250 Dr. Wollaston on double Images 



(Phil. Trans, for 1797, Tab. I. Fig. 3.) at the distance of 8 

 miles, the refraction seems to have been about 3'. 



All the appearances described by him, I am inclined to think, 

 arose from difference of temperature alone. He offers a con- 

 jecture, that evaporation might occasion the lower strata of the 

 atmosphere to have a weaker refractive power ; but, from the 

 following experiments, it seems to have a contrary effect. 



Exper. 7. I took a plate of glass, and, while I looked along 

 the surface, I poured upon it a small quantity of ether. 



A line upon the opposite wall, appeared instantaneously ele- 

 vated many minutes, and at times above \ a degree. 



This fluid being the most volatile, and most soluble in the 

 atmosphere, of any known liquid, produces the greatest effect ; 

 since the cold, during evaporation, conspires with the ether dis- 

 solved in the air, to increase the refractive power. 



Rectified spirit of wine also produces, from the same cause,, a 

 very considerable effect. 



Exper. 8. By moistening a board, 5 feet in length, with alcohol, 

 and observing the elevation of an object viewed over its surface, 

 I found the refraction to be about 15'. 



Exper. .9. I next made a similar experiment with water itself. 

 Of this, the effect was barely visible, when tried in the same 

 way; but, by means of a surface of 10 feet, and by viewing a 

 luminous point at a greater distance, the refraction became evi- 

 dent, and the object elevated above 3 minutes. 



In the course of these experiments, I tried whether confining 

 the saturated atmosphere, by boards on each side, would vary 

 the effect, and found the refraction in all cases much lessened; 

 and, when water was used, it became imperceptible; but, as 

 soon as the boards were removed, and a free current allowed to 



