Fig. 21 - PROFILE OF DRAIN C.

Horizontal Scale, 66 ft. to the inch.
Vertical Scale, 15 ft. to the inch.

1 to 17. Numbers of Stakes.
(82) etc. Distances between Stakes.
18.20 etc. Depths from _datum-line_ to surface.
2.50 etc. Depths of ditch.
20.70 etc. Depths from _datum-line_ to drain.

If, from too great depression of the lower parts of the field, or too
great distances for observation, it becomes necessary to take up a new
position with the instrument, the new level should be connected, by
measurement, with the old one, and the new observations should be computed
to the original plane.

It is not necessary that these levels should be noted on the map,--they are
needed only for computing the depth of cutting, and if entered on the map,
might be mistaken for the figures indicating the depth, which it is more
important to have recorded in their proper positions, for convenience of
reference during the work.

*The Depth and Grade of the Drains.*--Having now staked out the lines upon
the land, and ascertained and recorded the elevations at the different
stakes, it becomes necessary to determine at what depth the tile shall be
placed at each point, so as to give the proper fall to each line, and to
bring all of the lines of the system into accord. As the simplest means of
illustrating the principle on which this work should be done, it will be
convenient to go through with the process with reference to the main drain
_C_, of the plan under consideration. A profile of this line is shown in
Fig. 21, where the line is broken at stake No. 7, and continued in the
lower section of the diagram. The topmost line, from "Silt Basin" to "17,"
is the horizontal datum-line. The numbers above the vertical lines
indicate the stakes; the figures in brackets between these, the number of
feet between the stakes; and the heavy figures at the left of the vertical
lines, the recorded measurements of depth from the datum-line to the
surface of the ground, which is indicated by the irregular line next below
the datum-line. The vertical measurements are, of course, very much
exaggerated, to make the profile more marked, but they are in the proper
relation to each other.

The depth at the silt-basin is fixed at 2-1/2 feet (2.50.) The rise is
rapid to stake 3, very slight from there to stake 7, very r