. The elements of railroad engineering . times out of fivethe levelers reading will be the correct one, even to thou-sandths. More mistakes are made in reading the number offeet than the number of tenths. The leveler by first callingthe reading to the rodman will be certain to prevent such anerror, as it would at once be detected in the check experienced rodman can hold a rod practically plumb,and for all ordinary work his care is considered work requiring the greatest possible accuracy, such asbridge foundations, a hand level, which fits closely to theangle of the ro
. The elements of railroad engineering . times out of fivethe levelers reading will be the correct one, even to thou-sandths. More mistakes are made in reading the number offeet than the number of tenths. The leveler by first callingthe reading to the rodman will be certain to prevent such anerror, as it would at once be detected in the check experienced rodman can hold a rod practically plumb,and for all ordinary work his care is considered work requiring the greatest possible accuracy, such asbridge foundations, a hand level, which fits closely to theangle of the rod and carries two small spirit levels, is used toaccurately plumb it. In using a rod which is not self reading,all readings are taken with the target. 1279. Examples in Direct Leveling.—The princi-ples of direct leveling are illustrated in Fig. 291. Let A be the starting point, which has a known elevationof 20 feet. The instrument is set at B, leveled up, andsighted to a rod held at A. The target being set, the reading, 664 feet, called a backsight,is the distance which the pointwhere the line of collimation cutsthe rod is above the point A, andis to be added to the elevation ofLhe point A. + = called the height of instrumentand designated H. I. The instru-ment being turned in the oppositedirection, a point C is chosen,which must be below the line ofsight. This point is called a turn-ing point, and is designated b)^ theabbreviation T. P. Drive a pegat C or take for a turning point apoint of rock or some other perma-nent object upon which the rod isheld. The reading at this point isa foresight, and is to be sub-tracted from the height of theinstrument at B to find the ele-vation of the point at C. Let the rod reading be this reading is a foresight, itmust be subtracted from ,the height of instrument at B; - = , the ele-vation of the point C. Asthe ground rises abruptly,the rodman should slide therod to its full
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
