470 Mr. Babbage and Mr. Herschel's account of the 
TABLE II. 
No. of rev. 
performed. 
Times of their performance. 
Zinc 
interposed. 
Bismuth 
interposed. 
Copper 
interposed. 
Lead 
interposed. 
Tin 
interposed. 
0 
0.0 
0.0 
0.0 
0.0 
0.0 
I 
32.0 
31-5 
32.2 
32.0 
32.0 
2 
44-5 
44-7 
46.0 
46.0 
45-5 
3 
64.0 
54-3 
56.0 
56.0 
55 2 
4 
63.0 
64.7 
64.7 
64.0 
5 
72.0 
70.7 
72.7 
72.6 
7*-5 
6 
79.0 
77-5 
79 5 
80.0 
79.0 
7 
86.0 
84.0 
86.0 
86.0 
85.0 
8 
92.0 
89.8 
92.0 
92.0 
91.2 
9 
97-5 
95-5 
97-5 
98.0 
96.5 
lO 
101.6 
103.0 
103-5 
102.2 
5. The metallic plates here interposed, as also the wooden 
ones, were circular discs of 10 inches in diameter and half 
an inch in thickness, the metals being all cast for the pur- 
pose, the wooden disc serving for a pattern. Such only 
are arranged together as were made under such circum- 
stances as to be strictly comparable. It will be seen by 
these results that the various substances examined exert 
no sensible interceptive power, the slight excess of velocity 
in table 1. col. 1. when nothing was interposed, being evi- 
dently referable to the eddy caused in the air by the revolv- 
ing magnet. Glass in like manner had no effect ; but when 
the substance interposed was iron, the case was widely dif- 
ferent, the magnetic influence being greatly diminished by 
one, and almost annihilated by two thicknesses of common 
tinned iron plate, as the following table will shew. 
TABLE III. 
Revolutions 
performed. 
Time occupied. 
Paper 
One sheet of tinned 
Two sheets of tinned 
interposed. 
iron interposed. 
iron interposed. 
0 
0.0 * 
0.0® 
0.0® 
1 
2 
- 
89.7 
164.7 
I 
22.5 
128.2 
— 
- 
* 59-5 
— 
2 
31-5 
186.7 
— • 
- 
21 1.5 
— 
3 
38.5 
234*7 
— 
