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 28
the eye. If the two points on the wall coincide, the instrument iscorrect. If they do not, the mirror does not hang plumb, andthe point midway between the two is the true one. The instrument isrectified, or made tohang plumb, by meansof the pear - shapedpiece of lead seen at-tached to the…
Surveying
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