74 Mr. Christie on the mutual action of 
on the bottom of the screen P O, whatever torsion is given to 
the wire, thus forming a very accurate balance of torsion. 
The screen P O, having paper stretched tightly for the 
bottom and all the sides raised five inches, prevented currents 
of air, caused by the rotation of the magnets, affecting the 
disc or ring suspended over them. A B was so fixed that 
the axis CD, and the suspending wire e/were in the same 
vertical line, and with this line the centre of the graduated 
ring also coincided. 
On the mutual action of the particles of magnetic bodies during 
rotation. 
Having cut two discs of the same diameter from the same 
sheet of copper, I suspended one of them at the distance of 
an inch from the upper horizontal surfaces of the magnets, 
whose axes were vertical, and south poles upwards, and 
ascertained by means of this apparatus, the effects that would 
be produced by making the magnets revolve under it with 
their axes at various distances from the axis of rotation. I 
then observed the effect of making a circular cut through the 
disc, at the distance of an inch from its circumference; first, 
when a portion was left uncut; and next, when the outer ring 
was entirely separated from the remaining inner disc. The 
second disc was suspended by the same wire, and at the same 
distance from the magnets; and the effects which were pro¬ 
duced, by making successively circular cuts, at different 
distances from the centre, ascertained, whether the interior 
portions were removed or retained. 
The diameter of each of the discs was 8-4 inches ; the 
weight of that which I designate as I. was 5298 grains ; that 
