USE OF CONCRETE PIPE IN IRRIGATION. 



39 



tively. Under a 4-inch head over the center of these openings, they 

 will discharge 10. 15, 25, and 50 miner's inches, respectively. When 

 miner's inch boxes are first installed they should be checked against 

 a standard weir in order to test their accuracy. Under constant 

 heads, the miner's inch box measures water with fair accuracv but 



Fig. 15. — Weir box for controlling of head. 



where the heads are permitted to vary errors of 100 per cent may be 

 introduced. As in the case of weirs, spillways are used to prevent 

 large fluctuations in heads. One of these is shown in figure 18. 



Water registers. 



Wherever it is desired to keep a continuous record of the amount of 

 water passing a given point automatic registering devices known as 

 water registers are frequently used. These consist of a drum around 

 which the recording sheet is wrapped and both are attached to a 

 clock which causes them to revolve once in a period of 8 days or 

 more. The record is made on the sheet by a pen or pencil attached to 



