TERRESTRIAL REFRACTION. 



EXAMPLE I. 



The length of the measured line (Banswai^y and 

 jBej/o*oorJ being converted into an arc of a great circle, 

 gives 6' 34," 67 =A *, 



Again, the depression of the south extremity of 

 the line (near Bei/goo?") below the level of the north- 

 ern one, is 39. 7 feet, and using the proportion of 

 radius to tangent, we have for the angle which it 

 subtends 3' 25.' 75 -a. 



Lastly, on the 7 th of August, at 6"- 39' A. M. the 

 depression of the foot of the flag-staff near Bet/goor^ 

 was observed at Bansxvary, 6' 42" 66. 



But the height of the observer's eye above the 

 ground was 5. 67 feet (that of the instrument) which 

 to reduce to the ground, will require 29" 39 to sub- 

 tract : consequently, the corrected depression will be 

 6' 13". 9,7 zzD (the quantity entered in the tables). 

 Whence 



i A=3' 17" 34 

 + a-3 25 75 



6 43 09 

 -D-6 13 27 



r- 29 82 



The refraction entered in the tables. 



* The horizontal length of the measured line is SffJ99> 31 feet; 

 and when reduced to the level of the sea,::=39793> ?• This length 

 has been used, in this particular case, to obtain the contained arc ; 

 because the tables, by means of which the operation was performed, 

 were calculated to that distance from the centre of the earth. 



