vertical circle vernier. The other vial is thenparallel to the axis of rotation for the verticalmotion. The sensitivity of the plate level vialbubbles is about 70 sec of movement for 2 mmof tilt. Most engineer’s transits have a level vialmounted on the telescope to level it. Thesensitivity of this bubble is about 30 sec per 2-mmt i l t.CIRCLES AND VERNIERS.— The hori-zontal and vertical circles and their verniers arethe parts of the engineer’s transit by which thevalues of horizontal and vertical angles aredetermined. A stadia arc is also included with thevertical circle on some transits.The horizontal circle and verniers of the transitthat are issued to SEABEE units are graduatedto give least readings of either 1 min or 20 sec ofarc. The horizontal circle is mounted on the lowerplate. It is graduated to 15 min for the 20-sectransit (fig. 11-8) and 30 min for the 1-min transit(fig. 11-9). The plates are numbered from 0° to360°, starting with a common point and runningboth ways around the circle. Two double verniers,known as the A and B verniers, are mounted onthe upper plate with their indexes at circle readings180° apart. A double vernier is one that can beread in both directions from the index line. Theverniers reduce the circle graduations to the finalreading of either 20 sec or 1 min.Figure 11-9.-Horizontal scales, 1-minute transit.11-17
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