126 ROGEES AND McLEOD ON THE 



Letter of Transmission. 



harvard college observatory, 



Cambridge, Nov. 2, 1885. 



Dear Sir: Professor Rogers has completed his reduction of the observations made 

 in determining the difference of longitude between the observatories of McGrill College 

 and of Harvard College. I accordingly send you by express the papers containing the 

 results of his work. 



Yours respectfully, 



Edward C. Pickering. 

 Professor C. H. McLeod, 



Superintendent of the Observatory, McGill College, 

 Montreal, Canada. 



PART II. 



The instrument employed in the observations at the Cambridge station is a broken 

 transit made by Herbst, the mechanician of the Pulkowa Observatory at St. Petersburgh, 

 Russia. It is described on pages 28-29 in Vol. XVIII of the Annals of the Observatory. 

 It has been in use since September, 18Y0, and it has served as the type of all the instru- 

 ments of this class since constructed in the United States. 



The following instrumental constants were determined at the Pulkowa Observatory ; 



Value of 1 divLsioii of the level = 1".9.3 ± 0".01 

 Length of the bubble tube - =36 divisions. 



Inequality of pivots determined by level readings taken at every 5 degrees from 



— 60° to + 60° zenith distance : 



Circle East +0.082 in half divisions of the level. 



Circle West— 0.082 



Correction for Circle East := -|- 0".079. 



Value of 1 division of micrometer screw = 0'.0619. 



The reticule furnished with the instrument consisted of 11 spider line threads, but 

 this has been replaced by a glass retictile consisting of 25 threads arranged in groups of 5 

 threads each. The values for the reduction of the mean of each group to the mean of all 

 the groups, as obtained from 21 fully observed star transits between the declinations 



— 22' and + 72", with the circle east, are 



+ 421554, -f 2rl25'7, — 0^003, — 21^2'7'7, — 42^.550 



(The similar values for the Montreal transit, as obtained from 24 observations between 



— 22' and -f 65°, for lamp east, are + 32^753, + 15^707, -f O'.OOl, — 15^720, — 32^740). 



