Division of Water 247 



in the depth of the water over the lip of the spill -back must 

 make a perceptible difference in the discharge. 



Further than this, where the form of the opening is designed 

 to be made longer or shorter by means of a sliding valve accord- 

 ing as more or less water is desired, the amount discharged, 

 even when the head is maintained rigidly constant, is not 

 directly proportional to the length of the opening, because the 

 number of inches of margin upon which the resistance to flow 

 depends does not maintain a constant ratio to the cross -section 

 of the opening. The more margin there is in proportion to the 

 area of the opening, the greater must be the loss of discharge 

 through friction and contraction, so that the most exact and 

 generally satisfactory way of apportioning water among users 

 which has yet been devised, is that of bisecting the stream until 

 its volume has become suitable for individual use, and then sub- 

 dividing by time under some system of rotation. 



