. The elements of railroad engineering . L. 37° 2(>, is turnedat Sta. 20+38, making the length of the fourth course520 ft. Call Sta. 20 + 38, O. Produce A O 400 ft. to (9 as a center, with a radius O J\ describe the indefi- MAPPING. 747 nite arc P Q. The chord of 37° 20 is .6401, which, multi-pHed by 400 ft., gives ft., the length of the requiredchord. From /* as a center, with a radius of ft.,describe an arc intersecting the arc P Q in R. Join O and R,forming with O P the angle P O R = 37° 20. The end ofthe line 5 is at Sta. 25 + ^0, making the length of the lastcourse
. The elements of railroad engineering . L. 37° 2(>, is turnedat Sta. 20+38, making the length of the fourth course520 ft. Call Sta. 20 + 38, O. Produce A O 400 ft. to (9 as a center, with a radius O J\ describe the indefi- MAPPING. 747 nite arc P Q. The chord of 37° 20 is .6401, which, multi-pHed by 400 ft., gives ft., the length of the requiredchord. From /* as a center, with a radius of ft.,describe an arc intersecting the arc P Q in R. Join O and R,forming with O P the angle P O R = 37° 20. The end ofthe line 5 is at Sta. 25 + ^0, making the length of the lastcourse 542 ft. In a similar manner, plat the notes for linesb and c, which are given in Art. 1353. 1352. To Lay Off an Angle by its Bearing.—By this method of laying off angles, the direction of each lineis referred to the magnetic meridian, which maintains a con-stant direction, being a north and south line. The bearingof a line is the angle which the line makes with the magneticmeridian. In platting a land or railroad survey, a pencil. Fig. 329. line giving the direction of the magnetic meridian is drawnthrough each station at which a bearing is taken. The direction of the meridian may be given by means ofthe ordinary T square and triangles, and the angles laidoff either by a protractor or by tangents. The use of Tsquare and triangles in laying off angles by bearings is illus-trated in Fig. 329. A sheet of paper is fastened to a draw-ing board. It is well known that if the head of theT square 74S MAPPING. be kept firmly pressed against the side of the drawing board,as shown in the figure, the lines drawn along the straightedge will be parallel; hence, the lines drawn perpendicularto this straight edge by means of the triangles, as shown inthe figure, will be parallel. Either the parallels drawn along the straight edge of theT square or of the triangle may be used as the magneticmeridian, tliough the latter is preferable, as it brings thenorth end of the meridian at the top of
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
