Air-ships, lighter-than-air craft provided with means of propulsion and steering. The air-ship, unlike the aeroplane, is not dependent upon its engines for its power to remain in flight, but derives its sustentation from the hydrogen gas with which it is filled. Hydrogen, first weighed by Henry Cavendish in 1766, is the lightest gas known, being 14.47 times lighter than air. In the pure state it has a lifting force of 71.155 lb. per 1000 cu. feet, but for calculation purposes is usually assumed to contain 5 per cent of impurities, giving a 'lift' of approximately 68 lb. per 1000 cu. feet. Hydrogen is, when mixed with air, highly inflammable, and helium has therefore been suggested as a substitute. This has a lift, when pure, of about 65 lb. per 1000 cu. feet, but is only found in a few places in America and is therefore at present too expensive to be used in quantities. The lift of any given quantity of hydrogen depends upon the difference between its weight and that of an equal volume of air. As the amount, and therefore weight, of air contained in a given space varies with the barometric pressure and temperature, the lift of hydrogen given above varies also. These figures are based upon a temperature of 60° F. and a barometric pressure of 30 inches. As an air-ship rises from the ground, the density, and therefore pressure, of the air decreases, which causes the hydrogen in the envelope to expand proportionately. Rise in temperature has the same effect. When an air-ship ascends, the gas therefore expands, and at a certain point would burst the envelope were valves not provided to allow some of the gas to escape. It is important to realize that as the expansion occurs at a rate corresponding to the decrease in density no alteration in lift occurs so long as gas is not lost through the valves. This would continue indefinitely if the gas-chamber were capable of stretching indefinitely, but with the cotton-fabric used in practice a height is reached when gas commences to escape from the automatic valves. From this moment the lift of the air-ship begins to decrease. At a certain point this decrease will have reached such a point that the air-ship is 'in equilibrium', i.e. she weighs precisely the same as the volume of air she displaces. This is known as the 'maximum height'. Up to 10,000 feet it is roughly true that 1/30 of the lift is lost per 1000 foot rise.
The simplest form of air-ship is the non-rigid, which consists of a rubberized cotton-fabric gas-container (the 'envelope'), from which the 'car',
containing engines, crew, &c., is hung by flexible steel-wire ropes. To resist the bending moment introduced by the weight of the car, the envelope is inflated with hydrogen under pressure—usually about 25 mm. of water. So long as this pressure is greater than any local compression due to bending or loading in the fabric, the envelope will retain its shape. On coming down from a height, owing to the loss of gas, as already explained, the pressure will be reduced, and something must be done to restore it or the envelope will buckle. Fabric bags, known as 'ballonets', are therefore fitted inside the envelope, and as the air-ship descends air is forced into these bags, which supplies the lost pressure and maintains the shape of the envelope. The height to which a non-rigid air-ship can go, on returning from which the ballonets will be just full of air and the pressure the same as at starting, is known as the 'maximum ballonet height'. Ballonets are usually equivalent in volume to rather less than a quarter of the total volume of the air-ship—giving a maximum ballonet height of 6000 to 7000 feet. Usually from two to three ballonets are provided, according to the size of the air-ship. During the Great European War British non-rigid air-ships were constructed varying in size from a capacity of 70,000 cu. feet to 360,000 cu. feet. The former had one 75-h.p. engine, and the latter two of 375 h.p. each. Owing to difficulties in maintaining the shape and distributing the weight of the car over a long envelope, it is generally considered that 500,000 cu. feet probably represents the maximum size in which the non-rigid form of construction can be used. Above this size the semi-rigid type is used. In this case the envelope remains as in the non-rigid, but a girder or 'keel' is introduced between the envelope and the car, the weight of which is therefore taken by the keel and thence distributed to the envelope instead of being taken direct from the envelope as in non-rigids. There has been little development of non-rigids in Great Britain. The most prominent types are the Italian 'Forlanini', 'Verduzzio', and military air-ships. The keel, in all these examples, is not a rigid girder in the vertical sense, as it consists of a number of sections connected together by links. It is designed to resist compression only so long as it is held straight by the pressure of the envelope, and is not capable of taking a bending moment. When a size of about 1,000,000-cu.-foot-hydrogen capacity is reached it becomes economical to use the rigid method of construction. This is totally distinct from the other two types, as the non-rigid envelope is replaced by a rigid hull of sufficient strength to retain its shape without the assistance of any internal gas-pressure. The hull consists of a number of longitudinal members—usually built-up girders of 'duralumin', an aluminium alloy—connected together at distances of 25-30 feet by a number of 'transverse frames', or rings, forming bulkheads. The transverse frames are also of duralumin girders, and are braced by 'radical wires' running from the joints of these girders to a ring in the centre. Between each pair of these transverse frames is a gas-bag containing hydrogen. The gas-bags are made of rubberized cotton on to which is stuck 'gold-beater's skin', made from the lining of the intestines of an ox. This is done to prevent hydrogen leakage. This is necessary, as the fabric of the gas-bags of a rigid air-ship is lighter and contains less rubber than the envelope of a non-rigid.
