2 BULLETi:S^ 854, U. S. DEPARTMEIS^T OF AGEICULTURE. 
curing an enhanced degree of drainage, would not yield additional 
benefit in proportion to the increase in cost, and might not be justified. 
On the other hand, an inadequate installation might be so ineffective 
as not to justify even the small expenditure it would require. It 
is the engineer's high duty, therefore, in planning the general scheme 
of improvement for any drainage undertaking, to determine just 
what expenditure will yield a satisfactory return, and to so propor- 
tion the details ^s to secure the maximum benefit from the invest- 
ment. A deficiency at one point may reduce the effectiveness of 
the whole system, while the elaboration of one part out of proportion 
to the others might add materially to the expense without obtaining 
any benefit. 
The formulae in general use for computing the velocity of flovv^ 
in tile drains were proposed years ago, when all drain tile were small 
as compared with the larger sizes used to-day. Under the earher 
conditions, when other considerations had relatively large weight 
in determining the size of tile to be used, accuracy in computing 
carrying capacity was relatively unimportant; but nowadays drains 
12 to 48 inches in diameter are common, and accurate knowledge 
of the capacity is essential for economical design. 
Although many experiments have been made upon flow of water 
in iron, steel, concrete, and wood-stave pipes, the results are not 
directly applicable to tile drains. The tile usually are not nearly 
so regular in size and shape as are the other pipes mentioned, and 
specially noteworthy is the number and nature of the joints. While 
the other conduits are either of continuous construction or in 10 to 
20 foot lengths, drain tile are in lengths of only 1 to 3 feet. Fur- 
thermore, with clay tile the nature of the materials used and the 
methods of manufacture are the causes of some distortion in cross 
section; this is particularly noticeable where two lengths abut. The 
considerable unevenness at the joints, when multiplied by the greater 
number of joints, so greatly disturbs the flow of water as to make 
formulae devised for other kinds of conduit inapplicable to tile drains. 
Realizing the need for accurate knowledge regarding the flow of 
water in tile drains, plans for investigating this subject were made by 
the drainage division of the Bureau of Public Roads, early in 1915. 
The experiments so far made concern only the smaller sizes of tile, 
and this report therefore should be considered as a progress report 
of the investigation of the whole subject. 
SCOPE OF THE INVESTIGATION. 
Drain tile installed for agricultural improvement serve two some- 
what distinct purposes — as collectors of excess water and as conduits 
to convey the water to some more or less distant outlet, but usually 
both purposes are served coincidentally. The investigation herein 
