176 HISTORY OF 



against 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 

 furrow. 



The stream in all rivers is more rapid in pro- 

 portion as its channel is diminished. For in- 

 stance, it will be much swifter where it is ten 

 yards broad, than where it is twenty; for the 

 force behind still pushing the water forward, 

 when it comes to the narrow part, it must make 

 up by velocity what it wants in room. 



It often happens that the stream of a river is 

 opposed by one of its jutting banks, by an island 

 in the midst, the arches of a bridge, or some such 

 obstacle. This produces, not unfrequently, a back 

 current ; and the water having passed the arch 

 with great velocity, pushes the water on each 

 side of its direct current* This produces a side 

 current, tending to the bank ; and not unfre- 

 quently a whirlpool ; in which a large body of 

 waters are circulated in a kind of cavity, sinking 

 down in the middle. The central point of the 

 whirlpool is always lowest, because it has the 

 least motion ; the other parts are supported, in 

 some measure, by the violence of theirs ; and, 

 consequently, rise higher as their motion is 

 greater ; so that towards the extremity of the 

 whirlpool must be higher than towards the centre, 



