ANNOTATIONS AND ADDITIONS. 151 



are often heard to ripple audibly (filets de courant), are only per- 

 ceptible in a dead calm (caline plat). 



f 35 ) p. 36. " Increases the suffocating heat." 



I have observed in the Llanos de Apure, at the Guadalupe cattle 

 farm, the thermometer rise from 27 to 29 Reaumur (92. 7 to 

 9 7. 2 Fahr.) whenever the hot wind began to blow from the Desert, 

 which at such times was covered either with sand or with short 

 withered turf. In the middle of the sand-cloud the temperature was 

 for some minutes 35 R. (111 F.). The dry sand in the village of 

 San Fernando de Apure had a temperature of 42 R. (126 Fahr.). 



( M ). p. 37. :" The illusive image of a. coolj rippling, watery 

 mirror." 



The well-known phenomenon of the mirage is called in 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. iii. p. 184, Rhod. (p. 219, Wessel),. of extraordinary illusive 

 images, an African Fata Morgana, with most extravagant explana- 

 tions of the supposed conglomeration of the particles of air. 



(37) p> 37. (t The Melon-Cactus." 



The Cactus melo cactus is often 10 to 12 inches in diameter, and 

 has usually 14 ribs. The natural grop of Cactacese, the whole 

 family of NopaleaD of Jussieu, belong exclusively to the New Conti- 

 nent. The cactuses assume a great variety of shapes : ribbed and 

 melon-like (Melo cacti) \ articulated or jointed (Opuntise) ; forming 

 upright columns or pillars (Cerei); serpentine and creeping (Rhipsa- 



