LATITUDE OF MOKATTAM. 265 



has been used as the rate. Generally the interval is less than six hours, and 

 the " Adopted Rate " has been used. 



ZENITH DISTANCES OBSERVED AT MOKATTAM, AND INFERRED CO-LATITUDE. 



(Table XIX.) 



The Vertical Circle, or Altazimuth, used at Mokattam was similar in every 

 respect to the Honolulu instrument. It was mounted on a pier of masonry 

 founded on the solid rock, and was protected by a wooden hut with a 

 revolving roof. 



The mean corrections to the reading of the Vertical Circle for runs of the 

 four Micrometer screws for 100" were 



1874, November 4, +0-09 

 5, 0-76 



27, + o - o5 



28, 0-17 



The two levels parallel to the plane of the Vertical Circle were each 

 graduated with 30 divisions to one minute of arc. The Level Indication is 

 half the sum of the four readings. The Mercurial Barometer and the External 

 Thermometer, both by Messrs. Horne and Thornthwaite, were verified at 

 the Royal Observatory, Greenwich, and require no correction. The refractions 

 have been computed from the tables forming the Appendix to the Grecmuicli 

 Observations for 1853, and then reduced in the proportion of T0053 : 1. 



The comparisons by which the error of the Altazimuth Clock was obtained 

 are given in Table XVIII. The error is required only for computing the 

 Reduction to tJie Meridian. 



Co-latitude of Mokattam Station (Table XIX.) 



Mean of 6 Stars North of Zenith, 59. 58. 14-2 N. 

 6 South 59.58.13-1 



Adopted Co-latitude of the Altazimuth Pier, 59. 58'. 13"'6 N. 



G. L. T. 



