184 STEPPES AND DESERTS. 



Sanscrit the "thirst of the gazelle." (See my Relation 

 historique, T. i. pp. 296 and 625; T. ii. p. 161.) All 

 objects appear to hover in the air, and are at the same time 

 seen reflected in the lower stratum of air. At such times 

 the entire desert assumes the aspect of the wave-covered 

 surface of a wide spread lake. Palm trees, cattle, and 

 camels, sometimes appear inverted on the horizon. In the 

 French expedition to Egypt, the soldiers, parched with 

 thirst, were often brought by this optical illusion into a 

 state of desperation. This phenomenon has been remarked 

 in all quarters of the globe. The ancients were acquainted 

 with the remarkable refraction of the rays of light in the 

 Lybian Desert. I find mention made in Diod. Sic. lib. iiu 

 p. 184, Ehod. (p. 219, Wessel), of extraordinary illus- \ 

 images, an African Fata Morgana, with most extravagant 

 explanations of the supposed conglomeration of the particles 

 of air. 



(37) p . 19.__ The Melon-Cactus." 



The Cactus melo cactus is often 10 to 12 inches in 

 diameter, and has usually 1 4 ribs. The natural group of 

 Cactacese, the whole family of Nopaleae of Jussieu, belong ex- 

 clusively to the New Continent. The cactuses assume a great 

 variety of shapes : ribbed and melon-like (Melo cacti) ; articu- 

 lated or jointed (Opuntiae); forming upright columns or 

 pillars (Cerei) ; serpentine and creeping (Bhipsalides) ; or 

 provided with leaves (Pereskise). Many extend high up the 

 sides of the mountains. Near the foot of the Chimborazo, 

 in the elevated sandy plain around Riobaiuba, 1 have found 

 a new kind of Pitahaya, the Cactus sepium, even at a height 



