Dreyer — On Astronomical Transit Observations. 



49^ 



This regular Tariation is really surprising. There are several othe" 

 examples to be found in the Grreenwich observations, but none so 

 striking as the above-mentioned one ; for instance, the equation "W. 

 Ellis - Hogerson was — 



In 1846 

 „ 1847 

 „ 1849 

 ,, 1850 

 ,, 1851 

 „ 1852 

 ,, 1853 



- 0= 

 -0 

 + 

 + 

 + 

 + 

 + 



•11 

 •22 

 •12 

 •45 

 •36 

 •44 

 •62 



From these two examples we learn that it was I\Ir. Eogerson 

 ■whose error was gradually increasing in the course of years. On the 

 other hand, there may be found examples of a nearly perfect immu- 

 tability, but most commonly a personal equation will be found to 

 vary a little, withou.t following any distinct law. In order to illus- 

 trate how the relations of two observers can be at different epochs, I 

 have promiscuously taken the foUoAving specimens from the Green- 

 wich observations : — 



E^-E- 



\nd-Ear Method, j 



Chronograph Method. 





Dunkin 



Main 





Dunkin 



Stone 



Dunkin 



Year. 



and 



and 



Year. 



and 



and 



and 





Henr}'. 



Henry. 





Criswick. 



T. Ellis. 



Stone. 



1841. 





- 0^-09 



1855. 



- 0^-03 







1842. 



— 



- -01 



1856. 



-0 -10 







— 



1843. 



— 



- -02 



1857. 



-0 -10 



— 



— 



1844. 



+ 0^ 



30 



- -05 



1858. 



-0 -cs 



— 



— 



184.5. 



-0 



15 



- -12 



1859. 



- -13 



— 



— 



1846. 







00 



- -05 



1860. 



- -14 



+ 0^-02 



+ 0=-07 



1847. 



+ 



24 



-0 -03 



1861. 



- -15 



0. 00 



+ 0-13 



1848. 



- 



01 



- -04 



1862. 



- -lo 



+ '01 



+ -14 



1849. 







00 



- -05 



1863. 



- -16 



+ -02 



+ -14 



1850. 



- 



08 



-0 -11 



1864. 



-0 -12 



+ -04 



+ -13 



18.51. 



-0 



11 



- -11 



1865. 



-0 -12 



+ 0-06 



+ -09 



1852. 



- 



13 



-00 



1866. 



- ; 



+ -12 



+ -04 



1853. 



-0 



12 



+ -03 



1867. 



-0 -13 



+ -13 



+ -02 



1854. 



-0 -17 



— 



1868. 



- -10 



+ -13 



+ -05 









1869. 



-0 -11 



+ -17 



+ -03 



I 







1870. 



- -11 



- -01 



+ -17 



It has often been said, that a large personal error in many cases 

 can, by practice, be reduced to a considerably smaller one. C. "Wolf 



undertaken by Pape (Astr. Xachr., liv.), Dunkin (Monthly Notices, R.A.S., xx., 

 xxiv.j and Albrecht (Ueber die Bestimmung von Langendifferenzen, page 3). 



