27 



The velocity is obtained as explained above by placing the floats so they 

 will travel as nearly as practicable between the tags or marks. It is diffi- 

 cult to get the floats to travel straight down the stream and often the in- 

 accuracy due to this can not be avoided. When rod floats are used, rods of 

 different lengths are usually necessary. 



For an ordinary trapezoidal section a less accurate but more expedient 

 procedure is to obtain the total area of the cross section of the channel by 

 multiplying the depth by the average width (which is the sum of the top 

 width and bottom width divided by two) and to multiply the area, thus ob- 

 tained, by the average velocity. The average velocity is found as explained 

 above, using either surface floats or rod floats. 



In case the waterway is a flume the width in feet multiplied by the depth 

 of water in feet will give the cross section. 

 Current meter. 



The current meter generally used to measure the flow in canals consists 

 of a small wheel made up of vanes or cups connected to a rod about which 

 it revolves. The wheel when placed in the flowing water is set in motion 

 by the action of the current and the revolutions are transmitted to a record- 

 ing or sounding apparatus. By noting the time and the revolutions, the 

 number of revolutions per second are obtained. The velocities correspond- 

 ing to the number of revolutions are known for each current meter. 



A current meter is expensive and is not likely to be possessed by the 

 ordinary irrigator. For that reason a detail description of its use is 

 not given. 

 Rating station and rating flume. 



A rating station is a section of canal the discharge of which is known 

 for different depths of water. To obtain the relation of depth to discharge 

 it is necessary to rate the station. This is done by the measurement of 

 discharge for a wide range of depths, from which a table is prepared giving 

 the discharge at any depth. The rating' station should be selected where 

 the waterway has a straight reach of nearly uniform cross section above 

 and below and there should be no obstructions to interfere with the flow, 

 such as gates below it. The selected section should have a channel which 

 is not liable to change by scouring or silting. To insure a permanent sec- 

 tion a rating flume, which is a short section of flume, is often used. The 

 depth of water is indicated by a graduated rod placed on one side of the 

 canal or by graduations marked on the side of the flume. 



Automatic registers. 



The volume of water delivered over a weir or the discharge of a ditch 

 flume or creek obtained by measurements at a rating station or rating 

 flume will vary with the fluctuations in the water level. Where it is de- 

 sired to have a continuous record of the quantity of water, automatic 

 registers have been used. These registers are of different types. They 

 can, however, be classified in two classes. They all consist of a clock, a 

 float and a cylinder or drum to which is fastened a sheet of paper on which 

 the depth of water at different times is recorded by a pencil or pen. 



With one class of register the cylinder is placed vertically and is rotated 

 by a clock which gives it one revolution a week. The pencil is connected 

 to the float which is placed in a well or box built on the side of the weir 

 box or rating flume and connected with the water through an orifice. The 

 fluctuations in water level cause a rise and fall of the float and a corre- 

 sponding movement of the pencil which is recorded on the sheet placed on 



