the plumb bob away from yourself!”). When
the plumb bob is exactly on the line, the
instrument man calls out, “Good!” or “All
right!” The head chainman then marks the point
indicated by the plumb bob in the correct manner.
The first 100 ft have now been measured on the
given line of direction.
If the distance to be measured is long, the
chainmen will eventually proceed beyond the
scope of the instrument as it is then set up. The
instrument must then be shifted ahead to the last
point marked by the head chainman. When the
instrument has been set up over this point, the
telescope must be reoriented to the line of
direction. To do this, the instrumentman usually
plunges the telescope (rotates it vertically) and
backlights on a point on the line already laid off.
In taking backlights, the instrumentman is guided
by the rear chainman who holds on, or plumbs
over, the point. When the telescope has been
trained on the backsight point, it is again plunged.
The telescope is now again trained in the desired
direction.
Holding on a Point
If the point on the ground can be sighted
through the telescope, the chainman may simply
hold on the point; that is, hold a pencil point,
chaining pin point, plumb bob point, or some
other appropriate indicator on the point (fig.
12-4). Whatever the indicator may be, it is
Figure 12-4.-Indicators used for short sights.
essential that it be held in an exactly vertical
position. For short sights, it is also essential that
the shaft of the indicator be relatively slender so
that the vertical cross hair can be aligned with
sufficient exactness.
Plumbing over a Point
If intervening low growth or some other
circumstance makes it impossible for the instru-
mentman to sight the point on the ground, the
chainman must plumb over the point, using the
plumb bob and cord. If the distance is too far for
observation of the plumb bob cord, the cord
should be equipped with a plumb bob target, or
a range pole may be used. In the absence of a
target when using the plumb bob, you may tie a
piece of colored flagging to the cord, or you may
use a handkerchief, as shown in figure 12-5.
Some chainmen prefer to hold the plumb bob
and cord with the cord running over the
forefinger. Others prefer to have the cord running
over the thumb. If you are plumbing high (that
is, required to hold the cord at chest level or
Figure 12-5.-Using a handkerchief as a substitute for a
target on a plumb bob cord.
12-6
