These changes are graphically described by Humboldt in his "Views of
Nature," as they occur on the northern portions of South America, as
follows: "When, beneath the vertical rays of the bright and cloudless sun
of the tropics, the parched sward crumbles into dust, then the indurated
soil cracks and bursts, as if rent asunder by some mighty earthquake. The
hot and dusty earth forms a cloudy vail, which shrouds the heavens from
view, and increases the stifling oppression of the atmosphere; while the
east wind (_i. e._ trade-wind), when it blows over the long heated soil,
instead of cooling, adds to the burning glow.

"Gradually, too, the pools of water, which had been protected from
evaporation by the now seared foliage of the fan-palm, disappear. As in
the icy north animals become torpid from cold, so here the crocodile and
the boa-constrictor lie wrapped in unbroken sleep, deeply buried in the
dried soil. Every where the drought announces death, yet every where the
thirsty wanderer is deluded by the phantom of a moving, undulating, watery
surface, created by the deceptive play of the reflected rays of light (the
mirage). A narrow stratum separates the ground from the distant
palm-trees, which seem to hover aloft, owing to the contact of currents of
air having different degrees of heat, and therefore of density. Shrouded
in dark clouds of dust, and tortured by hunger and burning thirst, oxen
and horses scour the plain, the one belowing dismally, the other with
outstretched necks snuffing the wind, in the endeavor to detect, by the
moisture in the air, the vicinity of some pool of water not yet wholly
evaporated.

"Even if the burning heat of day be succeeded by the cool freshness of the
night, here always of equal length, the wearied ox and horse enjoy no
repose. Huge bats now attack the animals during sleep, and vampyre-like
suck their blood; or, fastening on their backs, raise festering wounds, in
which mosquitos, hippobosces, and a host of other stinging insects, burrow
and nestle. Such is the miserable existence of these poor animals, when
the heat of the sun has absorbed the waters from the surface of the
earth.

"When, after a long drought, the genial season of rain arrives, the scene
suddenly changes. The deep azure of the hitherto cloudless sky assumes a
lighter hue. Scarcely can the dark space in the constellation of the
Southern Cross be distinguished at night. The mild phosphorescence of the
Magellanic clou