of REFRACTION. 247 
BawE a ray coming from the bottom, and TxzE a ray 
coming from the top of the hill, falling upon the eye at 
E, in a direction between thofe of the other two rays; then 
it is manifeft, that fuch a difpofition of the rays will produce 
the obferved appearance. To elect this, there muft have been 
a very quick variation of the denfity of the air which lay be- 
tween the two curves y v E, x w E, fo as to increafe the curva- 
ture of the ray TxzE, after it cuts BwE in x, by which 
means, the ray T x z E, might fall between the other two rays. 
The phenomenon cannot be otherwife accounted for. As there 
are not, that I know of, any records of a phenomenon of this 
nature, the conftitution of the air muft have been fuch as but 
very rarely happens, or fuch an appearance would before have 
been taken notice of. 
THE phenomena which I faw at the fame place, and which 
I defcribed in the Philofophical Tranfactions of the Royal So- 
ciety for the year 1798, I explained upon the fame principle, 
that of a quick variation of denfity; and this was afterwards 
confirmed by fome very ingenious experiments made by Dr 
Wo.t.aston. Perhaps this phenomenon may afterwards be 
fubjected to an experimental illuftration. 
VI. 
