517 
and magnetic rotations. 
with sufficient delicacy to allow them to obey the minute 
forces under investigation, I made the following experiment. 
Experiment 18. 
A card of the same form and size as the brass needle de- 
scribed in Experiment 1 , covered with sealing-wax, having 
an agate cap in its centre, was balanced on the point of a 
sewing needle, cemented to a glass bridge, which rested on 
the screen. The wax card was about 1.5 inches above the 
leno screen ; under which, at the distance of .3 inch below, 
the copper plate was placed. The lamp was placed below the 
copper, at the distance of .75 inch from its edge. No motion 
was communicated to any part of the apparatus ; the waxed 
card needle rested at 168 0 . The lamp was lighted, and at 
the end of 75" the needle had moved 8° ; it gradually ad- 
vanced towards that part of the copper immediately above 
the lamp, and at the end of four minutes and a half became 
stationary at 87°. 5, a point not precisely over the lamp, but a 
little on one side of it. 
Experiment 19. 
The same apparatus and relative distances being used, no 
motion was employed ; a piece of red-hot iron, one inch long, 
by half an inch broad, was placed on the platform below 
the copper, instead of the lamp. The red-hot iron was placed 
under 210 0 , about one inch below the copper ; and the waxed 
card needle rested at 143 0 . The experiment began at 
4 h 23“ 
4 28 no effect. 
4 3 2 
The nearest end of the needle had moved over 47 0 , to a spot 
mdcccxxvi. 3 X 
