Part 16

AIRE ([=a]r), a river of England, W. Riding of Yorkshire, rising to the south-east of Penyghent and flowing in a south-easterly direction to join the Ouse above Goole, having passed through Leeds on its way; length, 70 miles. It is navigable up to Leeds, and forms an important portion of the Aire and Calder Navigation system, which connects Goole, Hull, &c., with Liverpool. The Calder enters the Aire at Castleford. The district specially known as _Airedale_ is the valley of the Aire above Leeds.--A large breed of terrier, of which there are several varieties, is known as the _Airedale terrier_, a strongly-built animal, rather long in the legs, with a hard, close coat.

AIRE, a river of France, in the Argonne region, a tributary of the Aisne.

AIRE-SUR-L'ADOUR ([=a]r-s[.u]r-l[.a]-d[:o]r), a small but ancient town of France, department of Landes, the see of a bishop. Pop. 3000.

AIRE-SUR-LA-LYS ([=a]r-s[.u]r-l[.a]-l[=e]), an old fortified town of France, department of Pas de Calais, 10 miles south-east of St. Omer. Pop. 5000.

AIR-GUN, a gun from which the bullet is propelled by means of compressed air. Until about the middle of the nineteenth century air-guns were made with a metal reservoir in the butt; this reservoir was charged with air by means of a pump, and although one pumping put in enough air for six or seven shots, the process of loading was awkward and laborious. The well-known 'Gem' air-gun was worked by means of a spring, which compressed the air; the great defect of this gun was that the barrel was used as a cocking-lever, and so was apt to become bent and inaccurate. The 'Gem' was a smooth-bore gun, and early attempts at rifled air-guns failed, as the pellet was apt to stick in the barrel, owing to the low velocity not allowing it to take the grooves. The 'Quackenbush' air-gun made an attempt to get over this difficulty; its slugs were felted, and the felt took the rifling and greatly increased the accuracy of the weapon, but, of course, the ammunition was much more expensive than ordinary air-gun pellets. The B.S.A. air-rifle is an excellent weapon which has overcome all the early difficulties of construction. It has a fixed barrel, a separate cocking-lever, and a rotating breech-plug, and the muzzle velocity of its 16-grain pellet is 600 feet per second, which compares not unfavourably with the 1000 feet per second of the 40-grain bullet of a .22 long-rifle cartridge. An air-gun is a splendid weapon for practising markmanship, as it is almost noiseless, and as its ammunition costs little. It does not need to be elaborately cleaned, as a miniature rifle does; an occasional oiling is all that it requires to keep it in order, and with care it should fire an indefinite number of shots without losing its accuracy.

AIROLO ([.a]-i-r[=o]'l[=o]), a small town of Switzerland, canton Ticino, at the southern end of the St. Gothard Tunnel, and the first place on this route at which Italian is spoken. Pop. 2000.

AIR-PLANTS, or EPIPHYTES, are plants that grow upon other plants or trees, apparently without receiving any nutriment otherwise than from the air. The name is restricted to flowering plants (mosses or lichens being excluded) and is suitably applied to many species of orchids. The conditions necessary to the growth of such plants are excessive heat and moisture, and hence their chief localities are the damp and shady tropical forests of Africa, Asia, and America. They are particularly abundant in Java and tropical America.

[Illustration: Fig. 1.--Air-pump (sectional view)]

AIR-PUMP, an apparatus by means of which air or other gas may be removed from or compressed into an enclosed space. It was invented by Otto von Guericke of Magdeburg about the year 1654, and described in 1657 by Gaspar Schott. An ordinary suction-pump for water is on the same principle as the air-pump; indeed, before water reaches the top of the pipe the air has been pumped out by the same machinery which pumps the water. An ordinary air-pump (see fig. 1) consists essentially of a cylinder or barrel with a piston and valves. The barrel is connected to the vessel from which the air is to be pumped. A is the vessel to be exhausted, C the air-pump cylinder, P the piston, VV valves in the piston, and O the connection to the vessel A. When the piston moves downwards from the position shown, it cuts off the connection with A by passing over O. The length L is made long enough so that O is kept covered up during the downstroke. The air filling the space D is compressed, and so lifts the valves VV and passes out through them. This goes on till the end of the downward stroke, when the volume is very small indeed. When the upward motion begins, the valves VV close, and the piston rises and creates a vacuum in D. When the piston rises sufficiently to uncover O (as in figure), air rushes from A into the highly-exhausted space D and fills it. The process is repeated indefinitely, and A is gradually exhausted.

