THE FLOW OF WATER IN DRAIN TILE. 



17 



corded the same depth as that shown by the piezometer 60 feet from 

 the tile entrance, without affecting the latter piezometer but caus- 

 ing the intermediate piezometers to register the same depth. 



MEASUREMENT OF MEAN VELOCITY. 



As stated before, the mean velocities obtained during the experi- 

 ments were determined by dividing the quantity of water passing 

 over the upper weir by the average cross-sectional area of the water 

 in the tile. For checking these results, velocities were determined 

 also by coloring matter and by the use of a voltmeter. Both potas- 

 sium permanganate and dyes were used. For injecting the colored 

 solution into the tile, the use of a large hard-rubber syringe proved a 

 satisfactory method. The voltmeter wt«s of the portable Weston 

 type (model 45) with a range of scale from zero to 1J volts; carbon 

 and zinc electrodes were used. To complete the circuit, a saturated 

 salt solution was inserted by the method employed in the color 

 tests. When the water saturated with salt passed the point where 

 the electrodes were placed in the tile, a current was set up whose 

 intensity was indicated by the voltmeter. When the volume of 

 saturated water had all passed, the needle of the voltmeter would 

 return to its original position. 



An observer noted the time the color was injected and also ob- 

 tained the times when the first and the last color passed the outlet. 

 The time the color spent in the tile was taken as from the instant of 

 injection to the mean between the first and the last of the appearance 

 at the outlet. This same method was used in the voltmeter tests, 

 the time being taken as from the instant of injection of the salt 

 solution to the mean between the time the needle of the voltmeter 

 began to register and the time when it returned to its original posi- 

 tion. Table 2 shows part of the results obtained in comparing the 

 values of the mean velocity as found by the weir, with those deter- 

 mined by color and voltmeter. 



Table 2. — Comparison of mean velocities as determined by various methods. 



Tile 



diameter. 



Velocity by 

 weir 

 (V). 



Velocity by 

 color 



Velocity by 



voltmeter 



(P.). 



V-Vc 

 V 



V- V v 

 V 



Inches. 



4 



4 



4 

 12 

 12 

 12 

 12 

 12 

 12 

 12 

 12 



Ft. per. sec. 

 2.168 

 2.133 

 1.866 

 1.236 

 1.124 

 4.551 

 3.711 

 2.251 

 2.493 

 2.769 

 2.104 



Ft. per sec. 

 2.227 

 2.167 

 1.898 

 1.293 

 1.211 

 4.782 

 3.691 

 2.320 

 2.656 

 2.723 

 2.272 



Ft. per sec. 



Per cent. 

 -2.7 

 -1.6 

 -1.7 

 -4.6 

 -7.7 

 -5.1 

 +0.5 

 -3.1 

 -6.5 

 +1.7 

 -8.0 



Per cent. 

























2.291 

 2. 500 

 2.764 

 2.235 



-1.8 

 -0.3 

 J 0.2 

 -6.2 



166597°— 20— Bull. 854- 



