caused by atmospherical Refraction. 249 



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

 level open road, with a gentle breeze blowing across it. A cur- 

 rent of air brings a cool stratum more closely in contact with 

 the heated surface ; and, since refraction depends on the incre- 

 ment or difference of density in a given small space, a very 

 moderate breeze will thereby 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 o, minutes, 

 at the distance of 1 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 upon 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 neglected to make any estimate of the quantity of re- 

 fraction. 



In the instance of the inverted vessel seen by Mr. Hud dart, 



Kk 2 



