TRANSVERSE SECTION OF THE STREAM. 215 



surface, they spread out and remain there, mingling with the water at 

 that level, and diminishing the velocity which would otherwise be 

 found there.* 



115. Transverse Section of the Stream. The 



form of the transverse section and the direction of the cur- 

 rent have such an effect upon the velocity at the surface, at 

 different distances from the banks, that there can be no 

 definite law of change. There is generally an increase of 

 velocity, as the distance from the banks is increased, until 

 the maximum point is reached. That portion of the river 

 where the water has its maximum velocity is called the line 

 of current or axis of the stream, and the deepest portion of 

 the stream is called the mid-channel. When the stream 

 bends, its axis is generally near the concave shore. 



It is observed that the surface of a stream, in any cross- 

 section, is highest where the velocity is greatest, which is 

 accounted for by the fact that, when the water is in motion, 

 it exerts less pressure at right angles to the direction of its 

 motion than when it is at rest, and therefore, where the 

 velocity is greatest the water must be highest, to balance 

 the pressure at the sides, where the velocity is less. 



It frequently happens that, while the mass of the water 

 in a river is flowing on down the river, the water next the 

 shore is running up the river. It is no unusual thing to 

 find a swift current and a corresponding fall on one shore 

 down stream, and on the opposite shore a visible current 

 and an appreciable fall up stream ; i. e., on one side of the 

 river the water is often running rapidly up stream, while on 

 the other side it is running with equal or greater rapidity 

 down stream. The apparent slope at every point is affected 

 by the bends of the river, and by the centrifugal force 

 acquired by the water in sweeping round the curves, and by 

 the eddies which form on the opposite side. The surface of 

 the river is not therefore a plane, but a complicated warped 



* Bncy. Brit., Vol. XII., p. 497. 



