164 



RAILROAD CURVES 



a flag at B, the lower clamp set, and by means of the lower 

 tangent screw the cross-hairs are made to bisect the flag exactly. 

 The vernier clamp is then loosened, the vernier set at zero, 

 and the telescope plunged. The line of sight will then be on 

 the tangent IP, and the deflection angles to K and C can be 

 turned off from this tangent, and the stations at K and C 

 located in the same manner that the stations at G and H were 



located from B, because the angle at IHB between the tangent 

 IH and the chord BH is equal to the angle EBH between the 

 tangent EB and the same chord. 



This method of setting the vernier for the backsight when 

 the instrument is moved forwards to a new instrument point 

 on the curve is sometimes called the method by zero tangent. 

 The essential principle of the method is that the vernier always 

 reads zero when the instrument is sighted on the tangent to 

 the curve at the point where the instrument is set, and the 

 deflection angles are made to read from the tangent to the 



