A treatise on levelling, topography, and higher surveying (1876)
Gillespie, W. M. (William Mitchell), 1816-1868, Staley, Cady, b. 1840, ed · New York, D. Appleton and company · leaf 141
lk along the line, lookingdirectly at a point on it, until the given point is seen by re-flection to coincide with the point on the line. A plumb-linelet fall from the eye will give the desired point. (193.) The Optical Square, Fig. 153. This is a box contain-ing two mirrors, fixed at an angle of… … (194.) To measure a Line, one End being inaccessible. A B be the required line, and B the inaccessible point. Let Fig. 154.
Surveying
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