371 WATER-TRANSPORT. 



and this may be secured b}- constructing,' a tank. This is an 

 artificial pond surrounded by strong embankments, which is fed 

 by underground culverts, or by a canal bringing water from the 

 upper part of the watercourse ; water may thus be collected in 

 the tank to strengthen the stream below it. 



There may be peculiarities of the locality which modify the 

 mode of construction of tanks, but they are much less variable in 

 this respect than dams. 



Figs. 213 and 21-4 represent a tank which has been constructed 

 at Wilgartswiesen in the Bavarian Palatinate. The reservoir A 

 is constructed between the floating stream t and a small mill- 

 stream 1)1. It is surrounded by strong embankments (d, d), 

 14 feet high, and is fed by the mill-stream, which bifurcates from 

 the watercourse above the reservoir, and is led along the hill-side 

 with a gentle fall, so that at a, it is about 10 feet higher than 

 the watercourse, which it rejoins after passing the mill M. 

 There are sluice-gates at a and b, the former for admitting the 

 water and the latter for its escape ; s, s is a cart-road along 

 which the wood is conveyed which is stacked at //, and there put 

 into the stream. This tank holds 280,000 cubic feet (8, OLIO cubic- 

 meters) of water, can be tilled once daily, and takes 2 hours 

 and 40 minutes to run dry, floating 42,000 stacked cubic feet 

 (1,200 cubic meters) of firewood. 



The embankments of tanks may be of earth or masonry, or 

 half earth, half masonry, as shown in section in tig. 214. Here 

 A represents the stone-masonry, B the earth-work, a the sluice- 

 valve, III the feeder, and t the watercourse. 



Tanks have been constructed at several places in Silesia, 

 Franconia, the Palatinate, &c., and are utilized in summer for 

 irriiratinfir meadows and cultivated lands. 



V. ]l'cirs. 



The works already described have for their oltject to increase 

 the quantity of water in a floating-channel, but as soon as the 

 accumulated water has run -off", the stream resumes its natural 

 level. 



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

 ])ermaneutly, and moderate its fall and velocity. They consist 



