On the double Images caufed hy Almofphencal RefraBion. 335 



A line upon the oppofite wall appeared inftantancoufly elevated many minutes, and at 

 times above half a degree. 



This fluid being the moft volatile, and moft foiuble in the atmofphere, of any known 

 liquid, produces the greateft efFeft ; fince the cold, during evaporation, confpires with the 

 ether diflblved in the air to increafe the refra£live power. 



Redlificd fpirit of wine alfo produces, from the fame caufe, a very confiderable 

 efFedl. 



Exper. 8. By moiftening a board, five feet in length, with alcohol, and obferving the 

 elevation of an objedt viewed over its furface, I found the refraftion to be 15'. 



Exper. 9. I next made a fimilar experiment with water itfelf. Of this, the efFe£l was 

 barely vlfible, when tried in the fame way ; but, by means of a furface of ten feet, and 

 by viewing a luminous point at a greater diftance, the refraftion became evident, and the 

 obje£l elevated above three minutes. 



In the courfe of thefe experiments, I tried whether confining the faturated atmofphere, 

 by boards on each fide, would vary the efFe£l:, and found the refraction in all cafes much 

 leflened ; and when water was ufed, it became imperceptible ; but as foon as the boards 

 were removed, and a free current allowed to pafs acrofs, the full efl^eft was again pro- 

 duced. The reafon of this difference appears to be, that the quicker evaporation 

 increafes the degree of cold, and the current brings greater diiFerences of denfity con- 

 tiguous. 



The ftate of rapid evaporation will fully account for the phenomenon witnefled by Mr. 

 Latham, who has defcribed (in the Phil. Tranf. for 1796, p- 357 ; or this Journal, II. 417) 

 an extraordinary elevation of the oppofite coaft of France, fo as to be feen from the 

 beach at Haftings, and other parts of Suflex. 



There is a faft of the fame kind ftated by De la Lande {jijlron. Tom. II.) who fays that 

 the mountains of Corfica (though at the diftance of more than 100 miles) are oecafionally 

 vifible from Genoa. 



It is probably owing to the fame caufe, that other objefts have been fometimes 

 feen, at fuch diftances that we (hould expe£l them to be intercepted by the curvature 

 of the earth 5 for it is evident, that whenfoever the evaporation over each mile of furface 

 occanons a refraftion of about one minute, the rays receive a curvature equal to that of 

 the ocean, fo that its furface will appear flat ; and the fpherical form of the earth will not 

 obftru£l horizontal vifion of obje£ls at any diftance. 



It ftill remained to explain the phenomena feen by Mr. Vince, as I had not hitherto 

 made an atmofphere capable of exhibiting images inverted, as well as elevated, by 

 increafed denfity. For, in the refra^ions produced in the 7th, 8th, and 9th experi- 

 ments, by evaporation at an cxpofed furface, I obferved the efiedl: was always greateft 

 in conta£k with the evaporating furface; any lower point a. Fig. 9, appeared brought 

 nearer to a higher point c, by the pencil of rays from a being more refraded at i, 

 than the pencil from c was refraded at d. Therefore, any rays palling from the eye 



Vol. IV. — October 1800. R r . « 



