372 WATER-TRANSPORT. 



of capital is invested, whilst others can harely raise the level of a 

 stream to its full strength. 



The more a water-course 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. 



In such cases, dams are sometimes constructed which allow of 

 a depth of water in the reservoir, at the dam, of 15-30 feet. 

 "Well constructed floating-channels with a small and uniform full 

 require much smaller dams. 



Large reservoirs are generally preferable to small ones, even 

 although 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. 



(k) Position of Dams. — The principiil dams are always made 

 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 li 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 utilize the first weak run of the water, and 

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

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

 possible. 



A place 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 mountainous 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. Sec. ; these will be described 

 further on, under the head of weirs. 



