COMSTOCK STUDIES IN ASTRONOMY 



67 



Vernier. 



170 

 170 10 



Watch. 



h. m. s. 

 4 5 30 

 4 6 27 



Vertical Circle = 32 8' 



d = 58" 



Vernier. 



170 10 

 170 20 



Watch. 



h. m. s. 

 4 23 18 

 4 24 17 



Vertical Circle = 28 46' 



^=39 



By a direct computation from the formula 1 



d = 57" cot (S + N) 



where N denotes the Bessel auxiliary, I find for the refrac- 

 tion in decimation at the time of these observations 59" 

 and 67" respectively, thus showing an agreement far within 

 the limits of error permissible in surveying practice. 



If, as is often the case, an accuracy of 20" is sufficient, 

 and the altitude of the sun is not less than 10, we may 

 dispense with the tabular values of N and write 



d = 2000 -r- hn 



where h is the altitude in degrees and the value of d is 

 given in minutes of arc. The error of this formula in the 

 preceding cases is 7" and 4", respectively. 



i Chauvenet, Spherical and Practical Astronomy, Vol. I, p. 171. 



