RAILROAD CURVES 163 



The distance of the P. C. or P. T. from the P. I. is called 

 the tangent distance, and the chord connecting the P. C. and 

 P. T. of a curve is commonly called its long chord. This term 

 is also applied to chords more than one station long. 



If / denotes the angle of intersection and R the radius of 

 the curve, then the tangent distance 



Laying Out a Curve With a Transit. When the angle of 

 intersection / has been measured and the degree of curve 

 decided upon, the radius of the curve can be taken from the 

 table of radii and deflections or it can be figured by the for- 

 mula 5,730 



. *"^~ 



The tangent distance is then computed and measured back 

 on each tangent from the P. I., thus determining the P. C. 

 and P. T. Suotracting the tangent distance from the station 

 number of the P. I. will give the station number of the P. C. 

 Ordinarily, this will not be an even or full station. The length 

 of the curve is then computed by dividing the angle / by the 

 degree of curve, the quotient giving the length of the curve 

 in stations of 100 ft. and decimals thereof. After having 

 found the length of the curve, compute the deflection angles 

 for the chords joining the P. C. with all the station points; 

 set the transit at the P. C.; set the vernier at zero, sight to 

 the intersection point, and turn off successively'the deflection 

 angles, at the same time measuring the chords and marking 

 the stations. The station of the P. T. is found by adding the 

 length of curve in chords of 100 ft. to the station of the P. C. 



If the entire curve cannot be run from the P. C. on account 

 of obstructions to the view, run the curve as far as the stations 

 are visible from the P. C. and run the remainder of the curve 

 from the last station that can be seen. Suppose that in the 

 10 curve shown in Fig. 5 the station at H, 200 ft. from the 

 P. C., which is at B, is the last point on the curve that can be 

 set from the P. C. A plug is driven at H and centered care- 

 fully by a tack driven at the point. The transit is now moved 

 forwards and set up at H. Since the deflection angle EBH is 

 10 to the right, an angle of 10 is turned to the left from zero 

 and the vernier clamped. The instrument is then sighted to 



