with a View to Irrigation. 261 



sary addition to every dam constructed across a river wlien 

 the bed has not sufficient tenacity to resist the action of the 

 water. This may be composed of logs notched upon the 

 sill pieces and covered with plank, or of rough logs notched 

 and pinned down upon the sleepers in close contact with 

 each other : it should extend far enough below the dam to 

 conduct the water away safely, and should have a row of 

 sheet piling in rear, as shown in figure 3. 



Where rough timber is plentiful, a dam, as shown in 

 figures 3 and 4, may be easily and quickly constructed 

 thus : two or three rows of rough sleepers are bedded across 

 the stream, and upon these rough logs are notched and 

 pinned at intervals of about five feet in the rear of the dam. 

 Over one of these sleepers another transverse log is notched 

 upon the first row of longitudinal timbers ; and if the dam 

 is a large one, perhaps a second transverse timber may be 

 required. The second row of longitudinal timbers is 

 notched upon the second row of transverse timbers, not 

 exactly over the first row, but just so much clear of it as to 

 allow of the end being notched and pinned upon the ground- 

 way or sleeper at the upper side of the dam, close alongside 

 of the first timber. Eow after row of timber is thus placed, 

 the dam constantly rising in rear by the thickness of a log 

 for each course, while in front all are brought down and 

 pinned to the ground-way. When the necessary height is 

 obtained, the top row of longitudinal timber may be laid 

 side by side in as close contact as possible, and the spaces 

 made good with small fascines, bark, &c.; or rows of trans- 

 verse logs may be placed at about three feet apart, and 

 planks spiked to them. The rear of the dam appears, as 

 shown in the sketch figs. 8 and i, of alternate rows of 

 longitudinal and transverse timbers to which planking is 

 spiked. 



Where timber is plentiful, and the river is deep, a dam 

 may be safely constructed to a great height of crib-work, 



