Division of Water 245 



have the mean velocity of the part remaining, and, having to 

 change its direction to one at right angles, its velocity is still 

 further checked in making the turn. The smallest users of water 



by this system, therefore, in- 



variably receive an amount 

 which is less than they are 

 entitled to use, while the larger 

 users receive more. In order 

 to reduce this inequality of 

 division, the practice of insert- 

 ing a weir-board in the canal 

 just above the divisor, so as to Fig ' 44 ' Oneform of ter divisor - 

 restore a more nearly equal velocity across the stream, is some- 

 times adopted; and if the canal is broadened above the measur- 

 ing-box, so that the water approaches the weir slowly and passes 

 over it smoothly without contraction, Carpenter states that the 

 method will give as satisfactory results as any with which he 

 is acquainted. 



The Use of Modules 



A module is denned as a means of taking out of a canal a 

 definitely specified quantity of water, measured as so many inches, 

 cubic feet per second, or other units, rather than the simple 

 division of a stream into a certain number of parts, as is the 

 case where the divisor is used. 



Two types of modules are employed, one based upon the 

 principle of the weir as a means for measuring water, and the 

 other on the laws governing the flow of water through orifices. 

 If it were readily practicable to establish and maintain any 

 desired pressure at a weir or an opening, water could be appor- 

 tioned for irrigation with satisfactory precision with the aid of 

 modules, but no method for doing this has yet been devised, 

 although much study through many centuries has been devoted 

 to it. 



The spill -box, invented by A. D. Foote, and represented in 

 Fig. 45, is, perhaps, as satisfactory a means for maintaining a 



