METEOROLOGY. 149 



are also seen about waterfalls and ship masts, forming a perfect 

 circle. Lunar rainbows are sometimes seen in the night time ; 

 but their colors are faint and indistinct In order to see a 

 rainbow, we must face a cloud and turn our back to the sun 

 or moon. The philosophical explanation of rainbows will be 

 found in works on natural philosophy and meteorology. 



MIRAGE. 



Mirage is an optical illusion often observed at sea, especially 

 in high latitudes ; it sometimes also appears on the land, par- 

 ticularly in Egypt and Persia: it is seen also on the margins 

 of rivers and lakes. It consists in the appearance in the air 

 over the surface of the sea, of multiplied images of objects on 

 the surrounding coast 



" It arises from unequal refraction in the lower strata of the 

 atmosphere, and causes remote objects to be seen double, as if 

 suspended in the air." These images are sometimes inverted: 

 ships, whale fisheries, and other objects, are sometimes descried 

 by means of mirage at considerable distances. The mathe- 

 matical theory of this phenomenon will be found in works on 

 optics, &c. 



SHOOTING STARS. 



These are common and well known meteors, some of which 

 resemble fire balls in every respect. We shall not attempt 

 any description or explanation of them, as their origin and 

 nature are involved in great obscurity and uncertainty. 



AEROLITES. 



These are mineral masses which fall to the earth apparently 

 from the upper regions of the atmosphere. They have a dark 

 or blackish color externally and a grayish hue internally. 

 They have a specific gravity more than three times that of 

 water : chemical analysis of one specimen shows its constituent 

 elements to be, iron, sulphur, silex, nickel, magnesia, and some- 

 times chromium. These meteors have, with some probability, 



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