10 REGULARITY OP THE INUNDATIONS. 



of several thousand square leagues receives. However m* 

 equal may be the quantity of rain that falls during severa, 

 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 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 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. Bivers 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 oi 

 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.* 



As long as, confounding the Bio Paragua of Esmeralda 



* 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, Meroe, and India, on the progress of 

 civilization along the valleys of the r vers. 



