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HISTOEY OF PHYSICAL ASTBONOMV.

any subject relating to astronomical science, when, as a preliminary opera-tion, it was found indispensable to execute an enormous mass of tediouscalculations. Indeed it is not to be expected, that any one can take suchan interest in the reduction of observations as the person who actuallymakes them, and hence it is reasonable to presume that he of all othersis best qualified for the execution of so laborious a task. Under a strongimpression of the imperfect state of all observations that were not imme-diately available for purposes of research, Mr. Airy, as soon as he wasappointed director of the Cambridge Observatory, introduced a system ofreduction as a normal part of his duties. The volume of results whichhenceforward issued annually from that establishment, formed a perfectmodel to the practical astronomer. Not only were the observations regu-larly reduced, but in so far as the planets were concerned, the results werecompared in every instance with the corresponding numbers calculatedfrom the most esteemed tables, so that the errors of the latter being thusexhibited in bold relief, served to invite rather than to repel the re-searches of the theoretical enquirer. This admirable system was shortlyafterwards introduced at Greenwich.

Another practice of great utility, which Mr. Airy adopted during hisdirectorship of the Cambridge Observatory, and which he followed upmore effectually upon his removal to Greenwich, was that of makingregular observations of the planets. This was a department of practicalastronomy, which for a long time had been very much neglected at theRoyal Observatory. The sun, moon, and principal stars were regularlyobserved as they passed the meridian; but, except during opposition, theplaces of the planets were very rarely determined. Mr. Airy’s atten-tion was strongly directed to the expediency of remedying this defect, bythe difficulty which he experienced in procuring suitable observations ofthe planets, while engaged in researches on physical astronomy. Theplanets are now invariably observed at Greenwich on every practicableoccasion, at whatever hour they pass the meridian.

From the intimate connexion which was found to subsist between thelunar theory and the problem of the longitude, the moon has formed anobject of unremitting attention at the Observatory of Greenwich, ever sinceits establishment in 1075. This constancy of purpose has not failed toproduce its due fruits, for it is universally acknowledged that, in so far asobservation is concerned, the present advanced state of the lunar theoryis almost exclusively attributable to the labours of the Greenwich astro-nomers. It is therefore an object involving in an especial degree thehonour of the Royal Observatory, to render the observations of the moonas accurate and as complete as possible. In pursuance of this object Mr.Airy has recently introduced the use of an altitude and azimuth instru-ment at Greenwich, for the purpose of making observations of the moon inany part of her course above the horizon. Tw r o distinct circumstancessuggested the expediency of deviating in this respect, from the plan of ob-servation hitherto pursued at the Royal Observatory. In the first place,it frequently happens, from the unfavourable state of the weather, that themoon cannot be seen when she is on the meridian. Secondly, althoughthe heavens should be perfectly serene, it is found to be impossible todiscern the moon on the meridian for several days before and after herconjunction with the sun, on account of the overpowering effulgence ofthe solar rays. It happeued from this circumstance that throughout anarc comprehending as much as one-third of her orbit, the moon hitherto