Davies, Charles, 1798-1876 · New York, Cincinnati [etc.] American book company · leaf 58
rescrewed firmly to the bars, bymeans of the screws a, b, c,and d. As the only use of this in-strument is to lay off rightangles, it is of the first im-portance that the lines of sight be truly at right angles. To ascertain if they are so, let the barAB be turned until its sights mark some distinct… … Fio. 35. 5^ ELEMENTS OF SURVEYING. [bOOK II, directed to the first object, the lines of sight are exactly atright angles. The sights being at right angles, if one of them be turnedin the direction of a given line, the other will mark the direc-tion of a line perpendicular to it, at the point where…
Surveying
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