to pass
a
depression
of the
surface
at No.
9.
C. 8. 41 ft. 2.83 1.16 14.86 " 11.37 " 3.49 ft
ft.
C. 9. 12 ft. do. .34 ft. 14.52 " 11.06 " 3.46 ft
C.10. 38 ft. do. .99 ft. 13.53 " 8.94 " 4.59 ft
C.11. 41 ft. 2.00 .82 ft. 12.61 " 8.52 " 4.19 ft
C.12. 41 ft. 1.09 .44 ft. 12.27 " 7.86 " 4.41 ft
C.13. 41 ft. do. .44 ft. 11.83 " 7.70 " 4.13 ft
C.14. 41 ft. do. .44 ft. 11.39 " 7.39 " 4.00 ft
C.15. 41 ft. do. .44 ft. 10.95 " 7.06 " 3.89 ft
C.16. 41 ft. do. .44 ft. 10.51 " 6.73 " 3.88 ft
C.17. 41 ft. do. .44 ft. 10.07 " 5.90 " 4.17 ft

NOTE.--The method of making the foregoing computation is this:

1st. Enter the lettering of the stakes in the first column,
commencing at the lower end of the drain.

2d. Enter the distances between each two stakes in the second
column, placing the measurement on the line with the number of the
_upper_ stake of the two.

3d. In the next to the last column enter, on the line with each
stake, its depth below the datum-line, as recorded in the field
book of levels, (See page 105.)

4th. On the first line of the last column, place the depth of the
lower end of the drain, (this is established by the grade of the
main or other outlet at which it discharges.)

5th. Add this depth to the first number of the line next preceding
it, and enter the sum obtained on the first line of the fifth
column, as the depth of the _drain_ below the datum-line.

6th. Having reference to the grade of the surface, (as shown by
the figures in the sixth column,) as well as to any necessity for
placing the drain at certain depths at certain places, enter the
desire