FLOATING. 



373 



position by piles, or row of piles behind which sunken fascines 

 or stones are placed. 



An overflow weir may be constructed either of wood or of 

 stone : Fig. 243 shows a section of a simple wooden weir with 

 a steep declivity ; Fig. 244, a weir with a slight fall. 



Stone - work is 

 naturally prefer- 

 able to wood in 

 constructing weirs, 

 and wherever coarse 

 stones are available, 

 a weir may be con- 

 structed as in Fig. 

 245. Weirs con- 

 structed of hewn 

 stone- masonry 

 (Fig. 246) are preferable to rough constructions, but unless 

 the watercourse has a rocky bed, piles must be driven in to 

 serve as a foundation under the weir. 



The efficacy of any weir is measured by the height to which 

 the water rises behind it, and the distance back to the point 



Fit:. iM.~,. Stone- overflow weir. 



Fi<_ r . 246. Stone weir. 



where the stream retains its former velocity, or ceases to be 

 slack- water. Hence, in order to improve thoroughly a stream 

 for floating, a succession of weirs should be constructed from 

 slack-water to slack-water ; in this way the average fall of 

 the stream will be reduced, a very important point in 

 floating. 



