DRAINAGE 189 



stake is 7.84 feet below the point where the disk has 

 been placed level with the instrument, which is 110.63 

 feet above datum; Station 6 is then 110.63 feet, minus 

 7.84 feet, or 102.79 feet above datum. In like manner, 

 the height of Stations 7, 8, etc., are successively 

 measured ; or when the earth at the stations is higher 

 than the level of the instrument, the operator must again 

 move the instrument, and, sighting back (getting the 

 " back sight ") to the point last measured, again find 

 the new height of his instrument. In this manner we 

 proceed to Station 20; that is, 2,000 feet from the 

 outlet. 



That we may now illustrate the method of getting the 

 new height of the instrument when going down grade, 

 and, where necessary for great accuracy of checking up 

 the work already done, we will begin at the upper end and 

 check the levels at the successive stations back to the 

 proposed outlet. By referring to Figure 81 it will be 

 seen that the elevation of the surface at Station 20 is 

 110.17 feet; and that the instrument standing in its 

 last position is 3.13 feet above the hub at this point, or 

 is 113.30 feet above datum. Now, turning again on 

 Station 19, the rodman reads 3.57 feet, and subtracting 

 this from the height of the instrument above datum, we 

 have as the height of Station 19, 109.73 ^ eet - Proceed- 

 ing to Station 18, the rodman reads 3,90 feet; at 17, 

 4.05 feet; at 16, 4.70 feet; at 15, 5 feet; at 14, 5.30 feet. 

 On account of the distance, it is now deemed advisable 

 to get a new height of instrument, and the instrument 

 is moved down to the neighborhood of Station 9. When 

 the instrument is properly leveled up and turned on Sta- 

 tion 14, the rodman finds his disk in line with the in- 

 strument at 2.47 feet. Since the height of the surface 

 at Station 14 is 108 feet and the instrument is found 

 to be 2.40 feet higher than at this point, the new height 

 of the instrument is calculated to be 110.40 feet. 



