The transit contains a graduated horizontal circle, referred to as the horizontal limb. The horizontal limb may be graduated clockwise from 0 through 360, as shown in figure 13-9, view A, or clockwise from 0 through 360 and also in quadrants, as shown in figure 13-9, view B; or both clockwise and counterclockwise from 0 through 360, as shown in figure 13-9, view C. The horizontal limb can be clamped to stay fast when the telescope is rotated (called clamping the lower motion), or it can be released for rotating by hand (called releasing the lower motion).

Figure 13-9.-Three types of horizontal limb graduations.

The horizontal limb is paired with another circle (the vernier plate), which is partially graduated on either side of zero graduations located 180 apart on the plate. When the telescope is in the normal (upright) position, the A vernier is located vertically below the eyepiece, and the B vernier, below the objective end of the telescope. The zero on each vernier is the indicator for reading the sizes of horizontal angles turned on the horizontal limb.

Figures 13-10 and 13-11 illustrate the method of turning an angle of 30 from a reference line with a transit.

1. With the transit properly set over the point or station, bring one of the horizontal verniers near zero by hand; clamp the upper motion; and, by turning the upper tangent screw, set one vernier 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 the reference line; clamp the lower motion; and, by using the lower tangent screw, set the line of sight on the marker.