75 
the particles of magnetic bodies, ^c. 
of II. was 5232 grains. I preferred making observations 
with discs having this difference in weight, to employing a 
file to reduce them to the same. In observing with the disc 
II. glass, weighing 66 grains, was added; and in every case 
where the weight of copper suspended was less than 5298 
grains, discs of wood and of glass were added to make up 
that weight, in order that the tension of the suspending wire 
might be the same in all cases. This wire was of hard brass 
of the size called by the wire-drawers No. 22, its length 45‘6 
inches. 
The disc I. being placed in the scale at the distance of one 
inch from the upper surfaces of two 12 inch magnets, these 
were adjusted in the frame G H, with their flat faces towards 
the axis of rotation, so that their axes were vertical, and at 
the distance 4'2 inches from the axis of rotation, or that these 
axes revolved directly under the edge of the disc. The tube 
L M was turned until there was no torsion in the wire when 
the index on the scale pointed to zero on the graduated ring. 
The magnets were now made to revolve in the direction of 
screwing with the angular velocity of 5 revolutions per 
second, corresponding to one revolution of the handle in two 
seconds, (which velocity was always carefully preserved in 
all the subsequent experiments,) and the time in which the 
disc performed one, two, three, &c. revolutions observed, 
until it began to revolve in a direction contrary to that of the 
magnets ; the instant when this happened was noted, and like¬ 
wise the number of revolutions of the index, and the degree 
where it pointed when it began to retrograde. The same was 
done when the magnets revolved in the direction of unscrew¬ 
ing, and the means in the two directions taken. Having no 
