DUTY AND DIVISION OF WATER 351 



special methods for measuring water. Ordinary gates, 

 placed at the heads of the laterals, were raised or 

 lowered by the water-master to send volumes of water, 

 equal to the eye, down the laterals to farmers owning 

 practically the same water-rights. Later, with the increas- 

 ing value of water, better measuring devices have been 

 adopted. 



The simplest devices for measuring the quantity of 

 water flowing in a channel are those known as weirs or 

 overfalls. A board is placed as a check across the stream. 

 Into the board is cut a notch through which the water 

 flows. The weir method of measuring water has been 

 investigated long and carefully, with the result that in 

 some form it may now be used safely and easily by the 

 farmer. The chief objection to the use of weirs in irriga- 

 tion is the tendency of the wen* to become filled with silt if 

 the water carries sediment. When this occurs, the meas- 

 urements are less reliable and the weirs must, therefore, 

 be cleaned frequently. 





Three kinds of weirs are used successfully for water 

 measurement. First, the rectangular weir, which is best 

 known and most certain, for it has received most study 

 by engineers. It is not, however, the simplest, because 

 the water flowing over the weir contracts at the bottom 

 and sides, and this contraction varies as the depth of the 



