501 
copper and other substances during rotation. 
A light needle, weight 42.5 grains, 6 inches long, on a 
pivot in a compass box, being placed over the plate, the rota- 
tion caused a deviation of 20'^ ; but when a heavy needle, 
weighing 197 grains, and of the same length, was similarly 
placed over the plate, it immediately revolved rapidly with 
the plate. 
A bar magnet, weighing 3 oz. 15 dwts. 19 grs. suspended 
by a wire. No. 20, revolved rapidly with the plate. 
A horse-shoe magnet, weighing nearly a pound, and sus- 
pended by the same wire, revolved with the disk. 
The following experiments were made with the view of 
ascertaining whether the effects increased nearly according 
to any power of the decrease of the distance. 
A strong needle, 6 inches in length, weighing 197 grains, 
and vibrating 22 times in a minute, delicately suspended on 
an agate within a rim accurately graduated, was placed with 
its centre exactly over that of the copper plate, and being ac- 
curately adjusted, so that the distance between the centre of 
the copper and that of the needle was such as I required for 
the observation, the copper was made to revolve rapidly 
(always as nearly as possible 12 times per second), and when 
the needle became stationary, the direction of its south end 
(being that most convenient for observation) was noted. This 
was done with the copper revolving in both directions, 
screw"' and ‘‘ unscrew.*" The direction of the south end 
of the needle was also observed before the rotation. 
