62 



tain, when you have not two barometers at the 

 two stations, and have to determine differences 

 of half a line of atmospheric pressure. 



It is probable, that the river loses part of the 

 quantity of it's waters in the cataracts, not only 

 on account of the increased evaporation owing* 

 to the dispersion of minute drops in the atmo- 

 sphere, but still more from nitrations into the 

 subterraneous cavities. These losses however 

 are not very perceptible, when we compare the 

 mass of waters entering into the raudal with 

 that which issues out near the mouth of the 

 Rio Anaveni. It was by a similar comparison 

 that the existence of subterraneous cavities in 

 the yellalas or rapids of the river Congo was 

 discovered. The pongo of Manseriche, which 

 ought rather to be called a strait than a fall, 

 ingulfs, in a manner not yet sufficiently ex- 

 plored, a part of the waters, and all the floating 

 wood of the Upper Maragnon. 



When, seated on the bank of the Oroonoko, 

 our eyes are fixed on those rocky dikes, the 

 mind inquires, whether, in the lapse of ages, the 

 falls change their form or height, I am not 

 much inclined to believe in such effects of the 

 shock of water against blocks of granite, and 

 in the erosion of siliceous matter. The holes 

 narrowed toward the bottom, the funnels that are 

 discovered in the raudales, as well as near so many 

 other cascades in Europe, are owing only to the 



