USE OF CONCRETE PIPE IN IRRIGATION. 37 



When weirs are formed by the use of concrete pipe, the usual prac- 

 tice is to employ pipe too small in diameter in proportion to the length 

 of the weir, thus departing at times very far from the standard weir, 

 and introducing errors which may amount in certain cases to 5 per 

 cent and more, a combination of this kind being shown in figures 13 

 and 14. If pipes are to be used for this purpose they should be of 

 large diameter and of thin shells so as to permit the largest pos- 

 sible distance between the end of the weir notch and the inner sur- 

 face of the pipe. According to standard dimensions and accurate 

 measurement a weir 1 foot long would require a pipe 65 inches in 

 diameter whereas if a 2-foot pipe is used for the same length of 

 weir the discharge may be too large by 5 per cent. 



In order that the water may approach the weir at a slow velocity 

 and be freed from cross currents and eddies, baffle boards are fre- 

 quently placed in the receiving chamber and the inlet pipe may be 

 placed in a vertical position and at right angles to the feed pipe. 



Another frequent cause of overregistering in the use of weirs is 

 the increase in head over the weir above the normal. This may be 

 partially overcome by having all excess water flow over spillways, 

 the longer the spillway the greater approach to normal conditions 

 being obtained. Another method is to keep a continuous record of 

 the flow through the weir notch by means of an automatic register. 

 In still other cases the head is kept constant by means of a gate 

 regulated by a float. Apart from the long spillway perhaps the 

 most effective way of maintaining a constant head over a weir is 

 shown in figure 15, which is a combination of the spillway and 

 valves. By this arrangement valve " b " in the main feed pipe when 

 closed causes the water to rise in one chamber of the box and flow 

 over spillway " a " into the other chamber, while valve " c " is regu- 

 lated to permit the desired amount of water to flow through the weir 

 " d " into the lateral pipe. It is thus apparent that any slight in- 

 crease in the depth of water in the spillway " a " will not materially 

 affect the measurement at " d." 



Miner's Inch Boxes. 



A device of this kind measures the quantity of water which flows 

 through a rectangular orifice under a constant head (fig. 16). The 

 dimensions of the orifice most commonly used in southern Califor- 

 nia are 4 inches deep and from 2 to 100 inches in length with a head 

 of 4 inches of water over the center of the orifice. The quantity of 

 water flowing through such an orifice is estimated to be 9 gallons 

 per minute for each square inch of orifice or the one-fiftieth part of 

 a cubic foot per second. The Azusa miner's inch box or hydrant 

 shown in figure 17 is fitted with a cast-iron plate which contains four 

 orifices all 4 inches deep and 2|, 3f , 6^, and 124 inches long, respec- 



