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azimuth, an arrangement which is necessary in the adjust- 
ments. The suspending wire passes round a small grooved 
wheel, on the axis of which the stirrup rests by inverted Ys; 
and the instrument is furnished with a series of such wheels, 
of different diameters, for the purpose of varying the interval 
of the wires. This interval is altered, at the upper extre- 
mity, by means of two screws (one right handed and the 
other left handed,) cut in the same cylinder; the wires being 
lodged in the intervals of the threads, and their distance re- 
gulated by means of a micrometer head. 
The third instrument is that used in determining the 
changes in the vertical component of the magnetic force. It 
is a magnet resting on agate planes, by knife edges, and 
brought to the horizontal position by weights. There is a 
small cross of wires near each end, and the changes of posi- 
tion of the magnet are read off by a pair of microscopes. 
From these changes of position, the changes of the ver- 
tical force are inferred, when we know the inclination at the 
place of observation, the azimuth of the plane in which the 
needle moves, and the angle which the line connecting the 
centre of gravity and centre of motion makes with the mag- 
netic axis of the needle. As the determination of this latter 
constant would require that considerable additions should 
be made to the apparatus, Mr. Lloyd has preferred to adjust 
the weights so that the angle in question shall be nothing. 
The weights are small brass screws moving in fixed nuts, one 
on each arm; the axis of one of these screws is perpendicular 
to the magnetic axis of the bar, and its movement conse- 
quently produces the desired adjustment; the other screw 
(the heavier) is parallel to the magnetic axis, and serves to 
adjust the magnet to the horizontal position. Each of these 
adjustments admits of an easy test. 
The supports of the magnet, and of the micrometers, are 
firmly attached to a massive slab of marble, which is ce- 
mented to the stone pillar on which it is placed; and a spirit- 
