. A manual of the principles and practice of road-making: comprising the location, consruction, and improvement of roads (common, macadam, paved, plank, etc.) and rail-roads . ad of the sights, a telescope is made parallel tothe level, and various contrivances to increase its delicacy andaccuracy are added, the instrument becomes the engineersspirit-level^ and is oui of the reach of the unprofessional read-ers for whom this volume is chiefly designed.* The same isthe case with the French reflecting level. • By whichever of these various means a level line hasbeen obtained, the subsequent opera
. A manual of the principles and practice of road-making: comprising the location, consruction, and improvement of roads (common, macadam, paved, plank, etc.) and rail-roads . ad of the sights, a telescope is made parallel tothe level, and various contrivances to increase its delicacy andaccuracy are added, the instrument becomes the engineersspirit-level^ and is oui of the reach of the unprofessional read-ers for whom this volume is chiefly designed.* The same isthe case with the French reflecting level. • By whichever of these various means a level line hasbeen obtained, the subsequent operations in making useof it are identical. Since the water level is easilymade and tolerably accurate, w^e will suppose it to be em-ployed. Let A and B represent the two points, the differ-ence of the heights of which is required. Set the instru-ment on any spot from w^hich both the points can be seen,and at such a height that the level hne will pass above thehighest one. At A let an assistant hold a staff graduated intofeet, tenths, &c. Turn the instrument towards the staff, * For its description and adjustments, see Davies Surveying, p. 140. LEVELLING. 97 Fig. look along the levelline, and note whatdivision on the staffit strikes. Thensend the staff to B,direct the instru-ment to it, and notethe height observedat that point. Ifthe level line pro-duced by the eye passes 2 feet above A and 6 feetabove B, the difference of their heights is 4 feet. Theabsolute height of the level hne itself is a matter ofindifference. If the height of another point, C, not visiblefrom the first station, be required, set the instrument so asto see B and C, and proceed exactly as with A and C be found to be 3 feet above B, it will be 4 — 3 = 1foot below A. If C be 1 foot below B, as in Fig. 43, itwill be 4 + 1=5 feet below A. The comparative heightsFisr. 43.
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectrailroa, bookyear1853
