231 
influence in the solar rays. 
so that another effect of the magnet was to diminish the time 
of vibration of the disc. If the acceleration in the time of 
vibration of the needle when exposed to the sun's rays arise 
from a similar cause, we ought also to find that the decre- 
ment of intensity in the needle corresponding to an increase 
of temperature, as deduced from the times of vibration in the 
sun and in the shade, is less than the decrement deduced 
from the times of vibration at different temperatures when 
both observations were made in the shade, or both in the 
sun, and likewise less than that deduced from the deviation 
due to torsion ; and this, from the observations, appears to 
have been decidedly the case. Probably, however, the acce- 
lerations in the times of vibration are only consequences of 
the diminution of the arcs, both in the case of the copper 
disc and of the needle vibrating in the sun : still this accele- 
ration will account for these differences in the decrements of 
intensity. 
Although we should not at present be able to point out 
the manner in which the rays of the sun tend to bring the 
needle to rest, the discovery of a fact proving magnetic influ- 
ence in the solar rays must, I think, ultimately lead to impor- 
tant conclusions respecting the phaenomena of terrestrial 
magnetism. On this account it is desirable that the mode of 
observing should be varied, and that observations should be 
made in climates more favourable for the purpose than our 
own. I regret that, in consequence of my absence from 
home, I could not repeat the experiments during the ex- 
tremely hot and clear weather of the past summer, as it is 
probable that the effects might have been even more decided 
than those which I have detailed. Possibly the effect might 
