Determining Grade. 



319 



adding .12 foot, to 7 feet, which is the height of the outlet, 

 for that of the second station; then .12 feet added to this 

 gives the third station and so on, thus: 



7, 7.12, 7.24, 7.36, 7.48, 7.60, 7.72, 7.84, 7.96, 8.08, 

 8.20, 8.32, 8.44. 



.** SOO 850 *<W 450 000 

 50 100 150 



FJQ. 137. Profile of ditch staked ready for digging, with depths for the 

 ditch at the several stations. 



If these numbers are subtracted from the hights of the 

 surface of the ground at the respective places the differ- 

 ence will be the depth the ditch must be dug at those 

 places, and the figures which are placed upon the finders 

 for the instruction of the men in digging. These figures 

 are given in the table in the column "depth of ditch." 



The experienced drainage engineer with accurate tele- 

 scope level makes the details of leveling, establishing the 

 grade and marking the grade pegs simpler than here given 

 but it is not safe for a farmer with a cheap level to follow 

 his methods. 



392. Changing from One Grade to Another It may hap- 

 pen in kying out the ditch that it is impracticable to fol- 

 low a single grade on account of having to dig too deep in 

 some places or of leaving the tile too close to the surface 

 in others. Suppose in the last profile (391) the ditch was to 

 be 500 feet longer and that in this 500 feet there had been 



