DRAINAGE 193 



be erased and other trials made, until the depth of the 

 proposed ditch at the various points is so arranged as 

 to make an effective system of drains without unneces- 

 sary expense. 



In case of branch drains, it is sometimes necessary to 

 make profiles of the several branches before finally 

 deciding upon the. grade and depth of the main drain, 

 showing their relations to the main drain, in order that 

 the entire plan of the drainage system may be so mapped 

 out as to make all parts fit together in a manner that 

 will insure practical slants, or grades, for carrying the 

 water from all portions of the land, and yet will not 

 necessitate deeper and more expensive drains than 

 necessary. 



The profile in Figure 83 shows the surface line of the 

 ground at each station on the line of a proposed ditch, as 

 determined by the use of the level, the leveling rod 

 and the calculations, Figure 81. This profile is 

 so made that the vertical lines represent the stations 

 100 feet apart, O representing the outlet. Each 

 horizontal line represents a foot in height. The hori- 

 zontal line A-B represents a line 100 feet above datum. 

 It may be seen from this profile that between Stations 

 O-5, 5-11 and 11-20 the profile of the ground 

 surface presents three different grades. While it 

 would be possible to give the ditch a uniform 

 grade between Stations O and 20, as indicated by the 

 dotted line, it would be impracticable, as it would bring 

 the ditch too near the surface between Stations 1-6 and 

 too deep between Stations 1016. To avoid this the 

 ditch is run on three separate grades, which give it a 

 nearly uniform depth and still allow sufficient slope to 

 carry off the water. 



By deciding upon the depth of the drain at each point 

 chosen for a change of grade, the amount of fall per 100 

 feet can be determined between each of two points of 



