the particles of magnetic bodies , &c. 85 
near the disc, which increased the final arc of torsion 2o°or 30°, 
or the values of a 10 or 15, as nearly as I could determine by 
making the magnets revolve under discs of wood and glass, 
of the same weight as the large copper disc. As I did not 
consider that the torsion could in all cases be determined 
within much less limits than this, I preferred giving the ob- 
servations as they were made, to applying a correction which 
was doubtful in its amount. 
When the magnets revolved at the distance 3-2 inches, the 
sum of the values of a is 444 ; and we may take this as the 
value of a when the magnets revolved under the disc and ring 
together at that distance. Now we have seen (Table II.) 
that when they revolved at the distance 3-2 inches from the 
axis under the uncut disc, the value of a. was 1528-7 ; so that 
here the magnetism developed was diminished by the circular 
separation in the ratio of 3-44 to 1. 
I now placed the disc II. in the scale, and having deter- 
mined the effects that were produced by making the magnets 
revolve under it, with their axes at the distance of 3-2 inches 
from the axis of rotation, their upper surfaces being at the 
distance of an inch from the disc, and their angular velocity 
5 revolutions in a second, the same as before ; I determined 
successively the effects produced by making circular cuts at 
the distances *7 inch, 1-2 inch, 1-7 inch, 2-2 inches, from the 
centre. I ascertained the effects both when the pieces in the 
interior were removed and when they were retained, except- 
ing in the first case of the small disc, 1-4 inch in diameter, 
which was removed, and glass of the same weight substituted 
in the first experiment, but was afterwards replaced, with the 
rings cut out, in the others. The differences in the results 
