122 



IRRIGATION FARMING. 



the stream of any required length. A float observation 

 gives only the velocity of a given small volume of water 

 which surrounds the float, and as different portions of 

 the small filament have very different longitudinal ve- 

 locities, it requires a great many float observations to 

 give as valuable information as may be obtained by run- 

 ning a current meter in the same filament for one minute. 

 The current meter method is the most accurate of ob- 

 taining sub-surface velocities ever yet devised. The 



river current meter used 

 on the geological sur- 

 veys in the West by the 

 United States govern- 

 ment surveyors is the 

 invention of J. S. J. 

 Lallie, who manufac- 

 tures them in Denver, 

 and is shown in Fig- 

 ure 42. 



In order to ascer- 

 tain the velocity of a 

 stream or ditch, lock 

 the gears in the meter 

 and note reading at 

 the pointers, which will be the first reading. Place the 

 meter in the stream or ditch and at the same instant the 

 gears are unlocked start a stop-watch. Then the meter 

 should be slowly moved from the top to the bottom of 

 the stream at least three times. At the end of these 

 movements the gears are locked, the watch stopped and 

 the second reading is made, and these together with the 

 time noted down. The difference between the first and 

 second ivadin^ i.s divided by the time which gives the 

 revolutions per second. The revolutions per second mul- 

 tiplied by the ratio will give the velocity of the stream 

 in feet per second. In the computations the following for- 



