36 BULLETIN 355; U. S. DEPAETMENT 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 multiphed by its rate of flow. This rate of flow in a tile line 

 will vary with the fall, the length of hne, 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-iiich tile 

 does not cost twice as much as 3-inch tile. Frequently 4-uich tile 

 can be bought as cheap, or nearly so, as 3-inch, although their water- 

 carrying capacity is nearly double that of the 3-uich. 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 EUiott (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 coUege 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 firet stake should 

 be numbered zero (0). (Ref. No. 8, pp. 58-63.) By determining 

 the difference in height between each succeeding pan* of stakes the 

 Hne of elevation of the surface of the ground is determined and may 

 be platted on horizontally ruled paper. When this Hne 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 f aU 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 wiU be the difference in 



