M.-P. VOL. I.] CRAWFORD CONSTANT OF REFRACTION. 193 



ADDENDUM. 



The table on page 189 shows a large range in the values 

 of A, viz., from -{-0.00662 to 0.00584. Upon plotting 

 these values, using the zenith distance z for abscissa, and 

 A for ordinate, it is easily seen that A varies quite uniform- 

 ally with the zenith distance. A straight line, inclined 

 about 145 to the zenith distance axis, and cutting'it at z = 

 about 55, appears to represent A very well. Therefore, 

 assuming Z to be the zenith distance for A = o, we can set 

 up an observation equation of the following type for every 

 star : 



loga=loga -[- [Z z]x, 



or 



loga Ioga = A = Zx zx= D zx , 

 where 



and where a is the a of the tables used (Pulkowa). 



Equations of this kind were, accordingly, formed and 

 solved for Z and x by the method of Least Squares. 



