The well known phenomenon of the _mirage_ is called in Sanscrit “the thirst of the gazelle.”[GY] All objects appear to float in the air, while their forms are reflected in the lower stratum of the atmosphere. At such times the whole desert 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 distraction. This phenomenon has been observed in all quarters of the world. The ancients were also acquainted with the remarkable 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.[GZ] Footnote 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 Cactaceæ, the whole family of the Nopaleæ of Jussieu, belongs exclusively to the New Continent. The Cactus assumes a variety of shapes, being ribbed and melon-like (_Melocacti_); articulated (_Opuntiæ_); upright-like columns (_Cerei_); of a serpentine or creeping form (_Rhipsalides_); or provided with leaves (_Pereskiæ_). 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 feet.[HA] Footnote 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, planetlike light, are now seen to scintillate in the zenith.[HB] All these phenomena are the result of the increased quantity of aqueous vapour floating in the atmosphere. Footnote 39: p. 16.—“_The humid clay soil is seen to rise slowly in a broad flake._” Drought produces the same phenomena in animals and plants as the abstraction of heat. During the dry season many tropical plants lose their leaves. The crocodile and other amphibious animals conceal themselves in the mud and lie apparently dead, like animals in cold regions who are thrown into a state of hybernation.[HC] Footnote 40: p. 17.—“_A vast inland sea._”