influence in the solar rays. 
221 
Needle exposed 
to the son ; or 
being in shade. 
Thermometer. 
Time of performing 
50 vibrations. 
Extent of arc of vibra- 
tion from zero 
at 50th vibration. 
5>h 3 H e 
60.9 
1 t 8.8 seconds. 
Not observed. 
Sun . . 
91.5 
1 18.0 
Not observed. 
Shade . . 
70.0 
1 1 8.8 
5 0 oo' 
Sun . . 
75-3 
1 18.0 
2 30 
Sun . . 
9°- 4 
1 18.4 
2 45 
Sun . . 
9 K 4 
1 18.0 
2 30 
Sun . . 
89.4 
1 18.4 
2 30 
Shade . . 
81.6 
1 18.7 
4 45 
From these observations it is evident, that although the 
needle vibrated rather more rapidly, yet it came more quickly 
to a state of rest when exposed to the rays of the sun, than 
when in the shade. The latter effect appears to have been 
produced independently of the heat of the rays ; since when 
the thermometer had risen only to 75.3, the terminal arc had 
decreased from 5 0 to 2 0 30'. These observations were made 
between noon and 1 11 30“ on the 4th of June 1824, and I had 
no opportunity of repeating them during that summer. 
As the apparent increase of intensity at an increased tem- 
perature, indicated by a small diminution in the time of vibra- 
tion, excited my surprise as much as the more rapid decrease 
of the arcs of vibration when the needle was exposed to the 
rays of the sun, I resolved to make use of a balance of torsion, 
in order to deduce the changes of intensity from the devia- 
tion caused by torsion, as well as from the times of vibration ; 
and that there might be less ambiguity in the results, I em- 
ployed a much more powerful needle, its weight being 197 
grains, and length 6 inches as before. This needle was sus- 
pended by a wire (No. 22) 22.9 inches long, and free from 
torsion when the needle pointed to zero. The observations 
MDCCCXXVI. G g 