[Illustration: Fig. 2.--Toepler Pump Fig. 3.--Sprengel Pump]

Air-pumps for compressing air are constructed on the same principle, but the valves act the reverse way. The bicycle pump is a well-known example of this form of pump. In the Fleuss or Geryk pump greater efficiency is attained by having layers of oil in the barrel and above the piston. In nearly all pumps for producing the high vacua necessary, e.g. for the electric glow-lamp and the X-ray tube, mercury is employed. In one form, the Toepler pump, a reservoir containing mercury is connected by a flexible tube to the receiver. (See fig 2. T tube connecting pump to vessel to be exhausted; R, reservoir, raised above A to drive air in B and C through D and out into open air; R is then lowered, and B and C fill with air from receiver. Process then repeated.) By alternately lowering and raising the reservoir, gas is first withdrawn from the receiver and then expelled through D, which also acts as a barometer. The process is repeated until the desired degree of exhaustion is reached. In a second type, the Sprengel pump, a stream of mercury from a reservoir situated above the vessel to be exhausted falls in drops through a narrow vertical tube which communicates with the vessel. (See fig. 3. A, reservoir; B, tube leading to vessel to be exhausted; C, bubbles of air carried down by mercury.) The air is entrapped between the falling drops of mercury, and is carried down and expelled with it. In the filter-pump, water is used instead of mercury, the pump being connected to an ordinary water-tap.

A more recent form, the Gaede pump, is of the rotary type. (See fig. 4. C, iron case; G, glass front; P two-chamber porcelain drum rotated counter-clockwise about axle A. As mercury leaves chamber R, air enters from receiver by tube T and opening B. When B is immersed, mercury enters and air is driven into case C and removed through tube S.) A porcelain drum, divided into two cells, rotates within an air-tight case more than half filled with mercury. Each cell has an opening which, when above the mercury surface, places the cell in communication with the receiver. When the opening is immersed, the entrapped air passes by another channel into the outer case, from which it is removed by another less efficient pump. The pump will reduce the pressure within a 6-litre bulb from 10 millimetres to .00001 millimetre of mercury in fifteen minutes. Langmuir's pump employs the principle of the aspirator. A current of mercury vapour passes from a mercury boiler past a tube communicating with the apparatus to be exhausted, and sucks the air from it; the mercury is condensed in the upper part of the pump, returns by side tubes to the boiler and leaves the extracted air in this condenser. A less efficient pump is employed to remove the air from the mercury condenser as it accumulates. This pump is said to be simple and rapid in action, and capable of exhausting an 11-litre bulb from atmospheric pressure to .00001 millimetre in eighty seconds.

[Illustration: Fig. 4.--Gaede Pump]

