16 BULLETII^ 854, U. S. DEPAETME:N-T OF AGRICIILTUEE. 
for the same depth of flow was run again. About 20 minutes were 
required to obtain the data for each depth of flow, the amount of time 
depending upon the grade of the flume. 
The readings for the first two and the last piezometers were not 
included to obtain the depth of flow in the drain. It should be 
remembered that water as it enters the drain has not the velocity it 
will acquire after traveling some distance; therefore the first two 
piezometers usuaUy recorded a depth slightly different from that of 
the piezometer 60 feet from the tile entrance or those of the succeed- 
ing piezometers. Even with a gradual, conical entrance to the tile 
drain (PI. Ill, figs. 1 and 2), the entrance velocity could not be easily 
regulated to be the same as the uniform velocity through the main 
portion of the tile. The average of the readings of the intermediate 
nine piezometers, less 0.09 foot, usuaUy was taken as the true depth 
of flow, although at times very erratic individual piezometer readings 
were obtained which were not used in obtaining the average. 
Only the upper weir readings were used in the final computations. 
It was found in the earlier experiments that after waiting some time 
for the lower weir box to fill to a steady height, the lower weir would 
read practicaUy the same as the upper weir, proving that there was 
no measurable loss of flow in passing tlirough the tile line. Hence, to 
save time in performing the experiments, it was decided not to wait 
for the lower weir to reach a steady reading. It may appear that^ 
in using only the upper weir readings to obtain the carrying capacity 
of the drain, too great a quantity of water was recorded due to. 
seepage into the earth adjacent to the drain, which would credit the 
tile with carrying more water than it actuaUy did carry. However' 
observation of the condition of the soil indicated clearly that tht 
soil became sufficiently saturated by the time steady flow was 
obtained in the tile, that there was no such loss, at least not in 
quantity that could have affected the results of this investigation. 
The use of the dam at the tile outlet did not affect the carrying 
capacity of the draia, for special care was taken not to allow any 
piezometer readings at the lower end of the flume to exceed the 
readings near the upper end. The dam merely assisted in obtaining 
a uniform depth of flow throughout the length of the drain. Thus 
the necessity of corrections for changing velocity heads due to 
decrease in the water cross-sectional area at succeeding piezometers 
near the outlet was eliminated. Without the dam and with a con- 
stant flow over the upper weir, the successive piezometers showed a 
continuous decrease in depth, and therefore increase ia velocity, 
toward the outlet of the tile line. In other words, the hydraulic 
gradient or water slope was greater than the grade of the tile. With 
no change in the amount of water passing over the weir, the height 
of the dam could be raised until the piezometer near the outlet re- 
