RAILROAD CURVES 



191 



Transit at P. Si'. The angles to be deflected are the same 

 as at P. Si. The station number of P. Si' is (39+79. 6) + 3 

 = 42 + 79.6. 



The Field Work. Run the two tangents to their intersec- 

 tion. Measure back from T the distances TC=TC' = 959.3 

 ft., and set stakes marked P. Si at C and C'. Set the transit 

 at C with the vernier at 0' ; sight on T and deflect the angles 

 (A) to locate the first spiral. When the stake at A (marked 

 P. 82) has been set, move to this point, set the vernier at 6 0', 

 backsight on C, turn the telescope until the vernier reads 

 0', and from this direction deflect the angles (B) to locate 

 the circular curve. When the stake B (marked P. 82) has been 

 set, move the transit to C', set the vernier at (X, backsight 

 on T, and deflect the angles (A) to locate the second spiral. 



SELECTION OF SPIRALS 



For a given velocity of train, in miles per hour, V, and the 

 degree of curve of the circular curve D c , the best length of 

 spiral, in stations is found by the following formula: 



L _ 



108,000 



EXAMPLE. Find the theoretically best length of spiral to 

 connect with a 6 curve, the maximum train velocity being 

 40 mi. per hr. 



SOLUTION. S u b s t i- 

 tuting the value of 40 

 for V and 6 for D c , 

 403X6 



108,000 



= 355.6 ft. 



Table of Minimum 

 Spiral Lengths. The 



accompanying table, 

 from Talbot's "Transi- 

 tion Spiral," gives the 

 values of a correspond- 

 ing to the least length of spiral that the engineer should 

 endeavor to insert. The spiral may be longer than the length 

 obtained from this table, but it should not be shorter, unless 

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