CERTAIN FORMS OF MIRAGE. 



115 



advance, and speedily vanishes altogether. Sometimes it is an 

 inverted representation of terrestrial objects which appears in the air ; 

 or rather, these same objects, several times reflected, appear to multi- 

 ply themselves. M. Tre'maux relates that he saw the latter form of 

 mirage in Nubia. He observed a row of doum-palms, which were 



A MlKAGE IN THE DESERT. 



about two thousand yards distant, repeated in several similar rows, 

 each with a like number of trees, so as to produce the effect of a 

 quincunx ; among these trees floated several seeming sheets of water. 

 We must remember, moreover, that the immensity, uniformity, 

 and vacuity of the Desert, singularly contribute to render optical 

 illusions frequent. The very serenity of the air assists in destroying 



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