volume and forcing the piston (B) to rise, which completes the
cycle. The engine was double-acting, another heating vessel
like A being connected with the upper end of the working cylinder
at F. The stages at which heat is taken from the furnace and
rejected to the cooler (C) are approximately isothermal at the
upper and lower limits of temperature respectively, and the cycle
accordingly is approximately ``perfect'' in the thermodynamic
sense. The theoretical indicator diagram is made up of two
isothermal lines for the taking in and rejection of heat, and
two lines of constant volume for the two passages through the
regenerator. This engine was the subject of two patents
(by R. and S. Stirling) in 1827 and 1840. A double-acting
Stirling engine of 50 horse-power, using air which was
maintained by a pump at a fairly high pressure throughout the
operations, was used for some years in the Dundee Foundry,
where it is oredited with having consumed only 1.7 lb. of coal
per hour per indicated horse-power. The coal consumption per
brake-horse-power was no doubt much greater. It was finally
abandoned on account of the failure of the heating vessels.
The type survives in some small domestic motors, an example of
which, manufactured under the patent of H. Robinson, is shown
in fig. 2. In this there is no compressing pump, and the main
pressure of the working air is simply that of the atmosphere.
The whole range of pressure is so slight that no packing is
required. Here A is the vessel in which the air is heated
and within which the displacer works. It is heated by a small
cokefire or by a gas flame in C. It communicates through a
passage (D) with the working cylinder (B) . The displacer (E)
which takes its motion through a rod (I) from a rocking lever
(F) connected by a short link to the crank-pin, is itself the
regenerator, its construction being such that the air passes
up and down through it as in one of the original Stirling
forms. The cooler is a water vessel (G) through which water
circulates from a tank (H). Ylessrs. Hayward and Tyler's
``Rider'' engine may be mentioned as another small hot-air
motor which follows nearly the Stirling cycle of operations.
An attempt to develop a powerful air-engine was made in
America about 1833 by John Ericsson, who applied it to marine
propulsion in the ship ``Caloric,'' but without permanent
success. Like Stirling, Ericsson used a regenerator, but with
this difference that the pressure instead of the volume of the air
remained constant while it passed in each direction through the
regenerator. Cold air was compressed by a. pump into a receiver,
where it was kept cool during compression and from which it
passed through a regenerator into the working cylinder. In
so passing it took up heat and expanded. It was then allowed
to expand further, taking in heat from a furnace under the
cylinder and falling in pressure. This expansion was continued
till the pressure of the working air fell nearly to that of the
atmosphere. It was then discharged through the regenerator,
depositing heat for the next charge of air in turn to take
up. The indicator diagram approximated to a form made up
of two isothermal lines and two lines of constant pressure.
In the transmission of power by compressed air (see POWER
TRANSMISSION) the air-driven motors are for the most part
machines resembling steam-engines in the general features of
their pistons, cylinders, valves and so forth. Such machines
are not properly described as air-engines since their function
is not the conversion of heat into work. Incidentally,
however, they do in some cases partially discharge that
function, namely, when what is called a ``preheater'' is used
to warm up the compressed air before it enters in the motor
cylinder. The object of this device is not, primarily, to
produce work from heat, but to escape the inconveniences
that would otherwise arise through extreme cooling of the
air during its expansion. Without preheating the expanding
air becomes so cold as to be liable to deposit snow from
the moisture held in suspension, and thereby to clog the
valves. With preheating this is avoided, and the amount
of work done by a given quantity of air is increased by the
conversion into work of a part of the supplementary energy
which the preheater supplies in the form of heat. (J. A. E.)
AIREY, RICHARD AIREY, BARON (1803-1881), British general,
was the son of Lieutenant-General Sir George Airey (1761-1833)
and was born in 1803. He entered the army in 1821, became
captain in 1825, and served on the staff of Sir Frederick
Adam in the Ionian Islands (1827-1830) and on that of Lord
Aylmer in North America (1830-1832). In 1838 Airey, then a
lieutenant-colonel, went to the Horse Guards, where in 1852
he became military secretary to the commander-in-chief, Lord
Hardinge. In 1854 he was given a brigade command in the
army sent out to the East; from which, however, he was
immediately transferred to the onerous and difficult post
of quartermaster-general to Lord Raglan, in which capacity
he served through the campaign in the Crimea. He was made
a major-general in December 1854, and it was universally
recognized in the army that he was the best soldier on Lord
Raglan's staff. He was made a K.C.B., and was reported upon
most favourably by his superiors, Lord Raglan and Sir J.
