to this line. The instrument, fixed on a tripod, and so adjusted
that it will turn to any point of the compass without disturbing the
position of the bubble, will, (as will its "line of sight,") revolve in a
perfectly horizontal plane. It is so placed as to command a view of a
considerable stretch of the field, and its height above the imaginary
plane is measured, an attendant places next to one of the stakes a
levelling rod, (Fig. 7,) which is divided into feet and fractions of a
foot, and is furnished with a movable target, so painted that its center
point may be plainly seen. The attendant raises and lowers the target,
until it comes exactly in the line of sight; its height on the rod denotes
the height of the instrument above the level of the ground at that stake,
and, as the height of the instrument above the imaginary plane has been
reached, by subtracting one elevation from the other, the operator
determines the height of the ground at that stake above the imaginary
plane,--which is called the "_datum line_."

[Illustration: Fig. 4 - MAP OF LAND, WITH SWAMPS, ROCKS, SPRINGS AND
TREES. INTENDED TO REPRESENT A FIELD OF TEN ACRES BEFORE DRAINING.]

Fig. 4 - MAP OF LAND, WITH SWAMPS, ROCKS, SPRINGS AND TREES. INTENDED TO
REPRESENT A FIELD OF TEN ACRES BEFORE DRAINING.

[Illustration: Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.]

Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.

[Illustration: Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.]

Fig. 6 - LEVELLING INSTRUMENT.(4)

[Illustration: Fig. 7 - LEVELLING ROD.]

Fig. 7 - LEVELLING ROD.

The next operation is to trace, on the plan, lines following the same
level, wherever the land is of the proper height for its surface to meet
them. For the purpose of illustrating this operation, lines at intervals
of elevation of one foot are traced on the plan in Fig. 8. And these lines
show, with sufficient accuracy for practical purposes, the elevation and
rate of inclination of all parts of the field,--where it is level or nearly
so, where its rise is rapid, and where slight. As the land rises one foot
from the position of one line to the position of the line next above it,
where the distance from one line to the next is great, the land is more
nearly level, and when it is short the inclination is steeper. For
instance, in the southwest corn