er of the plan, the land is nearly level to
the 2-foot line; it rises slowly to the center of the field, and to the
eastern side about one-fourth of the distance from the southern boundary,
while an elevation coming down between these two valleys, and others
skirting the west side of the former one and the southern side of the
latter, are indicated by the greater nearness of the lines. The points at
which the contour lines cross the section lines are found in the following
manner: On the second line from the west side of the field we find the
elevations of the 4th, 5th and 6th stakes from the southern boundary to be
1.9, 3.3, and 5.1. The contour lines, representing points of elevation of
2, 3, 4, and 5 feet above the _datum line_, will cross the 50-foot lines
at their intersections, only where these intersections are marked in even
feet. When they are marked with fractions of a foot, the lines must be
made to cross at points between two intersections,--nearer to one or the
other, according to their elevations,--thus between 1.9 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, soundin