THE FLOW OF WATER IN DRAIN TILE. 15 



ing the height of the water surface outside of the tile at the joint. 

 In some cases the water level outside the tile would remain steadily 

 0.1 or 0.2 foot higher on one side of the tile than on the other, and the 

 surface inside the tile would be very turbulent and would seem to 

 bear no relation to the elevation of the water surface outside the tile 

 joint. Such phenomena were most conspicuous when the depth of 

 flow was between half and full depth, and with the high velocities due 

 to the steeper slopes. The phenomena seemed to depend upon the 

 presence of air in the tile, as they disappeared largely when the tile 

 were completely filled, so that all air was excluded. 



METHODS OF CONDUCTING TESTS. 



A test was always begun at the least depth of flow. Six men were 

 needed to conduct a complete experiment at one grade, which required 

 from 3 to 6 hours, depending upon the number of depths of flow 

 tested. One man cared for the pump and engine, one read the upper 

 hook gage, a third was stationed midway the length of the flume at 

 a piezometer tube, another was stationed at the outlet to adjust the 

 height of the movable dam, and a fifth man read the lower hook gage. 

 The engineer in charge usually operated the valve controlling the 

 supply of water to the upper weir tank, and watched the upper 

 piezometer tubes. 



The engineer announced the depth of flow he desired to obtain to 

 the man stationed at the dam. The gate valve in the supply pipe 

 was partly opened and the piezometer readings noted. The dam was 

 then raised or lowered to secure the correct depth of flow at the 

 piezometer tube near the outlet, special care being taken not to get a 

 greater depth than desired there. The observer at the upper hook 

 gage called out the various gage heights at short intervals, that the 

 water supply might be regulated properly, and when the desired depth 

 in the tile was obtained, sufficient time was allowed to determine that 

 the depth over the weir was constant. The observations at the 

 upper, middle, and lower piezometers indicated when the flow was 

 steady throughout the tile line. When the flow was steady at the 

 proper depth, the signal was given and each of the tWo hook-gage 

 readers made record of the readings at his station every 30 seconds. 

 Meanwhile, the engineer in charge passed along the flume, recording 

 the readings of all piezometers in succession; the observer at the 

 lower end of the flume went to the upper end and then recorded the 

 piezometer readings in order, following just 2 minutes behind the 

 engineer's readings; the observer at the middle of the flume watched 

 the piezometer there to report if any considerable fluctuation indi- 

 cated that the test should be run again. If the depth over the upper 

 weir remained constant throughout the test, the engineer proceeded 

 to obtain the next depth of flow; if the weir readings varied, the test 



