370 



WOOD TRANSPORT BY WATER. 



iv. Tanks. 



Dams can be constructed across a stream only in narrow 

 mountain-valleys where they can rest on mountain-spurs on 



Fig. 241. Tank A supplied by river t. 



both sides, without being necessarily of any considerable 

 length. In wider and broader valleys with a slight fall, 



Fig. 242. Sluice-gate of the tank A. 



where there are meadows, cultivated lands and perhaps 

 houses, which a dam would obviously inundate, while its cost 



