6z On an unufual honzonlal IlefraB':oii of the Air. 



the raj'S of light ; let therefore onY., cmE, bvE, hivY., 

 be the curves defcribed. We have therefore to affign a caufe 

 which may bring ravs paffing above llie rays a rY,, />5E, to 

 the eye at E. Now, if there were no variation of the re- 

 frat^ive power of the air, a ray of light paffing through it 

 would dcfcribe a ftraight line; tlierefoe, the curvature of a 

 ray of light paffing through the atmofphere, depends ripon 

 the variation of the- refractive power of the air. If, there- 

 fore, wc fuppofe the air lying above aril, to vary quicker in 

 its refraftive power than the air throtigh which arfl paflTes, 

 the curvature of a ray proceeding above that of ^rE, will he 

 greater than the curvature of fir E 5 and upon this principle 

 we may conceive that a ray may dcfcribe the curve anK: and, 

 in like manner, if a quicker variation of refractive power 

 llionld take place above the curve anE, than in that curve, 

 a third ray may defcribe the cur\'e amE. The fame may 

 be faid for the rays bvE, btvE, diverging from l>. The al- 

 terations of the refraAive power may arife, partly from the 

 variations of its denfity, and partly from the variations of its 

 moifture; and the paffage of the rays through the boundary 

 of the fog may there futfer a very confiderable refraftion ; 

 for, from the motion of the fog, and that of the imaores above 

 mentioned, I have no doubt that the fog was a very confi- 

 derable agent in proclucing the phaenomena. When all the 

 caufes co-operate, I can eafily conceive that they may pro- 

 duce the effects which I have defcribed. If the caufe fl)ould 

 not operate in the tra£l; of air through which the curves anE, 

 ivE pafs, but ffiould operate in the traft through which 

 amE, btvE pafs, an ereft image would be vifible, but there 

 would be no inverted image ; andj (liould it operate in the 

 latter cafe, but riot in the former, there would be only an 

 inverted image. 



As the phaenomena are very curious, and extraordinary in 

 their nature, and have not, that I know of, been before ob- 

 ferved, I have thought proper to lay a defcripti&n of them, 

 with all the attending clrcljmftances, before the Royal So- 

 ciety. Thc\ appear to be of confiderable importance, as they 

 lead us to a knowledge of thofe changes to which the lower 

 parts of the atmofphcre are fomctimes fubject. If, when 



thef^ 



