Experiment station bulletins. 473 



four is sixteen. Sixteen is contained in twenty-five a little more than 

 one and one-lialf times and this represents I lie relative capacities of these 

 two sizes of tile for carrying water. 



The size of tile to be used in any instance will depend upon the area 

 from which it is to carry water and upon whether it is to carry away 

 only the excess of water due to rainfall on this area, or whether there 

 is added other water brought in by springs or surface drainage or seep- 

 age from adjacent areas. 



It is hardly advisable, all things considered, to use tile as small as 

 two inches in diameter. The following general statements are quoted 

 from 0. G. Elliott, than whom there is no more practical drainage en- 

 gineer in this country. These statements apply to average conditions; 



''When drains are laid so that there shall be a fall of three inches per 

 hundred feet, a three-inch, tile will drain five acres and should not be 

 of greater length than one thousand feet. 



A four-inch tile will drain twelve acres. 



A five-inch tile will drain twenty acres. 



A six-inch tile will drain forty acres. 



A seven-inch tile will drain sixty acres. 



A long drain has a less carrying capacity than a short drain of the 

 same size laid upon the same grade.'' 



It is seen that if a long drain is to be laid, and especially if this 

 drain be a main receiving water from laterals or other sub-mains, it 

 will be necessary from time to time to increase the size of the tile laid 

 as the drain approaches the outlet. Figure 5 illustrates this point. 



By giving careful attention to the capacity of the various sizes of 

 tile it is possible to exercise considerable economy in the use of the 

 tile laid in any system. , 



GRADE OR PALL. 



The business of any tile system is to furnish a means by which the 

 excess of water in the soil can find the most ready means of exit. 



Every system should be so laid that there is a gradual fall from the 

 extreme end of the drain to the outlet. This fall is usually spoken of as 

 the grade. 



It is desirable where possible to have a fall of as much as three inches 

 for every hundred feet of tile. A carefully constructed system will work 

 successfully on a much less fall than this. Two inches is a very com- 

 mon grade and in very flat areas a fall as slight as one inch per hundred 

 feet is used, and occasionally a fall of one-half inch is used. 



The less the fall the greater must be the care exercised in laying the 

 tile. 



The less the fall the less will be the capacity of the tile to remove the 

 water, and therefore the larger must be the tile used. 



Elliott says: "If we double the grade per hundred feet of the drain 

 we increase its carrying capacity about one-third." If this be true, then 

 if we lower the grade by half we should decrease the carrying capacity 

 by one-fourth. 



It is desirable to make the fall uniform throughout the length of each 

 line of tile. This is not always possible for reasons which win appear 

 later. Where changes in grade must be made it is still desirable to 



