A treatise on surveying, comprising the theory and the practice (1906)
Gillespie, William Mitchell, 1816-1868, Staley, Cady, 1840- · New York D. Appleton · leaf 391
90°, and walk along the line, looking directlyat a point on it, until the given point is seen by reflection to coin-cide with the point on the line. A plumb line let fall from the eyewill give the desired point. 811. The Optical Square (Fig. 543). This is a box containing two mirrors, fixed at an… … 812. To measure a Line, One End being Inaccessible (Fig. 544).Let A B be the required line, and B the inaccessible point. At A set off a perpendicular, A C, by Art. 810; then set theindex at 45°, and walk backward from A in the line of C A pro-longed, looking by direct vision at C, until you arrive…
Surveying
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