of the
instrument by means of set-screws. The instrument is firmly screwed to the
tripod, and placed at a point convenient for looking over a considerable
part of the highest land. By the use of the set-screws, the plane in which
the instrument revolves is brought to a level, so that in whatever
direction the instrument is pointed, the bubble will be in the center of
the glass. The line of sight, whichever way it is turned, is now in our
imaginary plane. A convenient position for the instrument in the field
under consideration, would be at the point, east of the center, marked
_K_, which is about 3 feet below the level of the highest part of the
ground. The telescope should stand about 5 feet above the surface of the
ground directly under it.

_The Levelling-Rod_, (See Fig. 7,) is usually 12 feet long, is divided
into feet and hundredths of a foot, and has a movable target which may be
placed at any part of its entire length. This is carried by an attendant,
who holds it perpendicularly on the top of the grade-stake, while the
operator, looking through the telescope, directs him to move the target up
and down until its center is exactly in the line of sight. The attendant
then reads the elevation, and the operator records it as the distance
below the _datum-line_ of the top of the grade-stake. For convenience, the
letterings of the stakes should be systematically entered in a small field
book, before the work commences, and this should be accompanied by such a
sketch of the plan as will serve as a guide to the location of the lines
on the ground.

The following is the form of the field book for the main drain _C_, with
the levels recorded:

LETTERING OF THE STAKE. DEPTH FROM DATUM LINE.
Silt Basin 18.20
C 1 15.44
C 2 14.36
C 3 12.85
C 4 12.18
C 5 11.79
C 6 11.69
C 7 11.55
C 8 11.37
C 9 11.06
C 10 8.94
C 11 8.52
C 12 7.86
C 13 7.70
C 14 7.39
C 15 7.06
C 16 6.73

The levelling should be continued in this manner, until the grades of all
the points are recorded in the field book.

[Illustration: Fig. 21 - PROFILE OF DRAIN C.]