Prof. Bessel on the Greenwich Observations. 293 



The greater number of the errors which have been pointed 

 out by the censor are merely accidental errors of the pen. 

 Errors of this kind are certainly disagreeable, and it would be 

 better if they could be entirely avoided ; but since all collec- 

 tions of observations in existence do contain such errors, they 

 clearly appear to be unavoidable. 



The ^rst class of errors mentioned in the Philosophical 

 Magazine contains the cases in which the mean deduced from 

 the readings of the two microscopes A and B differs from the 

 column in which that mean is assigned. Since there must be 

 some manifest oversight in all these cases, it may sometimes 

 be difficult to determine whether it is in the readings or in the 

 mean assigned ; but it will, in general, be easy to distinguish, 

 from the preceding or following observations of the same star, 

 where the error lies. 



The second class contains the differences between different 

 records of the same observation. These must be errors in the 

 copies sent to the press, and not in the readings of the micro- 

 scopes ; and they may generally be corrected by a compari- 

 son of the two passages : they sometimes extend to whole de- 

 grees, or to the tens of the minutes, and are then of no im- 

 portance ; for example, in the observations oiProcyon the 23d 

 February 1821, and of/3 Cepheit\\Q 8th December, where there 

 are errors of 30° and 5° respectively. 



The sixth class of errors contains the intervals between the 

 micrometer wires, as they are deduced fi-ora different observa- 

 tions of the same star. These are often dependent on errors 

 of the pen, as in the observation of Capella on the 7th February, 

 and in that of Sirius on the 8th, where there are errors of 5" 

 and of 40" respectively in the fourth wire : frequently also 

 they arise from inaccuracies of observation. In the former 

 case they are of no consequence whatever, being easily de- 

 tected at first sight ; in the latter they are fundamental imper- 

 fections ; but such imperfections are inseparable from the na- 

 ture of observations, and it would be ridiculous to expect from 

 an astronomer that he should perform impossibilities, All re- 

 gisters of observations exhibit inaccuracies of this kind; and if 

 any should be produced without them, it might with confidence 

 be asserted to be a forgery. The diligence of the astronomer 

 is proved, not by the perfect agreement in his tenths of se- 

 conds, but by the magnitude of his mean or his probable error; 

 and it would probably be difficult for the critic to prove that 

 this error is much greater in the Greenwich observations than 

 the nature of the instruments renders unavoidable. 



The errors of ihcfiflh class, which comprehends the diffe- 

 rences between the polar distances observed with two and with 



six 



