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A I R Y 233 smooth-bore tube, 15 inches or less in diameter. The full-calibre astonished his contemporaries. Under his direction a shell weighs 1000 lb, and carries a bursting charge of 600 lb of blasting gelatine, cut into the form of cheeses, titting the steel mural circle was soon erected, and regular observations envelope, and provided with a core of dry gun-cotton as a primer. were instituted with it in 1833. In the same year the Sub-calibie projectiles, 10 in. and 8 in., can also be used. In duke of Northumberland presented the Cambridge obsertheir case, rotation is given by vanes or fins attached to the'body vatory with a fine object glass of 12 inches’ aperture, of the shell. Air at 1000 lb pressure is stored in tubes close to the gun, and is supplied from primary reservoirs, to which it is which was mounted according to Airy’s designs and under directly pumped at a pressure of about 2000 lb. There is always, his superintendence, although the erection was not comtherefore, a considerable reserve of power available without pumping. pleted until after his removal to Greenwich in 1835. Pneumatic guns of this description (see figure) have been mounted for the protection of New York and San Francisco. With a full- Airy’s writings during this time are divided between calibre shell (1000 lb) these guns have a range of 2400 yards ; with mathematical physics and astronomy. The former are for a sub-calibre 8-in. shell (250 lb) the maximum range is 6000 the most part concerned with questions relating to the yards. The official trials showed remarkable accuracy. At 5000 theory of light, arising out of his professorial lectures, yards 75 per cent, of the projectiles fell in an area of 360 x 90 feet. among which may be specially mentioned his paper “ On When the gun was tried at Shoeburyness the accuracy was far greater than could be obtained with howitzer shells propelled by the Diffraction of an Object-Glass with Circular Aperture.” explosives. On account of the power of exploding the shell under In 1831 the Copley medal of the Royal Society was water, and thus securing a torpedo action, a direct hit upon a ship awarded to him for these researches in optics. Of his is not required, and the target offered is largely in excess of the astronomical writings during this period the most imdeck plan. The gun is, in fact, capable of replacing systems of portant are his investigation of the mass of Jupiter, his submarine mines with economy, and without the great objection report to the British Association on the progress of of interfering with a waterway. The only employment of the dynamite gun afloat has astronomy during the 19th century, and his memoir On been in the case of the U.S.A. gunboat Vesuvius, which an Inequality of Long Period in the Motions of the carries three in the bows. These guns are fixed at a Earth and Venus. His report is remarkable for the conconstant angle of elevation, and the range is regulated by ciseness and accuracy with which the condition of the the air valve, training being given by the helm. Thus science at the time is exhibited. The last section but mounted on an unstable platform, the accuracy of fire one of this report is devoted to “ A Comparison of the obtainable must evidently be much less than on shore. Progress of Astronomy in England with that in other The Vesuvius was employed during the Spanish-American Countries,” very much to the disadvantage of England. war of 1898, when on several nights in succession she This reproach was subsequently, to a great extent, reapproached the defences of Santiago under cover of dark- moved by his own labours. Airy’s discovery of a new inequality in the motions of ness and discharged three projectiles. Fire delivered under such conditions could not be sufficiently accurate to injure Venus and the earth is in some respects his most remarkIn correcting the elements of coast defences; but the shells burst well, and made large able achievement. Delambre’s solar tables he had been led to suspect an craters. (G. s. c.) inequality not embraced by those tables. For the cause Airy, Sir George Bidden (1801 1892), British of this he did not long seek in vain. Eight times the Astronomer Royal, was born at Alnwick on 27th July 1801. mean motion of Venus is so nearly equal to thirteen times He came of a long line of Airys who traced their descent that of the earth that the difference amounts to only the back to a family of the same name residing at Kentmere, 1/240th of the earth’s mean motion, and from the fact in Westmoreland, in the 14th century, but the branch to that the term depending on this difference, although very which he belonged, having suffered in the civil wars, small in itself, receives in the integration of the differential removed to Lincolnshire, where for several generations equations a multiplier of about 2,200,000, Airy was led they lived as farmers. George Airy was educated first to infer the existence of a sensible inequality extending at elementary schools in Hereford, and afterwards at over 239 years. The investigation by which this result Colchester Grammar School. In 1819 he entered Trinity was established w^s probably the most laborious that had College, Cambridge, as a sizar. Here he had a most been made up to Airy’s time in the planetary theory, and brilliant career, and seems to have been almost imme- was the first specific improvement in the solar tables made diately recognized as the leading man of his year. In in England since the establishment of the theory of gravi1822 he was elected scholar of Trinity, and in the follow- tation. In recognition of this work the medal of the ing year he graduated as senior wrangler and obtained Royal Astronomical Society was awarded to him in 1833. first Smith’s prize. On 1st October 1824 he was elected In June 1835 Airy was appointed Astronomer Royal in fellow of Trinity, and in December 1826 was appointed succession to Pond, and thus commenced that long career Lucasian Professor of Mathematics in succession to Turton. of wisely directed and vigorously sustained industry at This chair he held for little more than a year, being elected the National Observatory which, even more perhaps than in February 1828 as Plumian Professor of Astronomy and his investigations in abstract science or theoretical astrodirector of the new Cambridge observatory. Some idea nomy, constitutes his chief title to fame. The state of of his activity as a writer on mathematical and physical the observatory at the time of his appointment was such subjects during these early years may be gathered from that Lord Auckland, the first lord of the Admiralty, the fact that previous to this appointment he had contri- considered that “it ought to be cleared out,” while Airy buted no less than three important memoirs to the admits that it “was in a queer state.” With his usual Philosophical Transactions of the Royal Society, and eight energy he set to work at once to reorganize the whole to the Cambridge Philosophical Society. At the Cam- management. He remodelled the volumes of observations, bridge observatory Airy soon gave evidence of his remark- the library was put on a proper footing, the new (Sheepable power of organization. At the time of his appoint- shanks) equatorial was erected, and a new magnetic obserment the only telescope erected in the observatory was the vatory was built. In 1847 the altazimuth, designed by transit instrument, and to this he devoted himself with Airy to enable observations of the moon to be made whenvigour. By the introduction of a regular system of ever she might be visible (and not only on the meridian), selection and arrangement of his observations, and a care- was constructed. In 1848 Airy invented the reflex zenith fully worked out plan of reduction, he was able to keep tube to take the place of the zenith sector which had been his observations reduced practically up to date and employed up to that time. At the end of 1850 the great published annually with a degree of punctuality which transit circle of 8 in. aperture and 11 ft. 6 in. focal length S. L — 30