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was erected; this telescope is still the principal instrument of its class at the observatory. And finally, in 1859, a large equatorial of 13 in. aperture was erected. In his journal for that year he remarks, “There is not now a single person employed or instrument used in the observatory which was there in Mr. Pond’s time.” The great work of reducing the accumulated planetary observations made at Greenwich from 1750 to 1830 had been already commenced and was in progress when Airy became Astronomer Royal. Shortly afterwards he undertook the further laborious task of reducing the enormous mass of observations of the moon made at Greenwich during the same period under the direction, successively, of Bradley, Bliss, Maskelyne, and Pond, to defray the expense of which a large sum of money was allotted by the Treasury. The immediate result of this work, which was not completed till 1846, was to rescue from oblivion no less than 8000 observations of the moon, and to place them at the disposal of astronomers in such a form that they could be used directly for comparison with the theory and for the improvement of the tables of the moon’s motion. For this work Airy received a testimonial from the Royal Astronomical Society in 1848. The first-fruit of this work was soon seen in the discovery by Prof. Hansen, of Seeburg in Gotha, of two new inequalities in the moon’s motion. After completing this labour, and before engaging in any theoretical investigation in connexion with it, Airy made inquiries whether any other mathematician was pursuing the same subject, and learning that Prof. Hansen had undertaken this problem under the patronage of the king of Denmark, but that, owing to the death of the king, it was in danger of falling through for lack of funds, he applied to the Admiralty for the sum required to enable Prof. Hansen to complete his work. This request was immediately granted, and thus it comes about that Hansen’s famous Tables, de la Lune are dedicated to La Haute Amiraute de sa Majeste la Heine de la Grande Bretagne et d’lrlande. One of the most remarkable of Airy’s researches is his determination of the mean density of the earth. In 1826 the idea occurred to him of attacking this problem by means of pendulum experiments at the top and bottom of a deep mine. His first attempt, made in the same year, at the Dolcoath mine in Cornwall, failed in consequence of an accident to one of the pendulums; a second attempt in 1828 was defeated by a flooding of the mine, and many years elapsed before another opportunity presented itself. The experiments eventually took place at the Harton pit near South Shields in 1854. The immediate result of the experiment was to show that gravity at the bottom of the mine exceeded that at the top by 1/19286th of its amount, the depth being 1256 feet. From this he was led to the final value of 6'566 for the mean density of the earth as compared with that of water. This value is considerably in excess of that previously found by other methods, but from the care and completeness with which the observations were carried out and discussed, it is, as Airy himself says, “entitled to compete with the others on, at least, equal terms.” In 1872 Airy conceived the idea of treating the lunar theory in a new way, and at the age of seventy-one he embarked on the prodigious labour which this scheme entailed. A general description of his method will be found in the Monthly Notices of the Royal Astronomical Society, vol. xxxiv. No. 3. His method consisted essentially in the adoption of Delaunay’s final numerical expressions for longitude, latitude, and parallax, with a symbolic term attached to each number, the value of which was to be determined by substitution in the equations of motion. In this mode of treating the
question the order of the terms is numerical, and though the amount of labour is such as might well have deterred a younger man, yet the details were easy, and a great part of it might be entrusted to a mere computer. This work was published in 1886, when its author was eightyfive years of age. For some little time previously he had been harassed by a suspicion that certain errors had crept into the computations, and accordingly he addressed himself to the task of revising his work. But his powers were no longer what they had been, and he was never able to examine sufficiently into the matter. In 1890 he tells us how a grievous error had been committed in one of the first steps, and pathetically adds, “ My spirit in the work was broken, and I have never heartily proceeded with it since.” In 1881 Sir George Airy resigned the office of Astronomer Royal and resided at the White House, Greenwich, not far from the Royal Observatory, until his death, which took place on 2nd January 1892. A complete list of Airy’s printed papers, numbering no less than 518, will be found in his Autobiography, edited by his son, Wilfrid Airy, B.A., M.Inst. C. E. Amongst the most important of his works not already mentioned may be •named the following:—Mathematical Tracts (1826) on the Lunar Theory, Figure of the Earth, Precession and Nutation, and Calculus of Variations, to which, in the second edition of 1828, were added tracts on the Planetary Theory, and the Undulatory Theory of Light; Experiments on Iron-built Ships, instituted for the purpose of discovering a correction for the deviation of the Compass produced by the Iron of the Ships (1839); On the Theoretical Explanation of an apparent new Polarity in Light (1840); Tides and Waves (1842). He was elected a fellow of the Royal Society in 1836 and president in 1871, and received both the Copley and Royal medals. He was five times president of the Royal Astronomical Society, was correspondent of the French Academy, and belonged to many other foreign and American societies. He was D.C.L. of Oxford, and LL.D. of Cambridge and Edinburgh. In 1872 he was made K.C.B. In the same year he was nominated a Grand Cross in the Imperial Order of the Rose of Brazil; he also held the Prussian Order “ Pour le Merite,” and belonged to the Legion of Honour of France, and to the Order of the North Star of Sweden and Norway. (a. a. r.*) Aisne, a department in the N.E. of France. The northern part is crossed by branches of the Ardennes. The country is watered by the Somme, the Escaut, the Sambre, the Oise, the Aisne, and the Marne. Area, 2868 square miles. The department comprises 87 cantons and 841 communes, and its population in 1901 numbered 535,583, against 555,925 in 1886. The chief towns are Laon, the capital of the department, Chateau - Thierry, St Quentin, with important industries and 48,868 inhabitants in 1896, Soissons, and Yervins. Births in 1899, 12,164, of which 1450 were illegitimate; deaths, 12,077 ; marriages, 4251. In 1896 there were 1363 primary schools, with 76,645 pupils. Eight per cent, of the population was illiterate. Agriculture is highly developed, 1,717,107 acres being under cultivation in 1896, of which 1,197,950 acres were ploughland, 210,274 acres forest, and 197,688 acres in grass. In 1899 the produce of wheat was valued at £1,960,120 ; rye at £176,000 ; oats at £983,240. The production of mangold - wurzel and of potatoes is also considerable. The production of beetroot (1899) exceeded 26,000,000 hundredweights, placing the department second in this respect. The live stock numbered (1899) 781,060, of which 77,870 were horses. The value of milk products in 1899 was £1,012,000. There are no metals in the department of Aisne, but there is abundance of building-stone and of brick-clay. The industries of Aisne are weaving, St Quentin being renowned for its cambrics ; plateglass manufacture (St Gobain); glass-making (Folembray) ; but especially sugar-refining. With a production in 1899 of 171,000 tons, involving the labour of 10,500 men and women, Aisne takes the lead of all the departments of France in the sugar industry. Altai uki.
See Cook Islands.
