194 FARM DEVELOPMENT 



change. Then, by subtracting the depth of the ditch at 

 each point of change of grade from the elevation of the 

 surface of the ground above datum, the elevation, above 

 datum, of the bottom for the ditch at these points can be 

 placed in the column for Elevation of Drain. The dif- 

 ference in the elevation of the drain above datum be- 

 tween two points divided by the number of stations 100 

 feet apart gives the grade per 100 feet. By beginning 

 at the lower point and adding this amount to the height 

 of each station, the height of the next station is deter- 

 mined. The depth of drain at each station is then 

 determined by subtracting the height of the drain above 

 datum from the height of the surface of the ground 

 above datum at that station. These records can be copied 

 from the notebook upon stakes and thus show the depth 

 to dig the ditch at the respective stations. 



Deciding upon amount of slope or grade. In deciding 

 the most practical grade, or slant, of the line of the drain, 

 and the size of the ditch or the diameter of the drain tile, 

 rules are of some use, but very often these matters can 

 best be determined by one who has had practical ex- 

 perience under similar conditions. In rare cases, a steep 

 grade causes an open ditch to be gullied out by the 

 swift running water, and a more level grade with 

 an occasional waterfall buttressed with stones would 

 be better, but generally on flat lands the effort 

 should be to secure a rapid fall in the grade. In 

 some cases, for example, in the Valley of the Red 

 River of the North, in Holland, and in other very flat 

 countries, large open drains must, of necessity, be made 

 nearly level. It is astonishing how much water will be 

 removed by an open drain that has only a few feet of 

 fall per mile, or a tile drain that has only 2 to 4 inches fall 

 per 100 feet. Width of the ditch, or diameter of drain 

 tile must largely make up for lack of fall, since water 

 that has so little speed must be carried in a larger drain 



