110 IRRIGATION. 



it open as long as the stream is running into the caiial. 

 When the water is exhausted the pipe is again closed. 

 To prevent the water flowing over the dam, through any 

 accidental stoppage in the machinery, a branch of the 

 overflow pipe is carried down the face of the dam into 

 the canal. This apparatus is of very general use in the 

 Swiss Cantons, and in irrigating works elsewhere, and 

 works with regularity and precision. It is necessary that 

 the balance-trap be properly adjusted and looked after 

 occasionally. The worst that can happen in case of 

 accident is the overflow of the reservoir by the pipe into 

 the canal without harm. If the overflow is provided for 

 at the inlet by a pipe or a channel placed there, as already 

 suggested, this overflow pipe in the dam will not be 

 needed. In practice, however, it will be found safest to 

 have every guard against accident and consequent damage 

 to the works, and two outlets will be twice as safe as one. 

 If it is thought desirable, the waste-pipe in the dam may 

 be placed two inches above the level of the other outlet, so 

 that it will come into use only in case of a stoppage of 

 the lower one. The outlet pipe should be large enough 

 to discharge the water as least four times as rapidly as it 

 enters the reservoir ; so that the storage of two days flow 

 may be discharged in a night or during one cloudy day. 

 (Under no circumstance should the water be permitted 

 to escape during the day when the sun is shining). A 

 three-inch pipe will discharge nine quarts per second, 

 which would be more than enough to furnish two inches 

 of water to four acres in 12 hours. A pipe of this di- 

 ameter would therefore be of ample size for a 12-acre 

 meadow, giving a weekly watering to each 4 acres by 

 three discharges of the reservoir. 



A siphon is not always to be depended upon to dis- 

 charge a reservoir automatically. Sometimes the water, 

 when rising slowly and not filling the pipe completely, 

 trickles over and does not set the siphon in operation. 



