36 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
should be kept in mind. The quantity of water carried by any pipe 
or carrier of drainage is equal to the area of the cross section of the 
stream multiplied by its rate of flow. This rate of flow in a tile line 
will vary with the fall, the length of line, the size of tile, and the degree 
of smoothness of the inside of the tile. It is well to keep in mind 
that the cross sectional area of a tile varies directly as the square of 
its diameter. This means that, other things being equal, a 6-inch 
tile has about four times the water-carrying capacity of a 3-inch tile. 
It should also be kept in mind in this connection that the cost of tile 
does not increase in proportion to size. That is to say, 6-inch tile 
does not cost twice as much as 3-inch tile. Frequently 4-inch tile 
can be bought as cheap, or nearly so, as 3-inch, although their water- 
carrying capacity is nearly double that of the 3-inch. Again, the 
cost of digging the ditch and laying the tile, which is commonly of 
greatest consideration, is practically independent of the size of the 
tile to be laid. It is always best to be on the safe side with regard 
to the size of tile purchased for any drainage system. An estimate 
of the size of tile necessary for fields of different dimensions is given 
by Elliott (Ref. No. 8, p. 84). 
Where the size of tile, or anything else in connection with drainage, 
is difficult to determine, it is advisable to consult the State agricul- 
tural college or a drainage engineer. 
Laying out the drainage system. — After the lines along which tile are 
to be laid have been staked out by the use of laths or other stakes 2 
to 3 feet in length, placed 50 feet apart, short stakes, called grade 
stakes, should be driven even with the surface of the ground near 
the lath. (Ref. No. 8, pp. 48, 63-65.) A line of levels should 
then be run along the grade stakes, beginning with the lower end at 
the level of the outlet. In recording the levels this first stake should 
be numbered zero (0). (Ref. No. 8, pp. 58-63.) By determining 
the difference in height between each succeeding pair of stakes the 
line of elevation of the surface of the ground is determined 'and may 
be platted on horizontally ruled paper. When this line of elevation 
has been drawn the fall available can be determined. 
The next item to be found is the grade which can be used. (Ref. 
No. 8, pp. 68-74.) To do this, subtract the distance which the tile 
must be placed below the surface at the upper end, or at the point 
where the tile comes nearest to the surface, from the total height 
of the surface at that point above the outlet. This gives the fall 
which may be used by the tile through this distance. Ordinarily the 
gradient should be uniform, but where laterals join larger mains it 
is possible to use smaller gradients on the mains than are used on the 
laterals. Having determined the total fall in inches or hundredths of 
feet, divide this by the number of hundred feet in the length of tile 
to get the fall per hundred feet. Half of this will be the difference in 
