FLOATING. 569 



others can raise the level of a stream barely above its full 

 strength. 



The more a watercourse is encumbered with boulders and 

 rocks, the lower the dry-season level of its water and the 

 longer its course, the more plentifully should water be 

 supplied. Sometimes in such cases dams are constructed 

 which allow of a depth of water in the reservoir, at the dam, 

 of 15 to 30 feet. Well-constructed floating channels with a 

 small and uniform fall require much smaller dams. 



Generally large reservoirs are preferable to small ones, even 

 though they take a comparatively long time to fill, as their 

 effects are more proportional to their cost, and the floating is 

 more certain than where several small dams are constructed. 

 Very large dams have been made in Carinthia and the southern 

 Alps, and in Austria and Hungary. 



. (/i) Position of Dams. The principal dams are made always 

 in the uppermost parts of a mountain-valley, and their effect 

 reaches for several miles down, so that in many valleys no more 

 dams are required below the principal one. In other cases, 

 however, there are floating-channels with several small dams at 

 distances apart of from 1J to 2 miles. 



Dams are intended, as far as possible, to drive the drainage of 

 a locality into the watercourse which is used for floating. 

 Watercourses, however, contain least water near their sources, 

 but are here most in demand for floating purposes. It is there- 

 fore necessary to utilise the first weak run of the water, and 

 wherever it is possible to do so, a strong dam is erected near the 

 very top of a valley, so as to collect as much water above it as 

 possible. 



A site is therefore preferable for the principal dam 

 where the sides of the valley approach one another with 

 rocky walls, whilst above this gorge is a basin-shaped 

 expanse of valley. Such places are often found in mountain- 

 ous regions. 



Care must be taken that the water entering a reservoir is 

 fairly free from silt and gravel, which would soon render it too 

 shallow for use. Wherever this is not the case, special works 

 must be constructed to keep out the sand, etc. ; these will be 

 described further on, under the heading "Weirs." 



F.U. B B 



