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