A treatise on surveying, comprising the theory and the practice ; (1887)
Gillespie, W. M. (William Mitchell), 1816-1868, Staley, Cady, 1840- [from old catalog] ed · New York, D. Appleton and company · leaf 496
he supplement of the zenith-distance. Otherwise. Fix behind the horizon-glass a piece of white paperwith a small hole in it, and with a black line on it perpendicularto the plane of the arc. Then look into the mercury, so as to see in it the image of theline. Your line of sight is then vertical,… … 482 MARITIME OR HYDRO GRAPHICAL SURVEYING. line. Mark two points equally distant from the eye. Measure, bythe preceding method, the angles (3 and /?, which C A and C Bmake with the vertical C D. Then will half the difference of theseangles equal the angle which the slope makes with the horizon.…
Surveying
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