352 IRRIGATION PRACTICE 



water or the length of the weir increases. The necessary 

 corrections for this variation complicates the use of the 

 rectangular weir. Many irrigation engineers, however, 

 well acquainted with irrigation, insist that for practical 

 purposes no weir has taken the place of the rectangular 

 weir. Second, the trapezoidal weir, which has largely 

 replaced the rectangular weir in irrigation. The trape- 

 zoidal is like the rectangular weir, except that the sides of 

 the notch slope away slightly, making a trapezoidal, 

 instead of a rectangular, opening through which the water 

 flows. The sloping sides are intended to correct auto- 

 matically the loss of water due to the contraction. Over a 

 trapezoidal weir 2 feet long, all else being equal, twice as 

 much water is supposed to flow as over one 1 foot long. 

 This is not true of the rectangular weir. The first trape- 

 zoidal weir with this purpose in view was devised by the 

 Italian engineer Cippoletti, and the weir has been named 

 the Cippoletti weir by L. G. Carpenter, of Colorado, 

 who was first to call the attention of the American public 

 to this form of weir. In America, Canada, Australia and 

 other countries it is practically the only weir used by the 

 irrigator. Third, the triangular weir, the notch of which is 

 in the form of a triangle. Its chief advantage is that only 

 the depth of water flowing over the triangle needs to be 

 measured. Very satisfactory results are obtained by the 

 use of the triangular weir, and it is likely to increase in 

 favor. With our present knowledge, triangular weirs 

 seem suitable chiefly for small streams. 



Recently, Lyman has given careful and exhaustive 

 study to this subject, with the result that he has devised 

 methods whereby water may be correctly and easily 

 measured in flowing streams, without the use of weirs. 

 (Figs. 96, 97.) 



