FLOATING. 



375 



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 passing it. 



Thus we may have ground-weirs, below the surface of the 

 stream, over-flow 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 per- 

 fectly regulated. 



All these three 

 kinds of weirs are 

 employed in 



streams used for floating ; they are not only necessary to divert 

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

 these purposes besides for floating, but they also 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 position 

 by piles, or a row of piles behind which sunken fascines or stones 

 are placed. 



An over-flow weir may be constructed either of wood or of 

 stone ; fig. 215 shows a section of a simple wooden weir with a 

 steep declivity, and fig. 216, a weir with a gentle fall. 



Stone-work is naturally preferable to wood in constructing 



