THE EARTH. 177 



If the stream of a river be stopped at the sur- 

 face, and yet be free below ; for instance, if it be 

 laid over by a bridge of boats, there will then be 

 a double current j 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 bottom is swifter than at the 

 top, when, upon violent land-floods, the weight 

 of waters towards the source presses the waters 

 at the bottom, before it has had time to commu- 

 nicate 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 rub- 

 bing 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 ob- 

 ject ; 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 diminishes 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 



* Buftbn, vol. ii. p. 62. 

 VOL. I. M 



