vertical circle vernier. The other vial is then
parallel to the axis of rotation for the vertical
motion. The sensitivity of the plate level vial
bubbles is about 70 sec of movement for 2 mm
of tilt. Most engineers transits have a level vial
mounted on the telescope to level it. The
sensitivity of this bubble is about 30 sec per 2-mm
t i l t.
CIRCLES AND VERNIERS. The hori-
zontal and vertical circles and their verniers are
the parts of the engineers transit by which the
values of horizontal and vertical angles are
determined. A stadia arc is also included with the
vertical circle on some transits.
The horizontal circle and verniers of the transit
that are issued to SEABEE units are graduated
to give least readings of either 1 min or 20 sec of
arc. The horizontal circle is mounted on the lower
plate. It is graduated to 15 min for the 20-sec
transit (fig. 11-8) and 30 min for the 1-min transit
(fig. 11-9). The plates are numbered from 0° to
360°, starting with a common point and running
both ways around the circle. Two double verniers,
known as the A and B verniers, are mounted on
the upper plate with their indexes at circle readings
180° apart. A double vernier is one that can be
read in both directions from the index line. The
verniers reduce the circle graduations to the final
reading of either 20 sec or 1 min.
Figure 11-9.-Horizontal scales, 1-minute transit.