502 
Mr. Christie on the magnetism developed in 
Distance. 
4.0 in. 
3.5 In. 
3.0 in. 
2.5 in. 
2.0 in. 
Screw - - 
I 46W 
0 / 
3 20W 
6 20 W 
0 / 
14 30W 
0 t 
29 40W 7 Direction of south 
Unscrew - 
I 32 E 
3 08 E 
6 CO E 
13 50 E 
29 00 E 3 end of the needle. 
Mean - - 
I 39 
3 H 
6 10 
14 10 
29 20 
On diminishing the distance to 1.5 inch, the needle re- 
volved with the plate, and very shortly so rapidly, that it had 
the appearance of an entire circle. 
After this I replaced the needle by others which were lighter, 
letting every thing else remain the same, that is, the distance 
still 1.5 inch. 
Needle weighing 
42.5 grs. 
Needle weighing 
25 . s grs. 
Screw - - 
Unscrew - 
0 / 
24 40W 
25 20 E 
0 / 
10 30W 
10 40 E 
(I should mention that the needles were not at all neutralized). 
From the latter observations, it is evident that the effect 
produced depends upon the intensity of the magnetism in the 
needle employed; and this I think proves clearly that the 
effect arises from the magnetism induced in the copper from 
the needle itself. 
If we suppose the tang, of the deviation to vary as 
then 6 and being two deviations at the distances d and d! , we 
shall have n = 
log. a — log. a. 
Computing n from this, by a comparison of every two 
observations we have the following values of n ; 
