138 YIEWS OF NATURE. 



resembles a vast lake, whose surface undulates like waves, 

 Palm trees, cattle, and camels sometimes appear inverted in 

 the horizon. In the French expedition to Egypt, this optical 

 illusion often nearly drove the faint and parched soldiers to dis- 

 traction. This phenomenon has been observed in all quarters 

 of the world. The ancients were alsoacquainted with the re- 

 markable refraction of the rays of light in the Lybian Desert. 

 We find mention made in Diodorus Siculus of strange illusive 

 appearances, an African Fata, Morgana, together with still 

 more extravagant explanations of the conglomeration of the 

 particles of air.* 



(37) p. 15." The Melocactus." 



The Cactus melocactus is frequently from 10 to 12 inches 

 in diameter, and has generally 14 ribs: The natural group 

 of the Cactaceae, the whole family of the Nopalea3 of Jussieu, 

 belongs exclusively to the New Continent. The Cactus 

 assumes a variety of shapes, being ribbed and melon- like 

 (Melocacti) ; articulated (Opuntice}; upright-like columns 

 ( Cerei) ; of a serpentine or creeping form (Rhipsalides) ; or 

 provided with leaves (Pereskice}. Many extend high up the 

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

 in the sandy table-land around Riobamba, I found a new species 

 of Pitahaya (Cactus sepium\ even at an elevation of 10,660 

 feetf 



(38) p. 16. " The scene suddenly changes in the Steppe" 



I have endeavoured to describe the approach of the rainy 

 season, and the signs by which it is announced. The deep blue 

 of the heavens in the tropics is occasioned by the imperfect 

 solution of vapour. The cyanometer indicates a lighter shade 

 of blue as soon as the vapours begin to fall. The dark spot 

 in the constellation of the Southern Cross becomes indistinct 

 in proportion as the transparency of the atmosphere decreases. 

 and this change announces the approach of rain. The 

 bright radiance of the Magellanic clouds (Nubecula major and 

 Nubecula minor) then gradually fades away. The fixed stars 

 which had before been shining with a calm, steady, planet- 



* Lib. iii. p. 184, Rhod., p. 219, Weasel. 



t Humboldt, Bonpland, et Kunth, SynopiM Plantarum cKquinoct. 

 Orbis Ifovi, t. iii. p. 370. 



