THE FLOW OF WATER IN DEAIN 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 syi'inge proved a 
satisfactory method. The voltmeter was of the portable Weston 
type (model 45) with a range of scale from zero to 1^ volts; carbon 
and zinc electrodes were used. To complete the ckcuit, 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 
-Comparison of mean velocities as determined hy various methods. 
Tile 
diameter. 
Velocity by 
weir 
Velocity by 
color 
(Fc). 
Velocity by 
voltmeter 
T'- Vc 
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 
2.789 
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 
-1.1 
-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 
-10.2 
-6.2 
16659'; 
-20— Bull. 854- 
