94 IRRIGATION FARMING. 



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 X24 inches, capable of discharging 8,000 inches of 

 water, which runs into a weir, where the flow is meas- 

 ured in inches. This gate and weir control the flow of 

 water. On one side of the dam a spillway over solid 

 rock is provided for the overflow of the surplus water. 

 This is some four feet lower than the crest of the dam 

 and affords ample discharge for the superabundant 

 water. 



The Sweetwater dam, built across the mouth of a 

 cafion a short distance above National City, California, 

 and shown in the full-page photographic reprodu<5lion, 

 Fig. 19, is one of the boldest pieces of engineering in 

 the world. The dam is construdled as a crown arch, 

 and is the largest of its characfter in the world. It is 

 of solid granite and Portland cement, 46 feet thick at 

 the base and 12 at the top. It is 90 feet high at bed- 

 rock, 76 feet long at the base and 396 feet at the top. 

 The reservoir covers 700 acres, and has the enormous 

 storage capacity of six billions of gallons. The water 

 is discharged from the reservoir by means of a main 

 pipe 36 inches in diameter, and then by smaller pipes. 

 Much of the land under this system is high and rolling, 

 -but the head is sufficient to carry the water to the 

 highest portions. The dam gathers the rainfall from 

 186 square miles, and the capacity of the reservoir is 

 sufficient to hold a two years' supply for 10,000 acres. 



Cost and Capacity. — In calculating the cost of a 

 reservoir it is necessary to fix the value of a defined 

 volume of water for irrigation purposes. For conven- 



