RAILROAD CURVES 



173 



of 100 ft. At any other point of the spiral, the degree of curve 



is equal to the unit degree of curve multiplied by the distance 



of the point from the P. Si, also measured in stations of 100 ft. 



Let DC denote the degree of circular curve; D, the degree of 



-IT 

 \L 



- 





FIG. 1 



curve at any point of the spiral, distant / stations from the P. Si; 

 L, the total length of the spiral in stations; and a the unit 



degree of spiral. Then, a = ~T 



L 



and D = al 



The superelevation of the outer rail on the spiral is pro- 

 portional to J; it is zero at P. Si, and attains the value e, the 



