Laying Out Drains 479 



or 2.88 inches per 100 feet, it will be necessary to place 

 the bottom of the tile, at 600 feet from the outlet, 



6 X. 24 1.44 feet 

 higher than the outlet; that is, 



7+1.44 8.44 feet 



above datum plane ; but as the surface of the ground 

 at the 600-foot station is 11.36 feet above this plane, 

 as given in the table, it is clear that the ditch must 

 be dug at this place 



11.36 8.44 2.92 feet 



deep, as written on the finder stake in Fig. 157, and 

 as given in the table of field notes in the column 

 headed "depth of ditch." 



Since the grade line rises .24 foot per 100 feet and 

 .12 foot per 50 feet, the data in the table under 

 "grade line" are obtained by adding .12 foot to 7 

 feet, the distance of the outlet above datum, for the 

 50 -foot station ; twice 12 foot to the second or 

 100 -foot station, etc. 



The numbers in the column of differences are 

 obtained by subtracting the front -sight from the back- 

 sight, taken with each setting of the level, and these 

 differences, added to the height of the lower station, 

 give the elevation of the higher station above datum 



plane, thus: 



4 3.87 .13 feet; 



and this amount, added to the height of the back- 

 sight station, gives 



10 +.13 10. 13 feet 

 as the elevation of the 50 -foot station, and subtract 



