THE FLOW OF WATER IX DRAII^^TILE. 3 
reported, however, deals only with the discharge or carrying capacity 
of tile drains as conduits. No tests were made on sizes smaller than 
4 inches in inside diameter, as the use of smaller sizes now is con- 
sidered generally inadyisable, the small bore greatly increasing the 
danger of obstruction by sediment or by displacement. 
Laboratory methods are essential for securing definite results 
in such an investigatiouj in order that each factor influencing the 
flow may be varied through a considerable range, yet always subject 
to control, while the other factors are maintained constant. Only 
in this manner can each influence be measured separately. The 
factors influencing the velocity of flow in a tile drain are: the inside 
diameter of the pipe, the depth of the water flowing, the slope or 
grade of the water surface (which ordinarily is that of the tile line) , 
and the roughness and irregularity of the interior surface and of 
the joints. On tile lines installed for actual use in land drainage 
the grade of each line is fixed; most of the time they are empty or 
carry but little water; the amount of flow depends upon weather 
and seasons and can not be regulated for investigation; and when 
the flow is considerable the weather is likely to be bad, the roads 
practically impassable, and the ground surface covered with water — 
conditions that make it impossible to secure satisfactorily precise 
measurements in tile several feet under ground. 
The principal feature of the equipment for making the experiments 
was a wooden flume about 570 feet long, in which the tile were laid 
in earth exactly as drains are installed in the ground. The flume 
was adjustable to any grade up to 1.50 per cent (s= 0.015), without 
disturbing the tile. The depths of flow were observed by piezometer 
tubes hung on the side of the flume. Care was taken to make the 
tile lines truly representative of drains ordinarily well laid under field 
conditions. 
Experiments were made with all the usual commercial sizes of tile, 
both of clay and of concrete, from 4 to 12 inches inside diameter. 
Nine grades were used, from 0.05 to 1.50 per cent, for each size and 
kind of tile. For each size, kind, and grade it was desired to test 
depths of flow of one-fourth, three-eighths, and one-half the internal 
diameter of the tile, and other depths ranging from half fuU to full 
by successive increases of 5 per cent of the diameter. However, 
because of the practical difficult}^ of securing exactly any given depth 
of flow, the number of tests was considerably less than anticipated 
in the smaller sizes of tile. Also, the capacity of the pumping plant 
was not sufficient to fill the largest tile at the maximum grade. 
Tests were run, also, with the tile under slight internal pressure. 
In all, 824 separate tests were made, and from these a new formula 
has been devised for computing the flow in drain tile. • 
