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- 



