486 Proceedings of the Royal Irish Acadetny. 



In the different equations which, in the course of years, were found 

 between Bessel and AY. Struve, there was a regular variation: 



1814-8. B. - S. =-0=-04. 



1820-9. 



M 



-0-68. 



1821-1, 



); 



-0-80. 



1823-5. 





- 1-02. 



1834-5. 





- 0-77.* 



The two comparisons from 1821 and 1823 are indirect, derived 

 from the differences, Struve -Walbeck and Struve - Argelander, found 

 in Dorpat by direct comparison. The variation is very striking, and 

 this circumstance, as well as the considerable amount of the personal 

 equations, whose reality now was beyond doubt, showed the necessity 

 of examining this remarkable source of error in all its details. It is, 

 however, very seldom that personal differences are as large as between 

 Haskelyne and Kinnebrook, or between Bessel and his assistants. The 

 accordance between the different equations found in Konigsberg, 

 with respect to quantity and sign, makes it most probable that Bessel 

 observed about a second earlier than most astronomers do ; and he 

 would probably have agreed tolerably well with Alaskelyne, as the 

 difference B. — Kinnebrook, in the opposite case, would have amounted 

 to nearly 2^ 



As soon as the existence of the personal equations had been, 

 acknowledged in the scientific world, other astronomers began to make 

 researches in this dii'ection. First of all observatories, that in Altona, 

 directed by Schumacher, imitated those in Konigsberg and Dorpat, 

 and the following remarkable differences were found there in 1833 : — 



]S"ehus - WoKers - + 0^-73. 

 Petersen - ILiidler = + 0'-52.f 



These observations were made during a detenuination of longitude 

 by transport of chronometers, and since that time veiy few determi- 

 nations of longitude have been undertaken, without the observers hav- 

 ing compared their method of observing, as the whole amount of the 

 personal equations otherwise would make a part of the result. The 

 investigations of personality in observing transits, which have been 

 made on the occasion of determinations of longitudes, are very impor- 

 tant, and have produced many of the most reliable results we have 

 derived on this special subject. Besides, the plan for the observations 

 adopted in several observatories has rendered constant determinations 

 of the personal faults of the observers necessary ; and this has espe-- 

 cially been the case in Greenwich, where all the instruments are 

 used alternately by several observers. From the year 1838, the 

 volumes of the Greenwich Observations contain interesting discussions 

 on the equations between the different observers, which we shall often, 

 have occasion to quote in the following pages. 



* Konigsberger Beobachtungen, viii., pp. 5, 6 ; ibid. xx. p. 31. 

 t Astronomiscbe Xachiichten, xiii., Xo. 308 ; xhx., Xo. 1154. 



