mixture mainly of about 78 per cent by volume of nitrogen and 21 per cent
of oxygen. The latter is absolutely essential to animal life, while the
purpose chiefly served by the nitrogen appears to be to dilute the oxygen.
Oxygen is more soluble in water than nitrogen, and hence the air dissolved
in water contains about 10 per cent more oxygen than atmospheric air. The
oxygen therefore available for those animals which breathe by gills is
somewhat less diluted with nitrogen, but it is very much diluted with
water. For the various properties and phenomena connected with air see such
articles as _Atmosphere_, _Aeronautics_, _Air-pump_, _Barometer_,
_Combustion_, _Respiration_, &c.
AIR, in music (in It. _aria_), a continuous melody, in which some lyric
subject or passion is expressed. The lyric melody of a single voice,
accompanied by instruments, is its proper form of composition. Thus we find
it in the higher order of musical works; as in cantatas, oratorios, operas,
and also independently in concertos.--_Air_ is also the name often given to
the upper or most prominent part in a concerted piece, and is thus
equivalent to _treble_, _soprano_, &c.
AIR, or ASBEN. See _Asben_.
AIRA. See _Hair-grass_.
AIR BEDS AND CUSHIONS, often used by the sick and invalids, are composed of
india-rubber or of cloth made air-tight by a solution of india-rubber, and
when required for use filled with air, which thus supplies the place of the
usual stuffing materials. They tend to prevent bed-sores from continuous
lying in one position. They are also cheap and easily transported, as the
bed or cushion, when not in use, can be packed in small compass, to be
again inflated with air when wanted.
AIR-BLADDER. See _Swimming-bladder_.
AIR-BRAKE, a brake operated by air pressure, usually applied to brake,
simultaneously, all the wheels of a moving train. In the Westinghouse type,
by means of an ingenious 'triple valve' carried one on each carriage, the
train pipe is made to serve the dual purpose of supply and control. An
air-pump on the engine compresses air into the main receiver, from which it
flows through a reducing valve into the train pipe. The pressure, acting on
the under side of the triple valve, moves the valve to its extreme
position, thereby opening a passage to an auxiliary receiver on the
carriage and also putting the brake cylinder into communication with the
atmosphere. A spring in the brake cylinder keeps the brakes in the 'off'
position.
To apply the brakes, the pressure is lowered in the train pipe. The air
pressure in the auxiliary receiver reverses the triple valve, thus
admitting air to the brake cylinder and closing the outlet to atmosphere.
To remove the brakes, air from the main receiver is passed into the train
pipe, and the triple valve is restored to the 'off' position. See
_Traction_.--BIBLIOGRAPHY: W. W. Wood, _Westinghouse Air-brake_; R. H.
Blackhall, _Air-brake Catechism_.
AIR-CELLS, cavities in the cellular tissue of the stems and leaves of
plants which contain air only, the juices of the plants being contained in
separate vessels. They are largest and most numerous in aquatic plants, as
in the _Vallisneria spir[=a]lis_ and the _Victoria regia_, the gigantic
leaves of which latter are buoyed up on the surface of the water by their
means.--The minute cells in the lungs of animals are also called air-cells.
There are also air-cells in the bodies of birds. They are connected with
the respiratory system, and are situated in the cavity of the thorax and
abdomen, and sometimes extend into the bones. They are most fully developed
in birds of powerful and rapid flight, such as the albatross.
AIRD, Thomas, a Scottish poet and miscellaneous writer, friend of Professor
Wilson, De Quincey, and Carlyle, long editor of a newspaper in Dumfries;
born 1802, died 1876. He wrote _The Devil's Dream on Mount Aksbeck_; _The
Old Bachelor_, &c.
AIRDRIE, a municipal and parliamentary burgh of Scotland, in Lanarkshire,
near the Monkland Canal, 11 miles east of Glasgow, in the centre of a rich
mining district, with a large cotton-mill, foundries and machine shops,
breweries, &c., and collieries and ironworks in its vicinity. Pop. 24,160.
AIR-ENGINE, an engine in which air heated, and so expanded, or compressed
air is used as the motive power. A great many engines of the former kind
have been invented, some of which have been found to work pretty well where
no great power is required. They may be said to be essentially similar in
construction to the steam-engine, though of course the expansibility of air
by heat is small compared with the expansion that takes place when water is
converted into steam. Engines working by compressed air have been found
very useful in mining, tunnelling, &c., and the compressed air may be
conveyed to its destination by means of pipes. In such cases the waste air
serves for ventilation and for reducing the oppressive heat.
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.