of the bocas chicas, requires
various precautions, according as the waters are high or low. The
regularity of these periodical risings of the Orinoco has been long an
object of admiration to travellers, as the overflowings of the Nile
furnished the philosophers of antiquity with a problem difficult to solve.
The Orinoco and the Nile, contrary to the direction of the Ganges, the
Indus, the Rio de la Plata, and the Euphrates, flow alike from the south
toward the north; but the sources of the Orinoco are five or six degrees
nearer to the equator than those of the Nile. Observing every day the
accidental variations of the atmosphere, we find it difficult to persuade
ourselves that in a great space of time the effects of these variations
mutually compensate each other: that in a long succession of years the
averages of the temperature of the humidity, and of the barometric
pressure, differ so little from month to month; and that nature,
notwithstanding the multitude of partial perturbations, follows a
constant type in the series of meteorological phenomena. Great rivers
unite in one receptacle the waters which a surface of several thousand
square leagues receives. However unequal may be the quantity of rain
that falls during several successive years, in such or such a valley, the
swellings of rivers that have a very long course are little affected by
these local variations. The swellings represent the average of the
humidity that reigns in the whole basin; they follow annually the same
progression because their commencement and their duration depend
also on the mean of the periods, apparently extremely variable, of the
beginning and end of the rains in the different latitudes through which
the principal trunk and its various tributary streams flow. Hence it
follows that the periodical oscillations of rivers are, like the equality of
temperature of caverns and springs, a sensible indication of the regular
distribution of humidity and heat, which takes place from year to year
on a considerable extent of land. They strike the imagination of the
vulgar; as order everywhere astonishes, when we cannot easily ascend
to first causes. Rivers that belong entirely to the torrid zone display in
their periodical movements that wonderful regularity which is peculiar
to a region where the same wind brings almost always strata of air of
the same temperature; and where the change of the sun in its
declination causes every year at the same period a rupture of
equilibrium in the electric intensity, in the cessation of the breezes, and
the commencement of the season of rains. The Orinoco, the Rio
Magdalena, and the Congo or Zaire are the only great rivers of the
equinoctial region of the globe, which, rising near the equator, have
their mouths in a much higher latitude, though still within the tropics.
The Nile and the Rio de la Plata direct their course, in the two opposite
hemispheres, from the torrid zone towards the temperate.* (* In Asia,
the Ganges, the Burrampooter, and the majestic rivers of Indo-China
direct their course towards the equator. The former flow from the
temperate to the torrid zone. This circumstance of courses pursuing
opposite directions (towards the equator, and towards the temperate
climates) has an influence on the period and the height of the risings,
on the nature and variety of the productions on the banks of the rivers,
on the less or greater activity of trade; and, I may add, from what we
know of the nations of Egypt, Merce, and India, on the progress of
civilization along the valleys of the rivers.)
As long as, confounding the Rio Paragua of Esmeralda with the Rio
Guaviare, the sources of the Orinoco were sought towards the
south-west, on the eastern back of the Andes, the risings of this river
were attributed to a periodical melting of the snows. This reasoning
was as far from the truth as that in which the Nile was formerly
supposed to be swelled by the waters of the snows of Abyssinia. The
Cordilleras of New Grenada, near which the western tributary streams
of the Orinoco, the Guaviare, the Meta, and the Apure take their rise,
enter no more into the limit of perpetual snows, with the sole exception
of the Paramos of Chita and Mucuchies, than the Alps of Abyssinia.
Snowy mountains are much more rare in the torrid zone than is
generally admitted; and the melting of the snows, which is not copious
there at any season, does not at all increase at the time of the
inundations of the Orinoco.
The cause of the periodical swellings of the Orinoco acts equally on all
the rivers that take rise in the torrid zone. After the vernal equinox, the
cessation of the breezes announces the season of rains. The increase of
the rivers (which
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