with a View to Irrigation. 259 



The banks of the river or stream should be carefully 

 examined with reference to the quality of the soil of which 

 they are composed, and their power of acting as abutments 

 to the dam. 



When the site of the dam has been decided upon with 

 reference to tlie principal objects which it is intended to 

 answer, the necessary levels must be taken, and the height 

 of the structure determined. Upon this will depend, in a 

 great measure, (when materials are plentiful), the plan to be 

 adopted in forming the dam. 



In shallow rivers, when the bottom is rock, a dam of the 

 section shown in (plate xix.J fig. 1 maybe easily constructed 

 of 10 or 12 feet in height. The sill (a h) is bolted down to 

 the rock with fox-wedge bolts. The standard (5 c) is mortised 

 into this sill; and a brace {a c) is framed into the two, 

 making thus a strong vertical frame. When the dam is 

 high, a second brace may be inserted, and the horizontal 

 distances between the frames diminished ; but, in general, 

 8 or 10 feet may be allowed as a fair distance between these 

 frames from centre to centre. When the frames are securely 

 fixed, a facing of logs, roughly squared on the upper and 

 under sides, is laid in front of them, across the bed of the 

 stream. These should be got as long as possible, and 

 should break joint occasionally against a standard, to which 

 they should be sometimes pinned with a trenail, in order to 

 prevent their moving. 



When the water is intended to flow over this dam, the 

 space between the frames in the rear should be filled in with 

 blocks of rough stone, well wedged together, and laid in 

 steps, so as to break the fall of water on the bed of the 

 river in rear. If material of the proper quality cannot be 

 found, or if the time will not allow of its being quarried 

 and placed properly, this space may be filled in with earth 

 and rubble ; and logs being notched down upon the back 



