392 Scientific Notices — Miscellan^m, [NoV. 



ferences" between the polar distances observed with two and 

 with 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 y Draconis, 28th March, or slight 

 accidental 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 oift any new method of fixing the instrument so that it 

 should be subject 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 pro- 

 curing a firm foundation for the pillars, affords us only a com- 

 parative and not an absolute stability. The fixing of the 

 instruments at Greenwich has been such as to keep them for a 

 long time admirably firm; but at other times it has not been so 

 successful, as may be seen in the table of the place of the pole, 

 printed in the Nachrichten, 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 accidental errors of observation, nor are they of 

 material importance, as they may be readily determined by a 

 series of observations of the pole star, so complete as those 

 which are made at Greenwich. The accidental irregularities of 

 the polar distances, which remain after the correction of the 

 place of the pole, can be as little considered as an imputation 

 on the accuracy of the observer, as those of the intervals of 

 the micrometer wires. The truth of this remark is illustrated 

 in the Nachrichten, No. 73. 



The fourth class contains the differencCvS between the times 

 of transits observed with the transit telescope, and the mural 

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

 the 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 Avhich are made merely with a 

 view of determining its place with respect to the meridian. 

 We dught to acknowledge the occasional insertion of these 

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

 ment never deviates so much from the meridian as to affect the 

 Solar distances; but they are not intended for any other purpose, 

 either Bradley nor Maskelyne have ever noted the times of 

 the transits by their mural quadrant, although it was more 

 liable tb vKriktiorl than the mural circle. But to correct the 

 place <if thfe axis of this circle continually, so as to bring it 

 perfectly'irito the plane of the meridian, would certainly be of 

 no advaiifei^^ to the Greenwich observations. 



Other wbrs which are criticised, for example, those of the 

 nameJ»t)f'tW4^ fetes, of the hour or minute of their transits, and 



