repetition of Mr. Arago's experiments on magnetism y &c. ^6g 
, 3 . The rotation of the copper being performed with great 
regularity, it was evident that by noting the times of suc- 
cessive revolutions, we should acquire a precise and delicate 
measure of the intensity of the force urging it, provided we 
took care to neutralize the torsion of the suspending thread. 
To make the experiment strictly comparable proved however 
a matter of rnuch delicacy, as the slightest change in the 
distance of the plate from the magnet was found to produce 
a material alteration in the time of its gyration. 
4. Our first enquiry was directed to ascertain the effect of 
the interposition of different bodies as screens in cutting off 
or modifying the peculiar rotatory effect. The substances 
tried were, paper, glass, wood, copper, tin, zinc, lead, bis- 
muth, antimony, and tinned iron plate, The comparative 
effects of these may be seen by the following tabulated ob- 
servations, in making which we had the advantage of Mr. 
Barlow's and Mr, Christie’s presence and assistance, 
TABLE I, 
No. of revolut. 
■performed. 
Times of their performance* 
Nothing 
Paper 
Wood 
Antimony 
Antimony int. 
Antimony int. 
interposed. 
interposed. 
interposed. 
Interposed, l. 
3 d trial. 
3 d trial. 
0 
0.0 
0.0 
0.0 
0.0 
0.0 
0.0 
1 
34 
36.2 
37*2 
37-0 
36.0 
35-0 
2 
48 
51.0 
52.2 
51.0 
50.5 
50.0 
3 
59 
62. 0 
635 
62.0 
61.5 
61.5 
4 
68 
71.5 
73-0 
72.5 
71.0 
71.0 
S 
76.5 
80.0 
81.5 
80.5 
79.2 
79-7 
6 
83*5 
87.5 
89.0 
88.0 
86,5 
87.2 
7 
90 0 
95.0 
96.2 
95-3 
93-7 
94.0 
8 j 
97.0 
lOI.O 
103.0 
— 
100 0 
lOI.O 
9 i 
J 03-5 
107.5 
109.8 
108.0 
106.5 
107.5 
10 
109.0 
r 
II3-S 
115.5 
1 14.0 
1 12.5 
II3-7 
1 
