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 27
ning the square, then the other arm will belevel. (18.) Reflecting Levels. In these, the perpendicular to thedirection of gravity is not an actual line, but an imaginaryreflected line. It depends on the optical principle that a ray of lightwhich meets a reflecting plane at right angles is reflected… … PERPENDICULAR LEVELS. Fig. 8.
Surveying
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