FLOATING. 377 



much more suitable for floating, an important matter where 

 it contains much gravel and boulders. 



The most suitable places for weirs are narrow valleys between 

 rocky banks, as in such places, the water cannot damage the 

 banks of the channel and cause inundations, even when its depth 

 is considerably increased. 



In such places several consecutive weirs are generally required, 

 so that the watercourse in certain cases becomes regularly 

 terraced, with a succession of falls. As a rule, the number of 

 weirs should be proportional to the rapidity of the current and 

 the quantity of gravel and boulders in the stream. These weirs 

 are not all constructed at the same time, but by degrees, as the 

 space between any two of them becomes filled with silt and 

 gravel, and therefore the necessity arises for a new weir. 



Besides the above-mentioned weirs, others are also required 

 wherever any side-channel leaves the main stream to supply a 

 mill, &c. Booms for collecting the floating wood are also fre- 

 quently erected on weirs. The more remote the point where 

 the water from a side-channel is required, the higher must be 

 the weir which supplies it. 



It is evident that sand, gravel and boulders accumulating 

 behind the weirs constantly raise the bed of the stream, so that 

 the water will in time overflow its banks unless they are 

 sufficiently high. This is danger not only for the banks, but 

 also for the wood which is being floated and tends to leave the 

 stream and become stranded. If then a rush of high water 

 follows, much damage may be done to the riparian properties, 

 for which the manager of the floating will be held respon- 

 sible unless he has taken proper precautions. In all cases, 

 therefore, where such damage is to be feared, weirs should be 

 furnished with sluice-gates which may be opened when there is 

 danger of a flood. 



Fig. 219 shows a section through the middle of a weir in 

 which a sluice-gate is supplied (in o n) being the section of the 

 weir, and (o m) the sluice-gate with a sloping base (s ni), en- 

 closed by wooden horizontal walls ; this gate is closed when the 

 water-level is at its ordinary height, but can be opened in floods. 



More frequently, however, a ground-weir is erected with a 

 number of sluice-gates arranged side by side, so that the whole 



