As the equatorial rains take place in the flat country when the sun
passes through the zenith of the place, that is, when its declination
becomes homonymous with the zone comprised between the equator and one
of the tropics, the waters of the Amazon sink, while those of the
Orinoco rise perceptibly. In a very judicious discussion on the origin
of the Rio Congo,* (* Voyage to the Zaire page 17.) the attention of
philosophers has been already called to the modifications which the
periods of the risings must undergo in the course of a river, the
sources and the mouth of which are not on the same side of the
equinoctial line.* (* Among the rivers of America this is the case
with the Rio Negro, the Rio Branco, and the Jupura.) The hydraulic
systems of the Orinoco and the Amazon furnish a combination of
circumstances still more extraordinary. They are united by the Rio
Negro and the Cassiquiare, a branch of the Orinoco; it is a navigable
line, between two great basins of rivers, that is crossed by the
equator. The river Amazon, according to the information which I
obtained on its banks, is much less regular in the periods of its
oscillations than the Orinoco; it generally begins, however, to
increase in December, and attains its maximum of height in March.* (*
Nearly seventy or eighty days after our winter solstice, which is the
summer solstice of the southern hemisphere.) It sinks from the month
of May, and is at its minimum of height in the months of July and
August, at the time when the Lower Orinoco inundates all the
surrounding land. As no river of America can cross the equator from
south to north, on account of the general configuration of the ground,
the risings of the Orinoco have an influence on the Amazon; but those
of the Amazon do not alter the progress of the oscillations of the
Orinoco. It results from these data, that in the two basins of the
Amazon and the Orinoco, the concave and convex summits of the curve of
progressive increase and decrease correspond very regularly with each