224 



A MANUAL OF TOPOGRAPHIC METHODS. 



Table XXVIII. — Factors for reduction of transit observations — Continued. 

 Azimuth factor A=sin ( sec. 6. Star's declination ± 6. Inclination factor B = cos i sec. S. 



Table XXIX. — For reducing observations for latitude by Talcotfn method. 



[Extracted from Appendix 14. United States Coast and Geodetic Survey, Report for 1880.] 



Correction for differential refraction. — The difference of refraction for any pair of 

 stars is so small that we can neglect the variation in the state of the atmosphere at the 

 time of the observation from that mean state supposed in the refraction tables. The 

 refraction being nearly proportional to the tangent of the zenith-distance, the differ- 

 ence of refraction for the two stars will be given by — 



r— r'=57".J sin (z—z') sec.-.c 



and since the difference of zenith-distances is measured by the micrometer, the follow- 

 in. triable of correction to the latitude for differential refraction has been prepared 



