333 
The apparatus is likewise provided with a brass bar (also 
furnished with a collimator) for the purpose of determining 
the plane of detorsion of the suspension thread ;—a thermo- 
meter, the bulb of which enters the box, in order to deter- 
mine the interior temperature ;—and a copper ring, used in 
checking the vibrations. 
The instrument used in determining the horizontal com- 
ponent of the magnetic force is a magnet bar suspended by 
two parallel wires, and maintained, by the torsion of their 
upper extremities, in a position at right angles to the mag- 
netic meridian. The directive force due to the mode of 
suspension is known, when we know the weight of the sus- 
pended body, the interval of the wires, and their length; 
and the ratio of this force to the horizontal component of 
the magnetic force is given, when we know the angle through 
which the upper extremities of the wines have been turned, 
in bringing the suspended bar into the perpendicular posi- 
tion. The magnet being thus maintained in its position by 
the action of two forces, one of which (the torsion force) is 
constant, while the other (the magnetic force) is variable, it 
is manifest that its place will vary around its mean position ; 
and that these variations of angle are connected with the 
variations of the force, so that when the former are given, the 
latter are known. The variations of the angle are read off, 
as in the other instrument, by a collimator and divided scale; 
and the delicacy of the instrument is such, that (with the 
adjustment of the parts at present in use in the Observatory) 
we may estimate changes in the horizontal force amounting 
to s54¢pth part of the whole. 
The larger parts of this apparatus,—the box, the frame- 
work, and the support,—are precisely similar to those of 
the declination instrument. There are, however, several 
differences in the essential parts of the instrument, arising 
chiefly from the different nature of the suspension. The 
collimator is attached to the stirrup, and has a motion in 
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