128 Springs, Rivers, and the Sea. 



whole width of the river. The line in 

 which the water moves with the greatest 

 velocity is called the thread of the river, 

 and this thread seldom lies in the middle 

 of the river, but it generally comes near- 

 er to one side than the other, according 

 to the nature of the impediments, and 

 the configuration of the banks. The ve- 

 locity of rivers is likewise less at the 

 bottom of their channels, than at their 

 surface ; owing to the resistance which 

 the bed makes to the water as it flows. 



The running of rivers is upon the same 

 principle as the descent of bodies on in- 

 clined planes ; for water no more than a 

 solid can move on a horizontal plane, the 

 re-action of such a plane being equal and 

 contrary to gravity entirely destroys it, 

 and leaves the body at rest; here we speak 

 of a plane of small extent, and such as co- 

 incides with the curved surface of the 

 earth. But if we consider a large extent 

 or long course of water, then we shall find 

 that such water can never be at rest but 

 when the bottom of the channel coincides 

 every where with the curved surface of 

 the earth. In rivers that are made it is 

 usual to allow the fall of 1 foot in 300, but 



