372 



WOOD TKANSPORT BY WATER. 



accumulated water has run off, the stream resumes its natural 

 level. 



Weirs, on the contrary, are constructed to raise the water- 

 level permanently and moderate its fall and velocity. They 

 consist of a shallow dam erected across a stream, the top of 



which is either 

 slightly below, 

 even with or 

 above the water- 

 level, so that the 

 water must more 

 or less increase 

 in depth behind 

 the weir before 



Fig. 243. Wooden overflow weir. 



passing it. 



Thus we may 



have ground- weirs, below the surface of the stream; overflow 

 weirs, between the highest and lowest levels of the water, 

 and sluice-weirs which are provided with gates: in the latter 

 case, the quantity of water in the stream can be perfectly 

 regulated. 



All these three kinds of weirs are employed in streams used 



Fig. 244. Overflow weir with slight fall. 



for floating ; they are necessary not only to divert water to 

 mills and irrigation-canals, when the water is used for these 

 purposes besides for floating, but also to maintain a high 

 permanent level of water in a floating stream. 



The construction of ground-weirs is very simple, they may 

 be composed of a ridge of stones, a stem of a tree kept in 