Air-pumps are largely used in steam engineering, both on land and at sea, to extract the air which enters the condenser with the steam (see _Condenser_). Several varieties of air-pumps are in use. 1. The ordinary piston-pump (fig. 1) in which the piston extracts air by first sucking it into the cylinder and then expelling it to the atmosphere. The opening leading to the condenser is closed during the stroke in which the air is expelled. Two or three cylinders are usually provided on each air-pump set, the former type being known as a 2-throw pump and the latter a 3-throw pump. One of the best-known makes is the _Edwards_ air-pump. Piston air-pumps are driven either by the main engine through a suitable mechanism, or by a separate electric motor. The amount of power required to drive them varies with the size of the set, and with large engines of over 10,000 h.p. it is about 1/2 per cent or less. Vacua as high as 29 inches (Bar. 30 inches) can be readily maintained on large plants by this type of pump, provided the condenser is suitably designed. In well-maintained plants bad vacua are commonly due to deficient air extraction, which may arise from the low-pressure air-piping not being air-tight, or from the air-pump being too small. 2. The water-ejector type uses the momentum of a jet of water to extract the air entrained with it. Well-known types of this plant are the ordinary barometric jet-condenser and the _Leblanc_ air-pump. In the latter type, a rotating wheel, which carries vanes, forcibly throws sheets of water into a pipe communicating with the condenser. The sheets of water lie across the pipe, and the space between them is filled up with air sucked from the condenser. This water, with the entrained air, is thrown out, against the atmospheric pressure, by the momentum imparted to the water sheets by the rotating wheel. Very high vacua can be obtained with the Leblanc pump, but the power required to drive it is more than is required with a 3-throw piston-pump. (Cp. Sprengel pump above). 3. A steam-ejector is also used, a jet of steam taking the place of the sheets of water in the Leblanc type. Parsons' _augmentor condenser_ works on this principle. A small jet of steam sucks the air from the main condenser and compresses it into a small so-called augmentor condenser. The pressure in this condenser is a little higher than the pressure in the main condenser, but it is sufficient to enable an ordinary 3-throw pump to be used efficiently. The steam used to extract the air is condensed in the augmentor condenser by cold water, and the interior of the augmentor condenser is connected to the inlet of an ordinary 3-throw pump. The 3-throw pump is called upon to deal with the air at a slightly higher pressure than the condenser pressure, and the vacuum in the main condenser is improved by the drop of pressure which exists between the augmentor condenser and the main condenser. In a well-designed plant, for instance, a 3-throw pump might be used to maintain a vacuum of 29 inches in the augmentor condenser, while the steam jet would provide another 1/2 inch of vacuum, giving 29-1/2 inches vacuum in the main condenser. The _pressure_ in the main condenser is thereby reduced from 1 inch Hg. to 1/2 inch Hg.; a reduction of _one-half_. (Cp. Langmuir's pump above--using a mercury-vapour jet instead of a steam jet.)--BIBLIOGRAPHY: S. P. Thompson, _The Development of the Mercurial Air-Pump_; E. Hausbrand, _Evaporating, Condensing, and Cooling Apparatus_.

AIR-RAIDS. Apart from various sporadic bomb-dropping attacks by the Italians in Tripoli in 1913, the first air-raid proper was made by a Zeppelin on Antwerp during the investiture of that city by the Germans in 1914. Later on this new method of warfare was developed to a considerable extent by both sides during the Great European War, both air-ships and aeroplanes being used. Air-craft for this purpose have been likened to long-range guns, with the advantage of greater precision, because the target is in view, and very much longer effective range--the Germans, for example, used to raid London, and on one occasion Edinburgh, from bases situated in North Germany and on the Schleswig coast. Air-raids are of great value in affecting the _moral_ of the enemy country by bringing home the effects of war in its most terrifying aspect to the civilian population at home, and thus causing the dislocation of traffic and diminishing the output of munitions. Their practical value is in attacking and destroying munition-factories, army head-quarters, naval bases, &c., in addition to such important work as the demolition of ammunition-dumps, and cutting lines of communication behind the front.

Various protective devices against raiding aircraft have been invented. Among these are high-angle guns, capable of throwing shells to a height of some 30,000 feet, though possibly the most effective defence is small high-speed aeroplanes armed with machine-guns and capable of reaching great heights in a short space of time. For use at night, kite-balloons (see _Balloons_) are sent up in clumps connected together by cables. From the cables is suspended a network of steel wires, which is invisible to the hostile air-craft, and in which they may become entangled and so brought down. These have been raised to a height of as much as 12,000 feet. For raiding purposes two types of aeroplane--in addition to air-ships--have been developed. 'Day bombers' carry out raids in daylight at heights of 12,000 to 20,000 feet on points from 50 to 100 miles behind the lines. 'Night-bombers' are slower machines which raid well into the enemy's territory--up to 200 or more miles--at heights varying from 8000 to 12,000 feet. It is usual for night-raids to be carried out by squadrons of machines flying in formation, each machine carrying about a ton of bombs (in 1918). Air-ships can carry 5-10 tons of bombs to places up to 1000 miles distant from their bases.

During the last months of the war, our Independent Air Force dropped 500 tons of bombs on German objectives, and this raiding over a wide area of industrial Germany played no small part in causing that loss of spirit among the enemy which led eventually to their request for an armistice, and their virtual capitulation.

AIR-SHIPS

[Illustration]

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 60deg 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 '[Delta]'-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 Conquete 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 ([=i]l; 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_.