RAILROAD CURVES 169 



by the letters C and D. The stations are to be located in their 

 proper positions on the curve, between the obstructions, where- 

 ever it is possible to do so. In addition to this, it is customary 

 to mark with a tack or otherwise the point where the line of the 

 curve intersects each obstruction. 



Beginning at the point of curve A , which is at Station 3, the 

 curve can be run in as far as the first obstruction, which is the 

 building P, setting the stakes on the curve at Stations 4 and 5, 

 and a tack in the side of the building P at the point where the 

 line of curve intersects it, according to the deflection angle 

 as determined by its distance from Station 5. It is not possible 

 to proceed further in the regular manner, however, because 

 Station 6 cannot be seen from the P. C. Therefore, it is nec- 

 essary to locate Station 7 by deflection angle V'BE, from B or 

 Station 4, to determine the chord 4-7, which, in this case, 

 is a long chord of 3 stations, and to calculate the ordinates 

 D'D and C'C by substituting for a in the preceding formula 

 the value of MC = MD' = half a station or 50'. 



Fig. 8 shows also another method of passing a building, as S, 

 namely, by running an equilateral triangle FLG. In this 

 method, the instrument is set up at Station 8 and sighted back 

 to the P. C. Then, the telescope is reversed and the deflection 

 angle for Station 9 is turned off the same as if no obstruction 

 existed. The telescope will then be sighted on the line FG, 

 although the point G will not be visible. The angle GFL, 

 equal to 60, is then turned, and the point L is located so that 

 FL = FG = 100'. The instrument is next moved to L, and the 

 line LG is run, making 60 with FL. On this line the distance 

 Z,G=100' is measured, giving the point G, which is Station 9. 

 The transit is then set up at this point and sighted to L, and an 

 angle of 60 is turned off to the right, giving the direction of the 

 line 9-8, the intersection of which with 5 is marked. The 

 remainder of the curve may be run in the following manner: 

 Set the vernier at an angle equal to the deflection angle of the 

 chord 9-8 to the left from the zero; clamp the upper plate, 

 sight at the point set in the line 9-8 \ then clamp the lower 

 plate and set vernier at zero. The line of sight will then be in 

 the tangent at point 9, and by plunging the telescope the remain- 

 der of the curve can be run as if the point 9 were the P. C. 