Simpson. Airey was a quartermaster-general in the older
sense of the word, i.e. a chief of the general staff, but a
different view of the duties of the office was then becoming
recognized. Public opinion held him and his department
responsible for the failures and mismanagement of the
commissariat. Airey demanded an inquiry on his return to
England and cleared himself completely, but he never recovered
from the effects of the unjust persecution of which he
had been made the victim, though the popular view was not
shared by his military superiors. He gave up his post at
the front to become quartermaster-general to the forces at
home. In 1862 he was promoted lieutenant-general, and
from 1865 to 1870 he was governor of Gibraltar, receiving
the G.C.B. in 1867. In 1870 he became adjutant-general
at headquarters, and in 1871 attained the full rank of
general. In 1876, on his retirement, he was created a
peer, and in 1879-1880 he presided over the celebrated Airey
commission on army reform. He died at the house of Lord
Wolseley, at Leatherhead, on the 14th of September 1881.
AIR-GUN, a gun in which the force employed to propel the
bullet is the elasticity of compressed atmospheric air.
It has attached to it, or constructed in it, a reservoir of
compressed air, a portion of which, liberated into the space
behind the bullet when the trigger is pulled, propels the
bullet from the barrel by its expansion. The common forms of
air-gun, which are merely toys, are charged by compressing a
spiral spring, one end of which forms a piston working in a
cylinder; when released by a pull on the trigger, this spring
expands, and the air forced out in front of it propels the
bullet. Air-guns of this kind are sometimes made to
resemble walking-sticks and are then known as air-canes.
AIRY, SIR GEORGE BIDDELL (1801-1892), British Astronomer
Royal, was born at Alnwick on the 27th of July 1801.
He came of a long line of Airys who traced their descent
back to a family of the same name residing at Eentmere, in
Westmorland, in the 14th century; but the branch to which
he belonged, having suffered in the Civil wars, removed to
Lincolnshire, where for several generations they lived as
farmers. George Airy was educated first at elementary
schools in Hereford, and afterwards at Colchester Grammar
School. In 1819 he entered Trinity College, Cambridge, as a
sizar. Here he had a brilliant career, and seems to have
been almost immediately recognized as the leading man of his
year. In 1822 he was elected scholar of Trinity, and in the
following year he graduated as senior wrangler and obtained
first Smith's prize. On the 1st of October 1824 he was
elected fellow of Trinity, and in December 1826 was appointed
Lucasian professor of mathematics in succession to Thomas
Turton. This chair he held for little more than a year, being
elected in February 1828 Plumian professor of astronomy and
director of the new Cambridge observatory. Some idea of his
activity as a writer on mathematical and physical subjects
during these early years may be gathered from the fact that
previous to this appointment he had contributed no less than
three important memoirs to the Philisophical Transactions of
the Royal Society, and eight to the Cambridge Philosophical
Society. At the Cambridge observatory Airy soon gave evidence
of his remarkable power of organization. The only telescope
erected in the establishment when he took it in charge was
the transit instrument, and to this he vigorously devoted
himself. By the adoption of a regular system of work,
and a careful plan of reduction, he was able to keep his
observations reduced practically up to date, and published
them annually with a degree of punctuality which astonished his
contemporaries. Before long a mural circle was installed,
and regular observations were instituted with it in 1833.
In the same year the duke of Northumberland presented the
Cambridge observatory with a fine object-glass of 12 in.
aperture, which was mounted according to Airy's designs
and under his superintendence, although the erection was
not completed until after his removal to Greenwich in
1835. Airy's writings during this time are divided between
mathematical physics and astronomy. The former are for the
most part concerned with questions relating to the theory of
light, arising out of his professorial lectures, among which
may be specially mentioned his paper ``On the Diffraction of
an Object-Glass with Circular Aperture.'' In 1831 the Copley
medal of the Royal Society was awarded to him for these
researches. Of his astronomical writings during this period
the most important are his investigation of the mass of
Jupiter, his report to the British Association on the progress
of astronomy during the 19th century, and his memoir On an
Inequality of Long Period in the Motions of the Earth and Venus.
One of the sections of his able and instructive report
was devoted to ``A Comparison of the Progress of Astronomy
in England with that in other Countries,'' very much
to the disadvantage of England. This reproach was
subsequently to a great extent removed by his own labours.
Airy's discovery of a new inequality in the motions of
Venus and the earth is in some respects his most remarkable
achievement. In correcting the elements of Delambre's solar
tables he had been led to suspect an inequality overlooked by
their constructor. The cause of this he did not long seek in
vain. Eight times the mean motion of Venus is so nearly equal
to thirteen times that of the earth that the difference amounts
to only the 1/240th of the earth's mean motion, and from the
fact that the term depending on this difference, although very
small in itself, receives in the integration of the differential
equations a multiplier of about 2,200,000, Airy was led to
infer the existence of a sensible inequality extending over
240 years (Phil. Trans. cxxii. 67). The investigation that
brought about this result was probably the most laborious
that had been made up to Airy's time in planetary theory, and
represented the first specific improvement in the solar tables
effected in England since the establishment of the theory of
