Refraction tends to decrease the above correction as shown in 

 the figure. The curve of refraction depends somewhat upon the 

 density of the atmosphere, but the empirical valuation fixes the 

 radius at seven times that of the earth. The combined resultant effect 

 of curvature and refraction (BC in the figure) is the one usually 

 considered and is represented by the formula : 



R 



13: 



I) miles-. 



In the following table, the distance D, and the correction 

 C 4- R are given in feet, except where M represents miles. The 

 correction is always a minus quantity. (See also page 165). 



Consult C. & G. S. Report, 1883, pp. 289-321. 



