350 



IRRIGATION PRACTICE 



small canals or canals drawn from reservoirs, they who 

 take water out of a river have right only to a definite 

 proportion of the total flow. In a dry as in a wet year 

 this proportion remains the same. For instance, if there 



V % ' 



FIG. 96. Lyman rectangular weir. 



are 500 shares in such a canal company, each share will 

 receive TO is of the total flow in the canal whether the 

 flow be large or small. 



There are, therefore, two classes of measuring devices : 

 (1) Divisors, for the purpose of dividing streams into 

 halves, quarters or other fractions, independently of the 

 volume; (2) modules, for the purpose of measuring the 

 absolute volumes of water that flow through canals or 

 ditches. The divisors can be made satisfactorily only 

 after proper and satisfactory modules have been estab- 

 lished. 



A great variety of water-measuring devices exists. 

 In the beginning of modern irrigation there were no 



