29i Prof. Bessel on the Greervwich Observations. 



six microscopes, seem to me to have been introduced without 

 the least propriety: they are either insignificant errors of the 

 pen, as in the case of y Draconis 28th March ; or slight acci- 

 dental errors of observation, mixed with the changes of place 

 of the stars and of the refraction; or, lastly, changes of the place 

 of the pole on the instrument. For this last the observer can 

 by no means be responsible. Had the critic pointed out any 

 new method of fixing the instrument so that it should be sub- 

 ject to no alterations, he would have deserved the thanks 

 of all practical astronomers ; but the constant result of past 

 experience shows that the greatest possible care, in procuring 

 a firm foundation for the pillars, affords us only a comparative 

 and not an absolute stability. The fixing of the instruments 

 9t Greenwich has been such as to keep them for a long time 

 admirably firm ; but at other times it has not been so success- 

 ful, as may be seen in the table of the place of the pole, printed 

 in the Nac/iric/iten, No. 73 ; the differences between the latter 

 days of July and the beginning of August 1821 depending on 

 a change of this kind, so that they cannot be considered as ac- 

 cidental errors of observation ; nor are they of material impor- 

 tance, as they may be readily determined by a series of obser- 

 vations of the pole star, so complete as those which are made 

 at Greenwich. The accidental irregularities of the polar di- 

 stances, which remain after the correction of the place of the 

 pole, can be as little considered as an imputation on the accu- 

 racy of the observer, as those of the intervals of the micrometer 

 wires. The truth of this remark is illustrated in the Nachrich- 

 teti. No. 73. 



The fourth class contains the diffex-ences between the times 

 of transits observed with the ti-ansit telescope, and the mural 

 circle. The latter instrument, however, not being intended for 

 t4ie observation of transits, nor being ever actually so employed, 

 it would have been of no manner of use to seek for greater 

 accuracy in the memorandums which are made merely with a 

 view of determining its place with respect to the meridian. 

 We ought to acknowledge the occasional insertion of these 

 memorandums with gratitude, as they assure us that the in- 

 strument never deviates so much from the meridian as to affect 

 the polar distances ; but they are not intended for any other 

 purpose. Neither Bradley nor Maskelyne have ever noted the 

 times of the transits by their mural quadrant, although it was 

 more liable to variation than the mural circle. But to correct 

 the place of the axis of this circle continually, so as to bring it 

 perfectly into the plane of the meridian, would certainly be of 

 no advantage to the Greenwich observations. 



Other errors which are criticized, — for example, those of the 



names 



