160 
> _ 216 _ 24 
r 2 “315“ 35’ 
A negative tan a, when 
(648aj — 336) 2 Z 180*315 
300 2 87696\ 
$15 2 315 2 ) 
r • 2 > 372-2 
for x = snra Z. a = a x 
And also for x 7 
648 
300 
648 
i being taken so as to give a deviation between ai and a 2 , 
we shall have a possible solution in opposite direction to 
that for small magnet. 
3. 
L 
r 2 
1 , 
. 9 637 
« = sm -aZ. — 7 
945 
233 
945 
These examples only profess to show that when the inten- 
sity is suitably taken, the result specified by Dr. Joule is 
verified. I should expect to find, when the intensity is 
gradually changed through a wide range, the sign of the 
deviation also change, and then, after a certain continued 
increase of i (longer continued, as l bears a greater propor- 
tion to n) generally change back again. 
It will be noticed that must be 7 • 
r 2 315 
Now Dr. Joule points out that if L = actual length of 
needle, L approximately. Assuming this 
. 49 L 2 212-1 
" 64 315 
• 212,1 
“ r 2 ^49 SliT 
--7?. x -82 
r 7 
7*94 
And 7J inches x ■ 94=6 - 8 inches. 
Dr. Joule finds the effect begins at 6*5 inches. 
