will be deposited, (see page 91), and the drain,
thereby, obstructed. To provide against this, a silt-basin must be placed
at this point which will collect the silt and prevent its entrance into
the more nearly level tile below. The construction of this silt-basin is
more particularly described in the next chapter. From stake 7 to the main
silt-basin the fall is such that the drain will clear itself.

The drawing of regular profiles, for the more important drains, will be
useful for the purpose of making the beginner familiar with the method of
grading, and with the principles on which the grade and depth are
computed; and sometimes, in passing over very irregular surfaces, this
method will enable even a skilled drainer to hit upon the best adjustment
in less time than by computation. Ordinarily, however, the form of
computation given in the following table, which refers to the same drain,
(_C_,) will be more expeditious, and its results are mathematically more
correct.(16)

Fall. Depth
Feet and from
Decimals. Datum
Line.
No. of Distance Per 100 Between To To Depth of Remarks.
Stake. Between Feet. Stakes. Drain. Surface. Drain.
Stakes.
Silt 20.70 18.20 2.50 ft
Basin. ft. ft.
C. 1. 82 ft. 2 ft. 1.64 ft. 19.06 " 15.44 " 3.48 ft
C. 2. 39 ft. do. .78 ft. 18.28 " 14.36 " 3.83 ft
C. 3. 65 ft. do. 1.30 16.98 " 12.85 " 4.13 ft
ft.
C. 4. 51 ft. .56 .28 ft. 16.70 " 12.18 " 4.52 ft
C. 5. 43 ft. do. .24 ft. 16.46 " 11.79 " 4.67 ft
C. 6. 47 ft. do. .26 ft. 16.20 " 11.69 " 4.51 ft
C. 7. 32 ft. do. .18 ft. 16.02 " 11.55 " 4.47 ft Silt-Basin
here.
Made
deep at
Nos. 7
and 10