56 A HISTORY OF 



It often liappons that the stream of a river is opposed by on& 

 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 ve- 

 locity, pushes the water on each side of its direct current. This pro- 

 duces a side current, tending to the bank ; and not unfrequently 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. 



If the stream of a river be stopped at the surface, and yet be 

 free below ; for instance, if it be laid over by a bridge of boats, 

 there will then be a double current ; the water at the surface will 

 flow back, while that at the bottom will proceed with increased ve- 

 locity. It often happens that the current at the bottom is swifter than 

 at the top, when upon violent land-floods, the weight of waters toward 

 the source, presses the waters at the bottom, before it has had time to 

 communicate its motion to the surface. However, in all other cases, 

 the surface of the stream is swifter than the bottom, as it is not re- 

 tarded by rubbing over the bed of the river. 



It might be supposed that bridges, dams, and other obstacles in the 

 current of a river, would retard its velocity. But the difference they 

 make is very inconsiderable. The water, by these stoppages, gets an 

 elevation above the object ; which, when it has surmounted, it gives 

 a velocity that recompenses the former delay. Islands and turnings 

 also retard the course of the stream but very inconsiderably ; any 

 cause which diminishes the quantity of the water, most sensibly di- 

 minishes the force and the velocity of the stream. 



An increase* of water in the bed of the river always increases its 

 rapidity ; except in cases of inundation. The instant the river has 

 overflowed its banks, the velocity of its current is always turned that 

 way, and the inundation is perceived to continue for some days; 

 which it would not otherwise do, if, as soon as the cause was discon- 

 tinued, it acquired its former rapidity. 



A violent storm, that sets directly up against the course of the 

 stream, will always retard, and sometimes entirely stop its course. 

 I have seen an instance of this, when the bed of a large river was left 

 entirely dry for some hours, and fish were caught among the stones at 

 the bottom. 



Inundations are generally greater towards the source of rivers than 

 farther down ; because the current is generally swifter below than 

 above ; and that for the reasons already assigned. 



A little rivert may be received into a large one, \vithout augment- 

 ing either its width or depth. This, which at first view seems a para- 

 dox, is yet very easily accounted for. The little river, in this case, 

 only goes towards increasing the swiftness of the largei, aru j putting 



?.uf!bn, vol. li. p. 62. t Guglifl'mini 



