CONSTRUCTION OF DAMS. 209 



stream to a point at which the required level is reached. 

 This point should be found hy careful survey, before any 

 work is done, because the strength and size of the dam 

 must be proportioned to the pressure of the water con- 

 tained within it, and this is in a ratio with the hight of 

 head of the confined portion of the stream. This kind 

 of dam very rarely requires elaborate construction, but as 

 it is exposed to frequent erosive washing by the stream, 

 in floods, it should be built of such materials as are 

 known to bind well together. "When but little head is 

 required, a very simple dam of brush, stone, and earth, 

 will be sufficient. The work is commenced at the head of 

 the canal, which is first excavated to the proper depth, 

 up to the river bank where the head-gate is, (see a in the 

 figure), properly constructed. The building of the wing 

 dam is carried up the stream from this point. A few 

 piles driven into the river bed, three feet apart in a double 

 row would be advisable at this point, and for such a dis- 

 tance up the stream as may seem proper, as this point of 

 the dam is exposed to the greatest pressure, and is gener- 

 ally the weakest ; for the reason that in all earth-work 

 the junction of the old and new material is the most 

 difficult to consolidate evenly. If brush is to be con- 

 veniently procured, it may be interwoven between the 

 piles of each row, and rammed down compactly. Cross- 

 ties may be bolted or pinned to the piles, to prevent them 

 from spreading, and earth is then thrown between them. 

 Brush should be placed between the rows of piles, and 

 the brush should be placed so that the buts lie down 

 stream, and the fine part in the contrary direction. As 

 the earth is dumped into the dam it will cover the brush. 

 Afterwards coarse gravel or stone may be used to fill on 

 the outside. In this manner the dam is carried onwards 

 to the extremity where brush covered with earth will be 

 sufficient to divert the current where the difference 

 between the levels is but slight. 



