FLOW OF RIVERS. 13 



the whole of the water touches the bottom, in its 

 turn, in a shorter time, and thus meets with more 

 stoppage. This cannot happen so much with deep 

 water, because the film that touches the bottom is 

 much smaller in proportion to the bulk of water 

 that is above it. This shows why the water taken 

 across the breadth of a stream, at any given place, 

 runs more slowly in the shallow parts than in the 

 deeper ; and why again the rate of flow of brooks 

 and rivers is greater when they are swelled by floods. 

 And now you will see why, when, in very winding 

 rivers, suitable cuttings are made, which shorten 

 the channel, and for equal lengths increase the 

 fall, the level of the water above must be lowered. 

 The same cause, too, which in every case favours 

 the draining of the water from above the cutting, 

 will not fail, in case of sudden floods, to raise the 

 level in the part below for a time ; and this can 

 only be prevented by deepening or widening the 

 channel. 



The greatest check which the speed of running 

 water meets with is in large basins, such as lakes, 

 through which it has to run, and in which its 

 motion becomes almost nothing. If the size of a 

 lake is very great in comparison with the width of 

 a river that runs through it, any flood of the latter 

 that soon passes off can but slightly raise the level 

 of the former, and therefore cannot much increase 

 the outflow of water from the lake. Large lakes 



