The observations of stars at these four observatories are directed to the most accurate determination of the places of a limited number, and the deduction of their proper motions by comparison with the results obtained by Bradley, Piazzi (with an altazimuth by Ramsden at Palermo), and Groombridge; but at other observatories differential or zone observations of large numbers of stars have been made, with the object of making a com plete and tolerably accurate survey of the heavens, the rhomb or ring micrometer being used for this purpose. Among those who have devoted themselves to this work may he mentioned LaaaiIle at the cape of Good Hope, Lablade at Paris, Bessel at Konigsberg, and Argelauder at Bonn. These zone-observations are now being repeated with the transit-circle at a number of observatories, associated together for the purpose of getting far more accurate places than was possible with the equatorial. A large number of observatories, chiefly in Germany and America, are devoted to a very different class of observations—viz., differential observations with the equatorial (q.v.) of comets and small planets as referred to comparison-stars, and the search for such objects; whilst at other observatories, among which that of Pulkowa may be mentioned, the measurement of double stars with the micrometer is laid clown as the chief object. Of late years two new subjects have been introduced in the routine of observatory work—photography and spectroscopy. The former was carried on for many years at Kew observatory under Mr. de la Rue's auspices, and at his private observatory at Cranford, and the work is now being continued at Greenwich: the latter has been taken up at a number of Italian observatories, and particularly at Rome by P. Secchi, and it now forms part of the regular system at Greenwich; whilst the observatories at Paris, Berlin, and Vienna are equipped for these physical observations, and in America and Australia they are vigorously carried on at several observatories—Melbourne, in par ticular, being provided with a fourfeet equatorial reflector for this purpose, as well as for the examination of nebulae. The most important work of an observatory, however.
consists, not iu making observations, which are easily multiplied, but in reducing and publishing them—a task of far greater labor, and requiring far higher qualifications. Ifo•ever may be the observations, the method of eliminating their errors is the same in all cases, and similar mathematical considerations apply to their reduction, whether they lie meridian observations, micrometer measures, measures of photographs, or spectroscopic observations; and it is when such treatment is required in any inquiry that it should be undertaken at a public observatory, where this rigorous method will be applied.
The work of private observatories hardly admits of being specifi6d, though its general character has already been indicated; it may suffice to mention the observations of double stars and nebular by the two Herschels, Groombridge's catalogue of circumpolar stars, Smyth's double-star measures, Carrington's Redhill catalogue and solar observations, the nebular observations of Lord Rosse aid Mr. Lassell, De la Rue's long series of photo graphs, and the spectroscopic observations of Huggin's and Lockyer.
In addition to regular astronomical observations of all kinds, national observatories are usually charged with the distribution of time signals, and the rating of chronome ters for the navy—matters of great practical importance, especially in this country, where Greenwich time is communicated directly by telegraph to more than six hundred towns.