. A manual of the principles and practice of road-making : comprising the location, consruction, and improvement of roads (common, macadam, paved, plank, etc.) and railroads . Laying out its curves. 3. Staking out its side-slopes. RECTIFICATION. The minor irregularities, bends, and zigzags of the line(caused in part by the transverse balancing) may often beremoved by substituting for them one straight line, whichwill be the average of their deviations on either side. Aflagstaff being placed at one end of the line, an observer,at the other end, by signals directs assistants to place inline the
. A manual of the principles and practice of road-making : comprising the location, consruction, and improvement of roads (common, macadam, paved, plank, etc.) and railroads . Laying out its curves. 3. Staking out its side-slopes. RECTIFICATION. The minor irregularities, bends, and zigzags of the line(caused in part by the transverse balancing) may often beremoved by substituting for them one straight line, whichwill be the average of their deviations on either side. Aflagstaff being placed at one end of the line, an observer,at the other end, by signals directs assistants to place inline the rods which they bear ; and the points thus foundare marked by stakes, which are usually driven at everyhundred feet. In the case of long lines, through a coun- * Parnell, pp. 322, 433. lS6 THE LOCATION OF ROADS. try of forests, the use of the compass, or some other angu-lar instrument, is almost indispensable, for it is still an un-solved problem in engineering, how, without the aid ofthese, the Romans attained the wonderful straighlnesswith which they carried their roads over thickly-woodedhills and valleys, with such lofty disdain of the eifects of gravity. Fig. When a hill rising between two points, as A and B, pre-vents one being seen from the other, two observers C andD may place themselves on the ridge, as nearly as possiblein the line between the two points, and so that each can atonce see the other and the point beyond. C looks to B, andby signals puts D in line. D then looks to A, and putsC in line. C repeats his operation, and so they alter-nately line each other, continually approximating to thestraight line between A and B, till they at last find them-selves both exactly in it. When a wood, or some such obstacle, intervenes betweenthe two points, as in Fig. 61, a different method must beadopted. The direction from A to B not being exactlyknown, leaving a rod at A, set up another at C, as nearly inthe desired line as possible. Go on as far as the two rodsat A an
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectrailroa, bookyear1853
