THE EARTH, 85 



happens, that the surface of the water is higher than ma,y of the 

 grounds that are adjacent to the banks of the stream. If, after inun- 

 dations, we take a view of some rivers, we shall find their banks ap- 

 pear above water, at a time that all the adjacent valley is overflowed. 

 This proceeds from the frequent deposition of mud, and such like 

 substances, upon the banks, by the rivers frequently overflowing; and 

 thus, by degrees, they become elevated above the plain ; and the wa- 

 ler is often seen higher also. 



Rivers, as every body has seen, are always broadest at the mouth, 

 and grow narrower towards their source. But what is less known, 

 and probably more deserving curioshy, is, that they run in a more di- 

 rect channel as they immediately leave their sources ; and that their 

 sinuosities and turnings become more numerous as they proceed. It 

 is a certain sign among the savages of North America, that they are 

 near the sea when they find the rivers winding, and every now and 

 then changing their direction. And this is even now become an in- 

 dication to the Europeans themselves, in their journeys through those 

 trackless forests. As those sinuosities, therefore, increase as the river 

 approaches the sea, it is not to be wondered at that they sometimes 

 divide, and thus disembogue by different channels. The Danube dis- 

 embogues into the Euxine by seven mouths; the Nile by the same 

 number ; and the Wolga by seventy. 



The currents* of rivers are to be estimated very differently 'from 

 the manner in which those writers who have given us mathematical 

 theories on this subject, represent them. They found their calcula- 

 tions upon the surface being a perfect plain from one bank to the other : 

 but this is not the actual state of nature ; for rivers, in general, rise 

 in the middle ; and this convexity is greatest in proportion as the ra- 

 pidity of the stream is greater. Any person to be convinced of this, 

 need only lay his eye, as nearly as he can, on a level with the stream, 

 and looking across to the opposite bank, he will perceive the river in 

 the midst to be elevated considerably above what it is at the edges. 

 This rising, in some rivers, is often found to be three feet high ; and 

 is ever increased in proportion to the rapidity of the stream. In this 

 case, the water in the midst of the current loses a part of its weight, 

 from the velocity of its motion ; while that at the sides, for the con- 

 trary reason, sinks lower. It sometimes, however, happens, that this 

 appearance is reversed ; for when tides are found to flow up with vi- 

 olence Hgainst the natural current of the water, the greatest rapidity is 

 then found at the sides of the river, as the water there least resists the 

 influx from the sea. On those occasions, therefore, the river presents 

 a concave rather than a convex surface ; and, as in the former case, 

 the middle waters rose in a ridge, in this case they sink in a fur- 

 row. 



The stream of all rivers is more rapid in proportion as its channe) 

 is diminished. For instance, it will be much swifter where it is ten 

 yards broad, than where it is twenty ; for the force behind still push- 

 ing the water forward, when it comes to the narrow pait, it musi 

 make up by velocity what it wants In room. 



Rufibn. de Fleuves, passim, vol. ii 



