VERTICAL CIRCLE.— When reading theVERTICAL CIRCLE, turn the circle-selectorknob until its black line is vertical. Adjust themirror on the left standard and focus themicroscope eyepiece. You then go on in thefollowing way:1. Use the vertical circle tangent screw tomove the collimation level until the ends ofits bubble appear in coincidence (fig. 11-23)in the collimation level viewer on the leftstandard.2. Read the vertical circle and micrometerscale as described before. Be sure to have propercoincidence before you take the reading.3. The vertical circle graduations are num-bered to give a 00reading with the telescopepointing to the zenith. Consequently, the verticalcircle reading will be 900 for a horizontal sightwith the telescope direct and 2700 for a horizontalsight with the telescope reversed. Figure 11-23shows the view in the circle-reading microscopefor direct and reversed pointings on a target.These readings are converted to vertical angles asfollows:There are two separate occasions for settingthe horizontal circle of the theodolite. In the firstcase, the circle is set to read a given value withthe telescope pointed at a target. With thetheodolite pointed at the target and with theazimuth clamp tightened, the circle is set asfollows: Set the micrometer scale to read the unitminutes and the seconds of the given values. Then,with the circle-setting knob, you turn the circleuntil coincidence is obtained at the degree and tensof minutes value of the given reading. This settingnormally can be made accurately to plus or minus5 sec. After the circle is set in this manner, theactual reading should be determined.In the second case, the circle is set to a valueof a given angle. When a predetermined angle ismeasured, you first point the instrument along theinitial line from which the angle is to be measuredand read the circle. Add the value of the angleto the circle reading to determine the circle readingfor the second pointing. Set the micrometer scaleto read the unit minutes and the seconds of thevalue to be set on the circle. Then, you turn theinstrument in azimuth and make coincidence atthe degrees and tens of minutes value that is tobe set. The predetermined value can usually beset on the circle in this way to plus or minus 2 sec.ENGINEER’S LEVELThe engineer’s level is a widely used instrumentfor leveling operations. Its sighting device is a30 ± 3 variable power telescope, with amaximum length of 18 in. and with an erectingeyepiece. Some models use internal focusing,while others use external focusing objectiveFigure 11-23.-View of a vertical circle for direct and reversal pointings.11-27
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