the particles of magnetic bodies , &c. 95 
are, length 12-1 inches, breadth *9 inch, thickness ■ 36 inch, 
were fixed vertically in the frame G H, equally distant from 
the axis of rotation, their edges towards that axis, their south 
poles upwards and upper surfaces horizontal. The distance 
between the inner edges of the magnets was 10*05 inches ; 
between their outer edges 11*85 inches; so that their axes 
revolved exactly under the middle section of the ring, and 
they described a ring directly under the copper ring, of as 
nearly as possible the same horizontal dimensions. 
The screen P O being removed, the ring was lowered until 
it just touched without resting upon the upper surfaces of the 
magnets, when the distance between K and M, on the tube 
LM, was ascertained, 7-33 inches: adding *12 inch to this, 
7-45 inches would have been the distance between K and M, 
if the middle horizontal section of the ring could have coin- 
cided with the upper surfaces of the magnets. By subtracting 
in any case the distance between K and M from this number 
7 45 inches, I had very accurately the distance between the 
upper surfaces of the magnets and the middle horizontal 
section of the copper ring. After finishing the experiments 
the screen P Q was removed, and the ring again lowered 
until, as before, it just touched the upper surfaces of the 
magnets, when the distance between K and M was found to be 
7’ 34 inches, so that the wire could not have stretched during 
the experiments ; the small difference -oi inch must be attri- 
buted to slight inequalities on the surface of the copper. 
The magnets being made, as before, to revolve 5 times in 
a second, in the direction of screwing, and preserving this 
velocity very carefully in all cases, by making the intervals 
between the beats of the spring, b, on c , c ti exactly a second, 
