CO 



A HISTORY OF 



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 indication to the Europeans 

 themselves, in their journeys through those 

 trackless forests. As those sinuosities, there- 

 fore, 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 disem- 

 bogues 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 mathemati- 

 cal theories on this subject, represent them. 

 They found their calculations upon the sur- 

 face 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 rapidity 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 consi- 

 derably 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 pro- 

 portion 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 

 contrary reason, sinks lower. It sometimes, 

 however, happens, that this appearance is 

 reversed ; for when tides are found to flow 

 up with violence 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 pre- 

 sents 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 

 i arrow. 



The stream of all rivers is more rapid in 

 proportion as its channel is diminished. For 

 instance, it will be much swifter where it is 



a Button, de Fleuves, passim, vol. ii. 



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, tend- 

 ing 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 velocity. 

 It often happens that the current at the bot- 

 tom is swifter than at the top, when, upon 

 violent land-floods, the weight of waters to- 

 wards the source presses the waters at the 

 bottom, before it has had time to communi- 

 cate its motion to the surface. However, in 

 all other cases, the surface of the stream is 

 swifter than the bottom, as it is not retarded 

 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 wa- 

 ter, by these stoppages, gets an elevation 

 above the object; which, when it has sur- 

 mounted, it gives a velocity that recom- 

 penses the former delay. Islands and turn- 

 ings also retard the course of the stream but 

 very inconsiderably ; any cause which di- 

 minishes the quantity of the water, most 

 sensibly diminishes the force and the velo- 

 city of the stream. 



