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 491
direction,and when it is seen by reflection to coincide with the point on theline looked at directly, it will be in a line perpendicular to the givenline at the desired point. Conversely, to find where a perpendicular from a given pointwould strike a line : Set the index at 90°, and walk along the… … 707. The Optical Square (Fig. 521). This is a box containingtwo mirrors, fixed at an angle of 45° to each other, and therefore TEE PRACTICE. 477 giving an angle of 90°, as does the sextant with its glasses fixed atthat angle. It is used to set out perpendiculars. 708. To measure a Line, One End…
Surveying
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