536 



PHYSICAL GEOGRAPHY. 



kinds of animals ; occasionally they seem to be motionless, and in perfect quietude ; and 

 occasionally to be flying ; while immediately afterwards they themselves appear to be the 

 pursuers, and to make other objects fly before them." Milton might have had this passage 

 in his eye when he penned the allusion to the same apparitions : 



" As when, to warn proud cities, war appears 

 Waged in the troubled sky, and armies rush 

 To battle in the clouds ; before each van 

 Prick forth the airy knights, and couch their spears, 

 Till thickest legions close ; with feats of arms 

 From either side of heaven the welkin rings." 



The Mirage of the Desert. 



The mirage, adverted to in noticing the sandy deserts of the globe, is the most familiar 

 form of optical illusion. M. Monge, one of the French savans, who accompanied Buona- 

 parte in his expedition to Egypt, witnessed a remarkable example. In the desert between 

 Alexandria and Cairo, in all directions green islands appeared, surrounded by extensive 

 lakes of pure, transparent water. Nothing could be conceived more lovely or picturesque 

 than the landscape. In the tranquil surface of the lakes the trees and houses with which 

 the islands were covered were strongly reflected with vivid and varied hues, and the party 

 hastened forward to enjoy the refreshments apparently proffered them. But when they 

 arrived, the lake on whose bosom they floated, the trees among whose foliage they arose, 

 and the people who stood on the shore inviting their approach, had all vanished ; and 

 nothing remained but the uniform and irksome desert of sand and sky, with a few naked 

 huts and ragged Arabs. But for being undeceived by an actual progress to the spot, one 

 and all would have remained firm in the conviction that these visionary trees and lakes 

 had a real existence in the desert. M. Monge attributed the liquid expanse, tantalising 

 the eye with an unfaithful representation of what was earnestly desired, to an inverted 

 image of the cerulean sky, intermixed with the ground scenery. This kind of mirage is 

 known in Persia and Arabia by the name of Serab or miraculous water, and in the western 

 deserts of India by that of Tchittram, a picture. It occurs as a common emblem of dis- 

 appointment in the poetry of the orientals. 



In the Philosophical Transactions for the year 1798, an account is given by W. 

 Latham, Esq., F.R. S., of an instance of lateral refraction observed by him, by which the 



