237 
influence in the solar rays. 
3 s . 596 as the mean time of one vibration, when the needle 
vibrated from 90°, the terminal arc being I 9 f° and thermo- 
meter 102.3 ; and vibrated from 20°, the time of performing 
50 vibrations was 2 52.2, or the mean time of one vibration 
3.445, the terminal arc being 9 0 and thermometer 110. The 
intensities in the shade and in the sun, estimated by the times 
of vibration in the large arcs would be 1 and .995 ; hut esti- 
mated by the times of vibration in the small arcs 1 and .960. 
It appears from these observations, that in deducing the 
magnetic intensity in different parts of the earth from the 
vibrations of a needle, it is very essential to attend to the 
circumstance of the needle having been exposed or not to the 
rays of the sun during its vibration, as well as to the temper- 
ature in which the vibrations have been made. 
The very small increase in the times of vibration of the 
glass and copper needles, when exposed to the sun above 
those when screened, is to be attributed to the slight expan- 
sion of the needles themselves, and perhaps of the suspend- 
ing wires ; but this is too small at all to account for the ter- 
minal excess with these needles. I am aware that, had the 
copper needle been of the same weight as the magnetised 
needle, and the times of vibration of all the needles more 
nearly the same, in the series of experiments which I have 
given, the results would have been more strictly compara- 
tive ; but on the only day during which to the present time 
the sun has continued unclouded long enough for making a 
complete series with different needles, I was unprovided 
with any lighter copper ; and since I have formed needles of 
magnetised and unmagnetised sheets of copper and of glass, 
all of the same form and weight, and adjusted the suspending 
mdcccxxvi. I i 
