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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 jo^oth 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 



2g 



