10 
REGtriAEITY OF THE INUNDATIOHS. 
of SGVeral thousand square leagues receives. However ui> 
equal may be the quantity of rain that falls during several 
successive years, in such or such a valley, the swellings of 
rh'ers 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 prin- 
cipal 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 sen- 
sible imUcation 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. Eivers that belong entirely to the 
torrid zone display in their periodical movements that won- 
derful 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 decli- 
nation 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. Tlie 
Orinoco, the Hio IVfagdalena, 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 liio de la Plata direct their course, in the two 
opposite hemispheres, from the torrid zone towards the 
temperate.* 
As long as, confounding the Hio Paragpia of Esmeralda 
j A®*®’ •■•le Burrampooter, and the majestic rivers of 
Indo-China, direct their course towards the equator. The former flow 
from tlie temperate to the torrid zone. This circumstance of courses 
pursuing opposite directions {towards the equator^ and towards the 
temperate cl, mates) lias 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, Meroii, and India, on the progress of 
civilization along the valleys of the r vers. 
