VERTICAL CIRCLE.— When reading the
VERTICAL CIRCLE, turn the circle-selector
knob until its black line is vertical. Adjust the
mirror on the left standard and focus the
microscope eyepiece. You then go on in the
following way:
1. Use the vertical circle tangent screw to
move the collimation level until the ends of
its bubble appear in coincidence (fig. 11-23)
in the collimation level viewer on the left
standard.
2. Read the vertical circle and micrometer
scale as described before. Be sure to have proper
coincidence before you take the reading.
3. The vertical circle graduations are num-
bered to give a 00 reading with the telescope
pointing to the zenith. Consequently, the vertical
circle reading will be 900 for a horizontal sight
with the telescope direct and 2700 for a horizontal
sight with the telescope reversed. Figure 11-23
shows the view in the circle-reading microscope
for direct and reversed pointings on a target.
These readings are converted to vertical angles as
follows:
There are two separate occasions for setting
the horizontal circle of the theodolite. In the first
case, the circle is set to read a given value with
the telescope pointed at a target. With the
theodolite pointed at the target and with the
azimuth clamp tightened, the circle is set as
follows: Set the micrometer scale to read the unit
minutes and the seconds of the given values. Then,
with the circle-setting knob, you turn the circle
until coincidence is obtained at the degree and tens
of minutes value of the given reading. This setting
normally can be made accurately to plus or minus
5 sec. After the circle is set in this manner, the
actual reading should be determined.
In the second case, the circle is set to a value
of a given angle. When a predetermined angle is
measured, you first point the instrument along the
initial line from which the angle is to be measured
and read the circle. Add the value of the angle
to the circle reading to determine the circle reading
for the second pointing. Set the micrometer scale
to read the unit minutes and the seconds of the
value to be set on the circle. Then, you turn the
instrument in azimuth and make coincidence at
the degrees and tens of minutes value that is to
be set. The predetermined value can usually be
set on the circle in this way to plus or minus 2 sec.
ENGINEER’S LEVEL
The engineer’s level is a widely used instrument
for leveling operations. Its sighting device is a
30 ± 3 variable power telescope, with a
maximum length of 18 in. and with an erecting
eyepiece. Some models use internal focusing,
while others use external focusing objective
Figure 11-23.-View of a vertical circle for direct and reversal pointings.
11-27
