Figure 13-10.-Setting the vernier at zero-zero.The horizontal limb is paired with anothercircle (the vernier plate), which is partiallygraduated on either side of zero graduationslocated 180° apart on the plate. When thetelescope is in the normal (upright) position, theA vernier is located vertically below the eyepiece,and the B vernier, below the objective end of thetelescope. The zero on each vernier is the indicatorfor reading the sizes of horizontal angles turnedon the horizontal limb.Figures 13-10 and 13-11 illustrate the methodof turning an angle of 30° from a reference linewith a transit.1. With the transit properly set over the pointor station, bring one of the horizontal verniersnear zero by hand; clamp the upper motion; and,by turning the upper tangent screw, set onevernier at 0°, usually starting with the A vernier(fig. 13-10). Train the telescope to sight the marker(range pole, chaining pin, or the like) held on thereference line; clamp the lower motion; and, byusing the lower tangent screw, set the line of sighton the marker.Figure 13-11.-Setting an angle exactly on the vernier zero.2. Release the upper motion and rotate thetelescope to bring the zero on the A vernier in linewith th3 30° graduation on the horizontal limb,as shown in figure 13-11. To set the vernierexactly at 30°, use the upper tangent screw. Youmay use a magnifying glass to set the vernier easilyand accurately.3. Mark the next point with a marker, andfollow the procedures for establishing a point orstation.Similarly, you may use the procedures aboveto measure a horizontal angle by sighting on twoexisting points and reading their interior angle.In addition, the following hints may help youwhen you are taking horizontal measurements:1. Make the centering of the line of sight asclose as possible by hand so that you will not turnthe tangent screw more than one or two turns.Make the last turn of the tangent screw clockwiseto compress the opposing springs.13-10
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