HYDRAULICS. 203 



The velocity may also become nothing, by the declivity 

 becoming so small, that 



*** 



"'' - is less than -- 

 S 200000 



if _ is less than 3^7^, or than ^th of an inch 



to an English mile, the water will have a sensible 

 motion. 



293. In a river, the greatest velocity is at the sur- 

 face, and in the middle of the stream, from which it 

 diminishes toward the bottom and the sides, where 

 it is least. It has been found by experiment, that 

 if from the square root of the velocity in the middle 

 of the stream, expressed in inches per second, unity 

 be subtracted, the square of the remainder is the ve- 

 locity at the bottom. 



Principes (THydr antique, par Du BUAT, 67. 



Hence, if the former velocity be =: v 9 the velocity at 

 the bottom = v 2 Jv + 1 . 



294. It has been found by experiment, that the 

 mean velocity, (or that with which, were the whole 

 stream to move, the discharge would be the same 

 with the real discharge,) is equal to half the sum of 



the 



