of Edinhurgh, Session 1883-84. 547 
difference in the determined directions of the meridian was ob- 
served. 
A much more elaborate instrument, based on the same principle, 
has been constructed for the accurate measurement of magnetic 
declination. In it the coil is wound in two parts on a bronze frame 
(the form of which will be described below), of much smaller size 
than in the simple laboratory apparatus already described. The 
frame has hollow pivots, and is mounted on the Y’s of an altazi- 
muth instrument. Fig. 2 is taken from a photograph of the instru- 
ment. The pivots are made as nearly as possible of the same size 
as those of the telescope which belongs to the instrument, and the 
total weight of the frame nearly equals that of the telescope. 
A light mirror magnetometer stands between the two parts of the 
coil in the centre of the instrument, and is siq^ported by an inde- 
pendent tripod, as in fig. 2, or by a central pillar fixed to the base. 
At the middle of two sides of the frame narrow slits, a a, are pro- 
vided, through which the edge of the magnetometer mirror is to be 
sighted to bring it to a central position. It is centred with respect 
to the other horizontal and the vertical direction by sighting the 
face of the mirror through one of the hollow pivots, and making the 
clearance equal all round. 
The magnetometer is shown separately in tig. 3, The mirror 
with attached magnets is suspended by a single silk-fibre 40 cm. 
long. The upper end of the fibre is tied round the middle of a 
small rod of horn, whose Aveight is nearly equal to that of the 
mirror and magnets. This rod rests on two small V hooks pro- 
jecting down from the top of the magnetometer case. The hooks 
are united at the top, and can be turned or lowered by loosening a 
jam-nut. At the lower part of the magnetometer case a thin lens 
is fixed in front of the mirror. A little way above the mirror, a 
catch, consisting of a pair of inverted hooks, is fixed in the case, so 
that when the magnetometer is turned upside down, the mirror 
will be held by the catch, and the horn rod will hang free. 
This rod is then allowed to turn under the torsion of the fibre 
until that is completely removed. The V hooks are turned into 
the same azimuth as the hanging rod, and the magnetometer is 
then inverted, that is to say, restored to its normal position. It 
now hangs free, and without sensible initial torsion. It is next 
2 N 
A"OL. XII. 
