GATE STRUCTURES FOR IRRIGATION CANALS. 29 



so retarded along the edge of the ditch by grass, rocks, and other 

 friction elements that in the case photographed the water flowing 

 out of the small division at the left of the box had a velocity about 

 one-fifth that at the middle of the box. Also in the case shown in 

 the plate, the conditions of contraction are not such that the dis- 

 charge over the check or weir board is proportional to the length. 

 The division board should have been extended as far upstream from 

 the check board as the side walls so that the contraction of the cur- 

 rent would be completely suppressed by the time the water reached 

 the board, and then the discharge would be proportional to the length, 

 provided the velocity is uniform across the weir or check board. To 

 secure this last condition it is better to pool the water above the box 

 by widening and deepening the ditch or by installing baffle boards 

 in some form. The nearer still the water is above the box the more 

 nearly accurate is the division. 



As usually installed there is a very appreciable velocity toward the 

 box, and the diverted water is less than the figured proportion for the 

 reason that it is diverted at the side of the ditch while the greatest 

 velocity is near the middle. The only thing that can be said in favor 

 of this erroneous division is that the consumers at the head get less 

 water than they are entitled to, and in this way involuntarily contrib- 

 ute water to the ditch to help pay for the losses by seepage and evap- 

 oration below their gates. If the division were brought about exactly 

 as intended, then the stockholders at the lower end of the ditch would 

 have to stand the brunt of the losses. The best way, from a theoreti- 

 cal standpoint, is to determine what the losses in transmission actually 

 are and take them into consideration in determining the position of 

 any particular division wall. Figure 6 shows a wooden division box 

 designed to apportion out water quite accurately provided the water 

 has but a very low velocity toward the box. 



The box as designed by the writer and shown in figure 6 will deliver 

 water to 11 shares of stock out of a total of 36 shares. That is, the 

 net opening leading to the delivery box bears the same ratio to the 

 total width of opening as 11 bears to 36. Assuming the division wall 

 to be of 2-inch lumber, then the total width of opening is 70 inches. 

 On this basis the position of the division wall is found by the follow- 

 ing proportion : 



70 36 



where x is the desired width of the opening leading to the delivery 

 box, expressed in inches. Therefore 



70X11 770 

 X= -36— ° r 36 



