244 Irrigation and Drainage 



volumes of water if their dimensions are exactly the same as 

 shown at A and B. But if the division is made as at C, or in 

 any other manner, which makes the two arms in any .way 

 unlike, one will carry more water than the other. So, too, if 

 care is not taken to keep the main and the two branches clean 

 where the division is made, it will not be exact. 



When an effort is made to divide the main into two unequal 

 parts, or into an odd number of equal parts, the task becomes 

 an extremely difficult one, and one which is not likely to be 

 accomplished, and the attempt should be avoided. 



The cause of the difficulty is found in the fact that the waier 

 travels with the greatest velocity in the center of the stream 

 and diminishes in speed as the sides are approached, so that if 

 the main is divided into two branches which have cross -sections 

 in the ratio of 1 to 2, the larger arm will carry more than twice 

 the amount of the smaller one, because it must take a larger 

 share of the water moving in the central portion of the main. 

 Or if the main is divided into three equal laterals, then the 

 central branch is sure to carry more water than either of the 

 two taking the water from nearer the sides, and it is not prac- 

 ticable to so adjust the dimensions of these branches that with 

 varying volumes of water moving in the main the desired ratios 

 shall always be secured in the divisions. 



The Use of Divisors 



When it is desired to remove from a ditch a certain portion 

 of the amount of water which it is carrying, this is sometimes 

 attempted by means of an arrangement represented in Fig. 44, 

 called a divisor, in which the portion A is set into the channel 

 some fractional part of the whole width, determined by the 

 amount which it is desired to take out. Thus, if it was desired 

 to take out one-fifth of the stream, and the lateral had a width 

 of 40 inches, the divisor would be set in toward the center 8 

 inches. But from what has already been said, it follows that 

 less than one-fifth of the water can thus be removed, for the 

 two reasons, that the section of the stream removed does not 



