174 EXTRAOEDINAEY REFKACTION. [SECT. xvin. 



contemporaneously the elevation of the top of a mountain 

 above a point in the plain at its base, and the depression of 

 that point below the top of the mountain. The distance 

 between these two stations is the chord of the horizontal 

 angle ; and it is easy to prove that double the refraction is 

 equal to the horizontal angle, increased by the difference 

 between the apparent elevation and the apparent depression. 

 Whence it appears that, in the mean state of the atmo- 

 sphere, the refraction is about the fourteenth part of the 

 horizontal angle. 



Some very singular appearances occur from the accidental 

 expansion or condensation of the strata of the atmosphere 

 contiguous to the surface of the earth, by which distant 

 objects, instead of being elevated, are depressed. Some- 

 times, being at once both elevated and depressed, they appear 

 double, one of the images being direct, and the other in- 

 verted. In consequence of the upper edges of the sun and 

 moon being less refracted than the lower, they often appear 

 to be oval when near the horizon. The looming also or 

 elevation of coasts, mountains, and ships, when viewed across 

 the sea, arises from unusual refraction. A friend of the 

 author's, while standing on the plains of Hindostan, saw the 

 whole upper chain of the Himalaya Mountains start into 

 view, from a sudden change in the density of the air, 

 occasioned by a heavy shower after a very long course of 

 dry and hot weather. Single and double images of objects 

 at sea, arising from sudden changes of temperature which 

 are not so soon communicated to the water on account of 

 its density as to the air, occur more rarely and are of shorter 

 duration than similar appearances on land. In 1818, Cap- 

 tain Scoresby, whose observations on the phenomena of the 

 polar seas are so valuable, recognised his father's ship by 

 its inverted image in the air, although the vessel itself was 

 below the horizon. He afterwards found that she was seven- 

 teen miles beyond the horizon, and thirty miles distant. 

 Two images are sometimes seen suspended in the air over 



