DUTY AND MKASURKMENT OF WATER. 



157 



the stream of any required length. A float observa- 

 tion gives only the velocity of a given small volume of 

 water which surrounds the float, and as difierent por- 

 tions of the small filament have very different longi- 

 tudinal velocities, it requires a great many float obser- 

 vations to give as 

 valuable information 

 as may be obtained 

 by running a cur- 

 rent meter in the 

 same filament for one 

 minute. The cur- 

 rent meter method is 

 the most accurate for 

 obtaining subsurface 

 velocities ever de- 

 vised. The river cur- 

 rent meter used on 

 the geological sur- 

 veys in the west by 

 the United States 



government surveyors is the invention of J. S. J. 

 lyallie, who manufadtures them in Denver, and is 

 shown in Fig. 43. 



In order to ascertain 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 bot- 

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

 these movements the gears are locked, the watch is 



FIG. 43 — THE CURRENT METER. 



