148 HISTORY OF THE [1860-70 cognisant of all his efforts for the advancement of lunar theory (M.N., 20, 166). I was not aware myself of the amount of anxiety which the bad representation of the lunar motions by the Tables of Burckhardt had caused, more than twenty years ago, in the minds of some of the greatest of our astronomers, till, when I was preparing for my present task, the Astronomer Royal, with his usual kindness, put into my hands his correspondence on lunar theory, continued from the time when he was first called upon to occupy his present position, namely, the year 1835, to the present time. Even if I had not known how many great works he has brought to completion in the same period how he has revolutionised the theory and the practice of the construction of astronomical instruments, as well as of the making and reducing of observations how he has borne the chief labour in almost every Government commission for scientific purposes, of which we need only mention the Standards Commission how he has occasionally been engaged in optical researches, or in the writing of profound memoirs in some branch of abstract or mixed science, some of which adorn our own Transactions and all apart from and in addition to the direct duties of his office I should have thought the amount of care and labour which this subject of the lunar theory brought upon him, even before he took upon himself the responsibility of the reduction of the ancient Greenwich Ob- servations, a very serious addition to the heavy duties which are necessarily imposed upon him by the ordinary administration of the observatory. The reductions of the Greenwich Lunar Observations 1750 1830 were published in 1848, and the supplementary work bringing the reductions complete to 1851 was published in the summer of 1859. Main's words serve to indicate the respect in which Airy's activity was held at the beginning of the decade with which we are here concerned. His work enabled astronomers to rise above the inadequacy of the Lunar Tables of Burckhardt [1773-1825], and prepared the way for the great advance made by Hansen [1795-1874]. The fruits of his labour were being gathered at the beginning of this decade. It must have been an immense satis- faction to Airy when Hind, the Superintendent of the Nautical Almanac Office, was able to give him the results of the computa- tions of the moon's places from Hansen's Tables complete for the years 1847 to 1858. Adams's discovery of errors in Burckhardt's treatment of the moon's parallax enabled Airy to give a rigorously fair comparison of Burckhardt's and Hansen's Tables with the Greenwich Observations, conclusively in favour of the accuracy of Hansen's. " Probably in no recorded instance has practical science ever advanced so far in accuracy by a single stride."
