DUTY AND MEASUREMENT OF WATER. 119 



bank. Cut a notcfli in the board sufficiently deep to 

 all the water, and in length about two-thirds the 

 width of the stream. The bottom of the notch (B) in 

 the board, also the ends of the notch, should be beveled 

 on the down-stream side, and within one-eighth of tin 

 inch of the upper side of the board, leaving the edge 

 almost sharp. A stake (E) should be driven in the bot- 

 tom of the stream, several feet above the board, on a 

 level with the notch (B) this level being easily found 

 when, the water is beginning to spill over the board. 

 After the water has come to a stand and reached its 

 greatest depth, a careful measurement can be made of 

 the depth of water over the top of stake (E), in the 

 manner illustrated. Such measurement gives the true 

 depth of water passing over the notch, because if meas- 

 ured directly on the notch the curvature of water in 

 passing would reduce the depth. The line D is a level 

 from the bottom of the notch (B) to the top of the stake 

 (E), while the dotted line C represents the top of the 

 water, and the distance between the lines from the top of 

 stake gives the true depth or spill over the weir-board. 

 The lines in the sketch have the appearance of running 

 over the top of the board, when in fact they pass behind 

 it, but for the purpose of illustration the reader is sup- 

 posed to look through the board and the post. The sur- 

 face of the water below the board should not be nearer 

 the notch (B} than ten inches, that the flow will not be 

 impeded. Neither should the nature of the channel 

 above the board be such as to force or hurry the 

 water to the board, but should be of ample width 

 and depth to allow the water to approach the board 

 quietly. If the water passes the channel rapidly it 

 will be forced over the weir and a larger quantity will 

 pass than if allowed to spill from a large body moving 

 slowly 



