671 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



is very frequently to be seen ; but the inverted images are rarely well defined, unless 

 over a very even surface. One of the best situations for this purpose, is over a 

 level open road, with a gentle breeze blowing across it. A current of air brings a 

 cool stratum more closely in contact with the heated surface; and, since refraction 

 depends on the increment or difference of density in a given small space, a very 

 moderate breeze will thus render inversion more perceptible; but a strong wind will 

 reduce the temperature of the surface, and may make the heated stratum too thin 

 for any object to be seen through it from a distance. In one instance, when I saw 

 a refraction of about 9 minutes, at the distance of -y of a mile, a thermometer in 

 the sand was 101°; at 4 inches above, 82°; and at 1 foot above, 76 . 



Over water the evenness of the surface is favourable to the production of such 

 appearances; but, since the action of the sun is weak on a body so transparent, a 

 far greater extent of surface is requisite to produce any perceptible inversion. Being 

 at Bognor one bright morning, when the sea was calm, I had an opportunity of 

 observing the appearance of Selsea Bill, about 6 miles distant. The whole extent 

 of coast, when viewed with a pocket telescope magnifying about 16 times, appeared 

 inverted from one end to the other; and the lower part of a brick house on the 

 shore, was seen as distinct as the house itself. I judged the quantity of refraction, 

 in this case, to be about 2 minutes of a degree. This state of atmosphere appears 

 to be not very uncommon ; for, at Shanklin Chine, in the Isle of Wight, a few 

 days preceding, similar appearances were visible in several directions, but I neg- 

 lected to make any estimate of the .quantity of refraction. In the instance of the 

 inverted vessel seen by Mr. Huddart, (Phil. Trans, for 1797, %• 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 conjecture, that evaporation might occasion the lower strata of 

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

 periments, it seems to have a contrary effect. 



Exper. 7. I took a plate of glass, and, while looking along the surface, I poured 

 on it a small quantity of ether. A line on the opposite wall, appeared instantaneously 

 elevated many minutes, and at times above 4- 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 evident, and the object elevated above 3 minutes. In the course of these 



