20 Mr Harris on Magnetic Intensity by the 
shade under a closed receiver, and from which the air was not 
withdrawn, I noted the times and arcs corresponding to every 
10th vibration, the needle being liberated at an angle of 45° from 
the meridian. Some bright beams of sunshine were then thrown 
into the receiver, by means of plane mirrors, so as to impinge 
upon the needle, and the same observed again. In this case the 
heat was not considerable, and the experiment became very ma- 
nageable. 
The bar being allowed to rest for a short time, in order to 
recover its previous temperature, the same experiments were re- 
peated, after exhausting the receiver, so as to free the oscillations 
from any disturbing influence produced by the surrounding air. 
The general results I have collected in the following Table. 
TABLE II. 
OSCILLATIONS TAKEN IN AIR. OSCILLATIONS TAKEN IN Vacuo. 
ah 2 Exp. A. Exp. B. Exp. C. Exp. D. 
Shade, Temp. 62°. | Sunshine, Temp, 70°. Shade, Temp. 62°. | Sunshine, Temp. 70°. 
Mean Time of 200 Vibs. 24’.1” 23’ 56,6 DAL Oe DAr to 
| bale Time of 250 Vibs. 29'.2” 28' 54,5 30’ 13,5 30’ 24” 
| Cernainal Are, 14° 9° 22°, 5 22° 4 
These results, which I confirmed by many similar experiments, 
would lead us to infer, that, if the oscillations be taken in va- 
cuo, the differences in the ares of vibration in sunshine and in 
shade, may, under certain conditions, vanish, or nearly so; as 
also, that the time of a given number of vibrations, is upon the 
whole, in a void, rather increased than diminished. Hence it 
may be inferred, in the above instances, Ist, That the influence 
of the sun’s rays on a magnet oscillating in air, is to reduce more 
