OF THE AURORA BOREALIS ON THE MAGNETIC NEEDLE. 
115 
ance of the Aurora, having seen none since I saw it last. Four were over the 
Coreen hills, and the paths of all were nearly but not quite parallel to the 
streamers. As if in correspondence with the great brilliancy of the Aurora, 
they were very large and bright, fully equal at least to Jupiter. 
My attention having been sometimes directed to the diurnal variation of 
the needle, for the purpose of ascertaining whether any interference might 
be expected between it and the influence of the Aurora, I have been led by 
many observations (of the whole of which indeed I have no regular notes, but 
only of a few,) to conclude that the quantity of the variation depends on the 
brightness or gloominess of the weather. Thus, on the brightest day that has 
occurred since I received the apparatus, the 2nd of October 1829, therm. 52°, 
the diurnal variation at noon was 26' 20" W. On the two gloomiest days that 
have occurred, the 3rd and 4th of December, 1829, therm, about 42°, the 
diurnal variation at noon was 3' 20" and 3' 40" respectively. Again, some ob- 
servations in the month of January last, on bright days, when yet the earth de- 
rived little or no heat from the sun, owing to a close covering of snow and a 
hard frosty wind, — and others, on bright days likewise, when the earth, much 
cleared of snow, imbibed much heat, — would lead me to believe, that the 
direct radiating heat of the sun has more effect than the light in causing the 
increased variation, when the state of the earth’s surface is such as to imbibe 
that heat. 
