222 Mr. Christie on magnetic 
contained in the following table were made between 1 and 
2 o'clock after noon on the 30th of May, 1825 ; as before, 
the vibrations commenced 30° from zero. 
Needle exposed 
to the sun ; or 
being in shade. 
Thermo- 
meter. 
Index on the mi- 
crometer of 
torsion. 
Deviation 
of the needle. 
Time of perform- 
ing 50 vibrations. 
Extent of arc of vibra- 
tion from zero 
at 50 th vibration. 
4 S 
60 
o° 
- - - 
158.35 seconds. 
5 ° 45 ' 
60 
O 
- - - 
158.25 
5 4 ° 
6l 
360 
O 
O 
Os 
s C 
73 
360 
19 IO 
5 * ) 
82 
360 
19 l6 
^ o/D # 
80 
O 
- - - 
158.7 
3 Jo 
- 0 1 
a £ / 
85 
O 
- - - 
158.7 
3 IS 
3 </> f 
CO 
87 
O 
* 59-3 
3 05 
Here, as before, the terminal arc was in all cases dimi- 
nished when the needle vibrated exposed to the solar rays, 
but the increase of temperature caused a diminution of inten- 
sity, whether the intensity be estimated by the time of vibra- 
tion or by the deviation caused by torsion. Estimating it by 
the latter, we shall have, since the directive force varies 
directly as the torsion and inversely as the sine of the 
deviation, 
Directive force at temperature 6i° =; pn 4 ~ b= 1047.186 ; 
Directive force at temperature 82° = ^^^, = 1032.624. 
So that with an increase of temperature of 21 0 there appears 
to have been a decrease of intensity of 14.552. This would 
give the decrement of intensity .00067 for an increment of i° 
in temperature between the temperatures 6i° and 82°, the 
intensity at 6 1° being considered 1. From the experiments 
with powerful magnets, detailed in my former paper (Phil. 
Trans. 1825. p. 1.) it appears that, considering the inten- 
sity at 62° as 1 , the decrement of intensity corresponding to 
