37g 



WATER-TRANSPORT. 



Fifl. 21; 



weirs, and wherever coarse stones are available, a weir may be 

 constructed as in fig. 217. Weirs constructed of hewn stone- 

 masonry (fig. 218) are much preferable to rough constructions, 



and unless the water- 

 course 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 be- 

 hind it, and the distance back to the point where the stream 

 retains its former velocity, or ceases to be slack-water. Hence, 

 in order to thoroughly improve a stream for tioatiug, a succes- 



sion of weirs should be constructed from slack-water to slack- 

 water, and in this way the average fall of the stream will be 

 reduced, a very important point in floating. 



The slower the current, the further back the slack-water 

 extends ; in sluggish streams, weirs may reduce the velocity 

 of the stream too much for floating, and are only useful for 

 diverting mill-streams from the main watercourse. Wherever, 

 on the other hand, the current is rapid, it is evidently advan- 

 tageous to keep the water back, as much as possible ; for 

 independently of the advantages of a moderate current, the 

 banks and works to improve the floating are thus much better 

 secured against erosion, and the depth of the stream is rendered 



