G8 IRRIGATION FARMING. 



stand any natural pressure that may come upon it from 

 the impounded waters. 



In storing water for irrigation it is advisable to make 

 the slopes, particularly the inner one, more flat, and to 

 protect them, where they are likely to be washed, by rip- 

 rapping with rock, or slag, or lining with lumber. In 

 works of lesser magnitude pebblestones placed along 

 the water line will serve the purpose just as well. 



Masonry Work. In constructing a dam entirely 

 of solid masonry no definite rules can be given, as the 

 circumstances governing all cases will in no two instances 

 be alike, and we can only give the method by which a 

 substantial dam of this sort has been constructed. Let 

 us take for example the Bear Valley reservoir in South- 

 ern California. Into the solid rocks of a gorge the dam 

 is abutted and is built in a curve arching inward, form- 

 ing the arc of a circle, with a diameter of 335 feet, 

 illustrated very graphically in Figure 17. Its dimensions 

 are, on the top, 300 feet from the abutments, GO feet 

 from the bed rook of the creek in the highest point, and 

 conforming to the mountain slope on either side. The 

 foundation is 17 feet in width, running up to three feet 

 on the top, which is covered with huge blocks for coping. 

 The whole is built of vast granite blocks, which wm i 

 quarried near the margin of the lake and floated to the 

 wall on scows, while a derrick built on a raft placed them 

 in position. The best quality of Portland cement was 

 used for laying them, and all the interstices were filled 

 with beton, which was thoroughly tamped into place-. 

 until the whole structure is one homogeneous mass. 

 There are 3304 yards of rock work, on which 1300 bar- 

 rels of cement have been used. The cement was the 

 most expensive portion of the work. Beneath the dam 

 is a stone culvert for the outlet. This is closed by a 

 gate 21 by 24 inches, capable of discharging 8000 inches 

 of water which runs into a weir where the flow is i\; 



