10 Mr Harris on Magnetic Intensity by the 
the bar should be tested in this way. There will be little diffi- 
culty in effecting this, provided the bar is accurately made and 
tempered, and has been equally touched at first with magnets of 
the same force. 
17. For the purpose of an easy suspension, a very small hook 
is fixed to the small central hole. This is readily effected, by 
securing the eye of the hook to a fine doubled thread of silk, and 
then passing the silk either directly through the vertical hole, or 
otherwise out at the side, through the horizontal hole above men- 
tioned. In either case, a small knot tied in the silk, within the 
substance of the bar, secures the hook very effectually. 
18. The bar being suspended in this way, and the sliders 
placed at first in their original position, the inclination due to 
the polarity is corrected, by moving one of the sliders toward, 
and the other from, the centre, by nearly an equal quantity. The 
distances through which they require to be moved, in order to 
correct the inclination, are so little, that we may consider any 
error arising from change, in the angular inertia of the mass, as 
being indefinitely small, since the sliders are moved in opposite 
directions. Should it, however, be considered requisite, in any 
extreme case, we may find a series of points on each side the 
centre, in some of which the horizontal position may be obtained, 
and which would correspond to the same angular inertia with 
mathematical precision; since we have only to find a series of 
points, the sums of the squares of the distances of any two of 
which, from the centre of the bar, taking one on each side, give a 
constant quantity, the resistance of the sliders to motion being 
really, as the particles themselves, multiplied into the squares of 
their distances, from the axis of motion. 
In order to adjust the slides with accuracy, a fine pair of 
compasses, and a minutely divided scale, may be employed. 
19. In addition to these circumstances, incidental to the me- 
chanical conditions of a vibrating magnet, the silk thread of sus- 
pension requires also to be considered. It will be found, on ex- 
