. Railroad construction. Theory and practice . to introduce a substation at anypoint, its deflection from the PC may bereadily interpolated. The stations actuallyset from the PC are located as When the transit is set on anyforward station, backsight to any previousstation with the plates set at the deflectionangle for the station sighted at. Plungethe telescope and sight at any forwardstation with the deflection angle originallycomputed for that station. When theplates read the deflection angle for thestation occupied, the telescope is sightingalong the tangent at that station—whic
. Railroad construction. Theory and practice . to introduce a substation at anypoint, its deflection from the PC may bereadily interpolated. The stations actuallyset from the PC are located as When the transit is set on anyforward station, backsight to any previousstation with the plates set at the deflectionangle for the station sighted at. Plungethe telescope and sight at any forwardstation with the deflection angle originallycomputed for that station. When theplates read the deflection angle for thestation occupied, the telescope is sightingalong the tangent at that station—whichis the method of getting the forward tan-gent when occupying the PT. Even thoughthe station occupied is an unexpected sub-station, when the instrument is properlyoriented at that station, the angle readingfor any station, forward or back, is that originally computedfor it from the PC. In difficult work, where there are ob-structions, a valuable check on the accuracy may be found bysighting backward at any visible station and noting whether. Fig. 11. 20 RAILROAD CONSTRUCTION. §26. its dofleetion agrees with that originally computed. As anumerical illustration, assume a 4° curve, with 28° curvature,with stations 0, 2, 4, and 7 occupied. After setting stations1 and 2, set up the transit at sta. 2 and backsight to sta. 0with the deflection for sta. 0, which is 0°. The reading on is 2°; when the reading is 4° the telescope is tangent tothe curve, and when sighting at 3 and 4 the deflections will be6° and 8°. Occupy 4; sight to 2 with a reading of 4°. Whenthe reading is 8° the telescope is tangent to the curve and, byplunging the telescope, 5, 6, and 7 may be located with theoriginally computed deflections of 10°, 12°, and 14°. When oc-cupying 7 a backsight may be taken to any visible station withthe plates reading the deflection for that station; then w^hen
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