A 'Δ'-shaped keel runs along the interior of the ship, its weight being taken on the two bottom longitudinal girders. The chief function of the keel is to distribute the load of the various weights to the transverse frames of the air-ship. In it are slung the petrol-tanks, water-ballast tanks, bombs, &c., and living accommodation for the crew is also provided there. Along the bottom runs a walking-way from which access is gained to the cars and various parts of the air-ship. The cars containing the engines, wireless-cabin, and pilot's cabin are suspended from the transverse frames. Some of the cars, instead of being slung below the centre-line, are slung in pairs some little way up the side of the air-ship.
All air-ships are steered by means of rudders and, in the vertical sense, elevators, in precisely the same way as aeroplanes. Up to the end of 1919 speeds of 84 miles per hour had been reached and air-ships had climbed to 24,000 feet. The greatest distance covered in one flight was 4500 miles, while the longest time in the air was effected by R34 on her voyage to America, which occupied 108 hours—4 days 8 hours. Rigid air-ships of 2,750,000-cu.-foot capacity had been built with a length of nearly 300 feet and a gross lift of 60 tons. See also Aeronautics, Balloons.—Bibliography: L. Sazerac de Forges, La Conquête de l'Air; Santos Dumont, My Airships; Hildebrandt, Airships: Past and Present; Major G. Whale, British Airships: Past, Present, and Future.
Airy, Sir George Biddell, a distinguished English astronomer, was born at Alnwick, 27th July, 1801, and educated at Hereford, Colchester, and Trinity College, Cambridge, where he was senior wrangler in 1823. At Cambridge he was Lucasian professor of mathematics, and subsequently Plumian professor of astronomy and experimental philosophy, in the latter capacity having charge of the observatory. In 1835 he was appointed Astronomer Royal, and as such
his superintendence of the observatory at Greenwich was able and successful. He resigned this post with a pension in 1881. His important achievement is the discovery of a new inequality in the motions of Venus and the earth. He wrote much and made numerous valuable investigations on subjects connected with astronomy, physics, and mathematics. Among separate works published by him may be mentioned Popular Astronomy, On Sound and Atmospheric Vibrations, A Treatise on Magnetism, On the Undulatory Theory of Optics, On Gravitation. He died 2nd Jan., 1892. He left an autobiography, published in 1896.
Aisle (īl; from Lat. ala, a wing), in architecture, one of the lateral divisions of a church in the direction of its length, separated from the central portion or nave by piers or pillars. There may be one aisle or more on each side of the nave. The cathedrals at Chichester, Milan, and Amiens have five aisles, Antwerp and Paris seven, and that of Cordova nineteen aisles in all. The nave is sometimes called the central aisle. See Cathedral.
Aisne (ān), a north-eastern frontier department of France; area, 2838 sq. miles. It is an undulating, well-cultivated, and well-wooded region, chiefly watered by the Oise in the north, its tributary the Aisne in the centre, and the Marne in the south. It contains the important towns of St. Quentin, Laon (the capital), Soissons, and Château Thierry. In the European War (1914-18) severe fighting took place on the Aisne, and a great battle was fought on 12th Sep., 1914. General Nivelle's offensive on the Aisne began in April, 1917. Pop. (1921), 421,575.
Aïva′lik, or Kidonia, a seaport of Asia Minor, on the Gulf of Adramyti, 66 miles north by west of Smyrna, carrying on an extensive commerce in olive-oil, soap, cotton, &c. Pop. 21,000.