The singular phenomenon of these sand-spouts, of which we see something analogous on the cross roads of Europe, is especially characteristic of the Peruvian sandy desert between Amotape and Coquimbo. Such dense clouds of sand may endanger the safety of the traveller who does not cautiously avoid them. It is remarkable that these partial and opposing currents of air should arise only when there is a general calm. The aërial ocean resembles the sea in this respect; for here, too, we find that the small currents (filets de courant) in which the water may frequently be heard to flow with a splashing sound, occur only in a dead calm (calme plat).
[35]. p. 14.—“Increases the stifling oppression.”
I have observed in the Llanos de Apure, at the cattle farm of Guadalupe, that the thermometer rose from 92°.7 to 97°.2 Fahr. whenever the hot wind began to blow from the desert, which was covered either with sand or short withered grass. In the middle of the sand-cloud the thermometer stood for several minutes together at 111° Fahr. The dry sand in the village of San Fernando de Apure had a temperature of 126° Fahr.
[36]. p. 15.—“The phantom of a moving undulating surface.”
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]]
[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]]
[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.
[39]. p. 16.—“The humid clay soil is seen to rise slowly in a broad flake.”