caused by atmospherical Refraction. 247 



temperature alone, being in one instance more than double the 

 greatest horizontal refraction of the heavenly bodies ; in which 

 case, as the rays enter from a vacuum, the greatest possible 

 effect of the atmosphere might be expected. But it must be 

 remembered, that when a star appears in the horizon, its rays 

 intersect the superior strata of the atmosphere at an inclination 

 of several degrees, and that they pass but once through the 

 variations from rarity to density ; but, on the contrary, that in 

 the experiments with red-hot iron, the rays may pass actually 

 in the direction of the strata, and that they are refracted, not 

 only by their entrance from the denser into the rarer medium, 

 but the effect is doubled, since, the refraction caused by their 

 emergence is equal to that produced by their incidence. 



Although a stratum of air, heated by these means to so great 

 a degree, affords an erect, as well as an inverted, image of 

 objects seen through it, the more moderate warmth communi- 

 cated to it from bodies heated by the action of the sun upon 

 them, seems insufficient to produce both images; but the in- 

 verted image may generally be seen, when the sun shines upon 

 a brick wall, or other darker-coloured surface. 



While the eye of the observer is placed nearly in a line with 

 the wall, if another person, at 30 or 40 yards distance, extends 

 any object towards the wall, an image similar to it will appear 

 to come out to meet it. 



It would be difficult to ascertain with accuracy the degree of 

 rarefaction capable of shewing this appearance, but it may be 

 of some use to future observers, to mention the different degrees 

 of heat which I observed. 



In one instance, a thermometer in contact with the wall, 

 stood at g6° ; but, at f of an inch distance, 82 . 



mdcgc. K k 



