114 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



fall and the winter months. The maximum differences 

 observed were — 2°.o ( F) and -f- 2°.i (F). There was 

 one still larger difference, viz. — 3°.7 (F), which can 

 hardly be counted in the series, for it occurred on a poor 

 night, immediately after observing had been suspended 

 because of clouds and poor "seeing." The hot wave, 

 which caused the outside temperature to rise suddenly, 

 undoubtedly destroyed the "seeing." Although the 

 winter months present conditions not so favorable as those 

 of the summer months, nevertheless they also speak well 

 for the efficiency of the Lick Observatory Meridian 

 Circle house. 



4. Plan fo7' Observing. — The method of determining 

 the refractions here may be stated as being a quasi con- 

 verse to Talcott's method of determining the latitude. 

 Instead of eliminating the refractions to get the latitude, 

 the method is to determine the refractions by eliminating 

 the latitude, as follows: 

 Let 



z s = the zenith distance of a southern star, 

 z n = the zenith distance of a northern star, 

 z' s = the apparent zenith distance of the southern star, 

 z' n = the apparent zenith distance of the northern star, 

 8 S = the declination of the southern star, 

 S n = the declination of the northern star, 

 r s = the refraction of the southern star, 

 r n = the refraction of the northern star, 

 <p = the latitude of the Meridian Circle. 

 Then 



«n = S P + Zn = ?+(Zn+r») (i) 



K=<P— z s =f— (z' s -f-r s ) (2) 



S u — 8 8 =z' B + z' ll -{-r s +r n (3) 

 Let 



A=S-S S (4) 



B=z' s +z' n (5) 



Then 



or 



A=B + r s +r n (6) 



r B +r n =A— B (7) 



