490

HISTORY OF PHYSICAL ASTRONOMY.

the fourth the corresponding places are calculated from the best existingtables. In the fifth the observed and tabular places are compared to-gether, and the resulting errors exhibited.

When we consider that the observations which form the subject of thisimmense volume, were made by various individuals, with instruments verydifferent from each other, and by methods totally dissimilar, we may formsome conception of the difficulty of eliminating the systematic errors towhich they were liable, and assimilating them by legitimate principles soas to exhibit them in the character of one homogeneous collection ofresults. The execution of this great undertaking renders the Greenwichobservations of the planets from 1750 to 1830 immediately available forpurposes of theoretical research, leaving in this respect nothing further tobe desired.

The reduction of the Greenwich lunar observations from 1750 to 1830was undertaken at the public expense in consequence of a representationto that effect having been made to the Government by the British Asso-ciation, agreeably to a suggestion of Sir John Lubbock’s on the occasionof the meeting of the Association, which was held at Liverpool in 1837.The organization of the plan of reduction, as w T ell as the superintendenceof its execution, was in this instance also confided to Mr. Airy. In con-sequence of the multitude of inequalities by which the moon’s motion isaffected, the undertaking was one of stupendous magnitude. In order toform some idea of the labour expended in its execution, it may be men-tioned that upwards of 8000 places of the moon were deduced from theobservations, and compared with the corresponding places calculated fromthe tables. An extraordinary force of computers was employed for severalyears at the Royal Observatory in these calculations. The results werefinally published in the year 1848, in two quarto volumes*. Mr. Airy,who is distinguished no less as a theoretical than as a practical astronomer,has made them the subject of profound discussion, with a view to obtaina more accurate knowledge of the moon’s motion, and has announced theconclusions at which he arrived in a paper which appears in the seven-teenth volume of the “ Memoirs of the Astronomical Society.”!

Allusion has already been made to Pond’s method of determining thezenith distance of a celestial body at one transit by a combination of

* The calculations connected with this great undertaking were superintended by Mr.Hugh Breen, who had the melancholy satisfaction of just living to see its completion.Mr. Airy, who bears honourable testimony to his skill and accuracy, states, at the begin-ning of the first volume of the published results, that after a short illness he expired onthe morning of April 1, 1848, only a few hours having elapsed after the last supple-mentary tables had been sent to the press.

+ Allusion has already been made to some of the more important results of these re-searches in a former part of this work. Another very interesting confirmation of them ismentioned by Prof. Hansen, in a letter to the Astronomer Royal, dated June 27, 1850.(Mo. JProc. ~Ast. Soc., June, 1850.) Mr. Airy had deduced + 1'.721 as the correctionof Damoiseau’s value of the secular motion of the lunar node. Prof. Hansen, havingcaused one of his assistants to compute, by the modern tables, the nineteen ancient, eclipsesrecorded by Ptolemy in the Almagest , found by a comparison of the results with the re-corded observations, that the correction of the same element should be + 1'.643. Theclose agreement of these two values will appear very remarkable, when the complexityof the moon's motion is taken into account. Professor Hansen states that they bothagree as nearly as could be desired, with the corresponding value deduced by him fromthe theory of gravitation. The result of these different researches completely refutes thestatement recently put forth by Prof. Seiffarth of Leipsic, to the effect that the ancienteclipses indicated the necessity of a considerable correction of the motion of the lunarnode.