspended by two portions of the same thread, and is maintained by a rotation of the 
upper extremities in a position at right angles to the magnetic meridian. The principal 
adjustments are the following, for the purpose of bringing the magnet, when sus- 
pended by the double thread, into a position at right angles to the magnetic meri- 
dian : — 1st. To bring the line of detorsion of the threads to coincide with the magnetic 
meridian. This is effected by suspending the magnet by a single filament of untwisted 
silk ; the telescope is turned until the centre division of the scale is on the wire ; 
the instrument is clamped ; the magnet is then introduced carefully with its bifilar 
suspension ; the vernier of the torsion circle is turned until the centre division is 
again on the vertical wire of the telescope, and the plane of detorsion is in the mag- 
netic meridian. The telescope which is fixed to the instrument is then turned 90° in 
azimuth ; the vernier of the torsion circle is turned in the same direction through 
an angle (^;) equal to 60°. The centre division of the scale is brought on the vertical 
wire by increasing or diminishing the interval of the threads ; which is effected by 
turning the milled head of the screw-cylinder. The telescope is then turned back 
90°; the torsion circle being turned back through the angle ; if the adjustment 
has been made correctly, the line of detorsion is again in the magnetic meridian ; 
* Some ■want of similarity to the Singapore curve at those stations -where any difference is observable, may 
possibly arise in some degree from -want of the observations extending over a sufficient number of days ; but 
not altogether, for at Madras, where a difference is perceptible, the declinometers were observed for more than 
a month. 
2 Q 
MDCCCLI. 
