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 withdrawn, I noted the times and arcs corresponding to every 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. 



Bar30 .400. 



Oscillations taken in Air. 



Oscillations taken in Vacuo. 



Exp. A. 



Shade, Temp. 62°. 



Exp. B. 



Sunshine, Temp. 7<f. 



Exp. C. 

 Shade, Temp. G2°. 



Exp. D. 

 Sunshine, Temp. 70°. 



Mean Time of 200 Vibs. 

 Whole Time of 250 Vibs. 

 Terminal Arc, 



24M" 



29'.2" 

 14° 



23' 56",6 



28' 54",5 



9° 



24' 10", + 



30' 13,5 

 22°, 5 



24' 15" 

 30' 24" 

 22° + 



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 arcs 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, 1st, That the influence 

 of the sun's rays on a magnet oscillating in air, is to reduce more 



