OBSEBVATIONS WITH THE ZENITH TELESCOPE. 



281 



In so far as the problem of latitude variation is concerned extremely precise values 

 of the stellar co-ordinates are not required, nor is it important to reduce them to a uni- 

 form system, in the present case, however, a preliminary reduction of the observations 

 showed very appreciable changes in the value of the micrometer during the progress of 

 the work. These were probably due to the wearing of the screw as time went on. It is 

 believed that the prejudicial effect of this change has been practically removed by deriv- 

 ing the screw value from the latitude observations, employing intervals sufficiently small 

 to warrant regarding it as constant during the interval. 



If the latitude stars could have been selected so that the plus and minus values of 

 the micrometer correction in each group should exactly balance, no harm would follow 

 if small errors of the screw were disregarded. Tins condition cannot, however, be 

 fully satisfied, at least not for any considerable! time, as precession would soon destroy the 

 equilibrium if such existed. 



In case (his method of treatment is to be free from objection, the Declinations 

 employed must have a high degree of precision. For this reason the matter lias received 

 a, greater amount of attention than would have been necessary otherwise. 



It is hoped, moreover, that the results may prove valuable in lines of investigation 

 other than that which was its immediate object. Willi this end in view the Right 

 Ascensions have been reduced with the same care and thoroughness as that given to I he 

 Declinations. This part of the investigation is the work of Mr. Evans. 



The system adopted is that of Anwers, as found in I ho Astronomische Nachrichten, 

 Vol. 134, p. 33. It is not to be understood that this system is considered superior to 

 thai, of Newcomb ; but since Auwers' system has boon more commonly employed in (his 

 class of investigations, it was thought best to retain it here for tin; sake of uniformity. 



Authorities. 



In the list of catalogues employed it is not thought necessary to give more than the 

 names of those which are generally well known. The works themselves should be found 

 in every astronomical library. In case they are not readily accessible, further informa- 

 tion regarding them may be obtained from Newcomb's New Catalogue of Fundamental 

 Stars, Davis' Declinations and Proper Motions of Fifty-six Stars, and Boss' Declinations 

 of /''/'.red, Stars. 



In case of all catalogues for which Auwers gives systematic corrections, these have 

 been applied. In other cases such corrections have been derived indirectly through the 

 systems of Newcomb or Boss. Where recourse has been had to other methods, mention 

 of the process is found in the accompanying notes. In some of the recent catalogues the 

 uncorrected places have been employed. For the sake of completeness and uniformity, 



