140 



EOGEES AND McLEOD ON THE 



mined very frequently from the collimators. In the series of observations extending from 

 18*70 to IStS, the value of Bessel's constant n was derived from equations of the following 

 form, in which the nomenclature given above is followed : 



= j; + tan Ô, n + (JT + m) 

 = Jo' + tan â.^ n + (JT + m) 

 Q = J„^+ tan ô,n+ (JT + m] 

 = /]* + tan (Î' n + (JT + «0 

 = J/ + tan S" 7i + (JT+m) 



Ai /) 1 A." — A:- 



= 



= 



= 



tan Ô" — tan â., 



A' -A 



: +'h 



tan tf, — tan tf, 



n = the value adopted for the evening- 

 tan rf"— tan rf, ' ■' of observation. 



After the most careful discussion of the observed values of the collimation for each 

 year, it was still found impossible to satisfy the relation : — 



which should hold if every source of error was taken into account in the fundamental 

 equation. These relations are given in the following table. 



Year. 



1871 . . . 

 1872... 

 1873... 

 1874... 

 1875... 

 187r>... 

 1877... 

 Means. 



— .008 



— .003 

 + .001 



— .001 

 .000 



+ .002 



— .001 



— .001 



s. 

 — .002 



.000 



+ .008 



+ .010 



+ .00.5 



+ .007 



+ .006 



+ .005 



s. I s. 



+ .015 I + .012 



+ .006 I + .013 



— .002 1 — .004 



— .006 



— .001 



— .002 



s. 



— .036 



— .008 



— .005 



— .003 

 + .001 



— .014 



— .002 I — .009 ! + .006 — .001 



— .006 1 — .017 + .005 — .004 



— .004 I — .025 + .007 — .001 



— .002 I — .017 + .007 , — .003 



"i — "2 



+ .003 

 + .008 



.016 ; + .011 



It will be seen from this table that the value of the constant », which satisfies the 

 observations near the equator, fails to satisfy those situated near the zenith by an average 

 amount of nearly 0'.02. Since this variation does not appear to be dependent either on 

 the position of the instrument or upon the season of the year, it must probably be referred 

 to the personality of the observer. 



Secondly, during the past year, the writer undertook, in conjunction with Mr. S. C. 

 Chandler, assistant at the Observatory, a series of simultaneous observations with the 

 Meridian Circle, and with a new instrument invented by him and named " The Almucantar," 

 for the purpose of testing a new form of level attached to the meridian circle, as well as for 

 the purpose of making a direct comparison of the two instruments in the determination of 

 absolute time. Another question of special importance to Mr. Chandler, and for which 



