. 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 . of length. Measure on a vertical line thence drawn, five-tenths of an inch, representing five feet of height. Thisfixes C. Join C to B. Proceed in like manner for allthe levels. The distances may be vi^ritten horizontally in their ap-propriate places, and the heights or depths of the ground(above or below the datum line) vertically, along thelines which represent them, as in the figure. 5. ESTABLISHING THE GRADES. The gr
. 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 . of length. Measure on a vertical line thence drawn, five-tenths of an inch, representing five feet of height. Thisfixes C. Join C to B. Proceed in like manner for allthe levels. The distances may be vi^ritten horizontally in their ap-propriate places, and the heights or depths of the ground(above or below the datum line) vertically, along thelines which represent them, as in the figure. 5. ESTABLISHING THE GRADES. The grade of a line is its longitudinal slope, and isdesignated by the proportion between its length and thedifference of height of its two extremities. The ratio ofthese two quantities gives it its name, as we have seen;the road being said to have a grade of one in thirty whenit rises or falls one foot in each thirty feet of the profile of a proposed route has been made,a grade-line is drawn upon it (usually in red) in sucha manner as to follow its general slope, but to average itsFig. irregular elevations and depressions, as in the figure. Theratio between the whole distance and the height is then to ESTABLISHING THE GRADES. 105 be calculated. If, as in the figure, it rise 100 in 4000, thegrade is one in forty, flatter than our assumed limit of onein thirty, and the line will be a satisfactory one, if on cal-culation it be found that the cuttings about equal the fill-ings. If either be much in excess, the grade is altered toequalize them, as will be explained under the next if the grade be found steeper than the limit, as whenit ascends the face of a hill with a rise of 100 feet in1500, or a slope of one in fifteen, either the hill must becut down, or, which is generally preferable, the length ofthe line must be increased so as to equal 100 x 30 = best method of obtaining this increased length, or development, (whether by a zigzag or by a singleob
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectrailroa, bookyear1853
