circular plate carrying verniers, telescope
standards, plate-level vials, and a magnetic
compass. The spindle is accurately fitted to
coincide with the socket in the lower plate spindle.
A clamp is tightened to hold the two plates
together or loosened to permit the upper plate to
rotate relative to the lower plate. A tangent screw
permits the upper plate to be moved a small
amount and is known as the UPPER MOTION.
The standards support two pivots with adjustable
bearings that hold the horizontal axis and permit
the telescope to move on a vertical plane. The
vertical circle moves with the telescope. A clamp
and tangent screw are provided to control this
vertical movement. The vernier for the vertical
circle is attached to the left standard. The
telescope is an erecting type and magnifies the
image about 18 to 25 times. The reticle contains
stadia hairs in addition to the cross hairs. A
magnetic compass is mounted on the upper plate
between the two standards and consists of a
magnetized needle pivoted on a jeweled bearing
at the center of a graduated circle. A means is
provided for lifting the needle off the pivot to
protect the bearing when the compass is not in use.
LEVEL VIALS.— Two plate level vials (fig.
11-6) are placed at right angles to each other. On
many transits, one plate level vial is mounted on
the left side, attached to the standard, under the
Figure 11-8.-Horizontal scales, 20 second transit.
11-16
