Milage, 451 



which constitutes mirage. In explanation of this phenomenon, 

 it may be remarked, that, as soon as the soil becomes heated, 

 the lower stratum of air is also affected by the calorific in- 

 fluence. Numerous aerial currents are established, and, an 

 undulatory motion taking place in the air, distant objects, 

 become changed in form, and variously distorted and broken. 

 If, when these changes are going on, a calm should prevail, 

 and the mass of atmosphere upon the plain remain at rest 

 while the stratum in contact with the ground becomes gra- 

 dually heated, mirage will arise. In such cases, the observer 

 will see distant objects in their natural positions and forms ; 

 but, beloxv them, these images will be seen reversed, and the 

 spectator believe that he is looking at a reflection from the 

 surface of a body of water. The sky, also, joins in com- 

 pleting the illusion, its images being reflected in the same 

 manner. The whole visible appearances, the French philo- 

 sophers who visited Egypt remark, are, indeed, the same as 

 those usually exhibited by water. All the laws by which the 

 observer has been accustomed to judge of the existence of 

 water, viewed at a distance, are here called into action ; and 

 the man of science as well as the peasant alike find themselves 

 deceived." 



In this description of the phenomenon, it is stated that 

 distant objects are seen in their proper positions ; and, at the 

 same time, under them their reversed images, as if reflected 

 from the surface of water. The sky, also, i^s in the same 

 manner reflected ; the whole appearances, in short, being the 

 same as those usually exhibited by water ; and the explanation 

 is, that these reversed images are owing to reflection from an 

 invisible mirror ; that is, from a supposed reflecting surface 

 at the place where two strata of different densities are in 

 contact : which (speaking from memory) is, I think, the 

 theory of the French philosophers referred to as explaining 

 the phenomenon. 



I have seen this kind of mirage, I dare say, a thousand 

 titnes ; and have often attentively observed it for a length of 

 time, with a view of accounting for it; and I have to say, 

 that the appearances, though like^ are 7iot the same as, those 

 usually exhibited by water. The images seen by me were 

 always very indistinct ; though, generally, their colours were 

 tolerably bright, their outlines were undefined, and they more 

 or less approached to a triangular form, the base being upper- 

 most ; appearing, on the whole, very much as if reflected 

 from water slightly ruffled by a light breath of wind. The 

 images were never reversed^ but invar iahly erect: though this 

 was not always easily made out ; for, from the constant form 



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