UO Astronomical and Nautical Collections. 



The sixth class of errors contains the intervals between the mi- 

 crometer wires, as they are deduced from different observations 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" re- 

 spectively in the fourth wire ; frequently also they arise from in«* 

 accuracies of observation. In the former case they are of no con- 

 sequence whatever, being easily detected at first sight; in the 

 latter they are fundamental imperfections; but such imperfections 

 are inseparable from the nature of observations, and it would be 

 ridiculous to expect from an astronomer that he should perform 

 impossibilities. All registers 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 

 seconds, 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 the fifth class, which comprehends the differences 

 between the polar distances observed with two and with six micro- 

 scopes, 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 accidental errors of ob- 

 servation, mixed with the changes of place of the stars and of the 

 refraction, or, lastly, changes of the place of the pole on the instru- 

 ment. For this last the observer can by no means be responsible. 

 Had the critic pointed out any new method of fixing the instru- 

 ment 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 

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

 parative and not an absolute stability. The fixing of the instru- 

 ments 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 



