9 and 3.3, the 2-foot
and 3-foot contour lines must cross. The total difference of elevation,
between the two points is 3.3--1.9=1.4; 10/14 of the space must be given to
the even foot between the lines, and the 2-foot line should be 1/14 of the
space above the point 1.9;--the 3-foot line will then come 3/14 below the
point 3.3. In the same manner, the line from 3.3 to 5.1 is divided into 18
parts, of which 10 go to the space between the 4. and 5. lines, 7 are
between 3.3 and the 4-foot line, and 1 between the 5-foot line and 5.1.
[Illustration: Fig. 8 - MAP WITH CONTOUR LINES.]
Fig. 8 - MAP WITH CONTOUR LINES.
With these maps, made from observations taken in the field, we are
prepared to lay down, on paper, our system of drainage, and to mature a
plan which shall do the necessary work with the least expenditure of labor
and material. The more thoroughly this plan is considered, the more
economical and effective will be the work. Having already obtained the
needed information, and having it all before us, we can determine exactly
the location and size of each drain, and arrange, before hand, for a rapid
and satisfactory execution of the work. The only thing that may interfere
with the perfect application of the plan, is the presence of masses of
underground rock, within the depth to which the drains are to be laid.(5)
Where these are supposed to exist, soundings should be made, by driving a
3/4-inch pointed iron rod to the rock, or to a depth of _five_ feet where
the rock falls away.
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