Ball — Researches on Annual Parallax. 



357 



Micrometric Measurements of the Difference of Declination letween 61 (B) 

 Cygni and BD + 38°, 4351, on 16th November, 1878. 



Screw I. 



Screw II. 



Screw I. 



Screw II. 



Screw I. 



Screw II. 



Screw I. 



Screw II. 



63698 



40627 



55783 



32139 



64318 



40047 



55599 



32446 



63753 



40598 



55857 



32134 



64163 



40177 



55562 



32506 



63791 



40537 



55902 



32063 



64175 



40237 



55501 



32549 



63823 



40537 



65921 



32074 



64097 



40257 



55483 



32633 



55639 



32356 



64217 



40184 



55906 



32143 



63557 



40863 



55666 



32353 



64215 



40142 



55697 



32293 



63293 



41103 



55732 



32268 



64202 



40113 



55716 



32303 



63267 



41088 



55753 



32262 



64213 



40095 



55708 



32348 



63154 



41228 



Each of these eight columns had first to be reduced to seconds. 

 This is accomplished by adding together the four larger numbers in 

 each column, and subtracting from the result the sum of the four 

 smaller numbers. JS. g. for the first column this result is 32"275. This 

 has to be multiplied by one-eighth of the value of the micrometer 

 Screw I., at the temperature of the instrument (39°"9). The result 

 thus found is 36"*292. In a similar manner the movement of Screw II. 

 in the second column gives 37""224. Thus the first set of measures on 

 the night in question gives for the apparent total distance the result 

 73"'516. This must, however, receive a correction of + 0"'026 on 

 account of refraction, and of - 0"'031 on account of reduction for aber- 

 ration, precession, and nutation, so that the final result is 73""511. In 

 ,a similar manner we have for each of the other sets of observations on 

 the night in question 73"-705, 73"-539, 73"-453. The mean of the 

 whole four is 73"-552, which is accordingly taken as the final result 

 of the observations for this one night. 



In many cases the number of complete measures was less than four, 

 so it has been found convenient, when estimating the weight attached 

 to each night's work, to use the number of complete measures which 

 have been obtained. Thus, for the night just considered, the weight 

 is taken as 4. 



The following Table contains the mean results of the observations 

 mi the several nights which are included in the present series : — 



