4 2 Mr . Hey’s Account , &c. 
Thefe circumftances were perhaps merely accidental ; though, 
in our prefent date of imperfect knowledge refpefting thefe 
meteors, it may not be amifs to take notice of them. In one 
of the inftances related by Dr. Huxham, the wind was 
N.W. by N. ; in the other it was eaft. 
Before I conclude this account, already I fear too much pro- 
trafted, let me hazard a conje&ure refpeaing the white colour 
and ftationary continuance of fome of thefe arches. Experi- 
ments in elearicity, made with what is called an exbaufted 
receiver, fhew, that the colour and motion of the elearic fpark 
vary in proportion to the rarity of the air in the receiver. The 
more the air is rarified, the more moveable and coloured is the 
elearic aura paffing through it. On the contrary, the colour 
of the fpark approaches to whitenefs, and moves with greater 
difficulty, as the air is admitted. Will this obiervation ferve 
in any meafure to account for the difference in colour and 
motion of thefe elearic arches, for fuch 1 prefume to Call 
them? May we not fuppofe the more coloured and trail - 
fient corufcations of aurora borealis to be made in the rarer 
parts of the atmofphere, while the more white and ftationary 
ones poffefs the denfer parts * ? The whiteft arches which I 
. fa w were the moft fixed; that feen April 12. was the mod 
coloured, and had the moft internal motion. 
I am, &c. 
WILLIAM HEY. 
* The luminous arch feen by Mr. Swinton, Oft. 12, 1766, the edges of 
which were in a vertical pofition, feems clearly to have been formed in the lower 
part of the atmofphere : for, while “ the upper or exterior limb was white and 
“ refplendent, the lower was' obfeure, and fcarcely diftinguifliable uom the 
a clouds,” 
