of numerous miracles which he performed. Aidan died on the 31st August 651. AIDE-DE-CAMP, a confidential officer attached to the "personal" or private staff of a general. In the field he is the bearer of his chief's written or verbal orders, and when employed as the general's mouthpiece, must be implicitly obeyed. In garrison and quarters his duties are more of a social character-he superintends the general's household, writes and answers invitations, &c. To increase their state, colonial governors and the Lord-Lieutenant of Ireland have aides-de-camp with functions analogous to those of equerries to royalty. Officers above the rank of captain are seldom taken as aides, and none of less than two years' service. The sovereign, as head of the army, may have an indefinite number of aides-de-camp. In 1874 there were thirty-three military aides-de-camp; of these, twelve, taken from the militia, were honorary, the remainder, from the regular army and marines, were chosen for distinguished war services. The appointment carries with it promotion to the army rank of "full" colonel. The Queen has also at present (1874) eleven naval aides-de-camp, in compliment to the sister service; but the appointment is more especially of a military character. An admiral's aide-de-camp is his flag-lieutenant.

AIDIN, or GUZEL-HISSAR, a town of Turkey in Asia, in the pashalic of Anatolia, about 70 miles S.E. of Smyrna. It is beautifully situated near the river Meander, and is the residence of a pasha. Since 1866 it has been connected with Smyrna and Ephesus by rail. On a neighbouring height are to be seen the ruins of the ancient Tralles. Aidin is a place of very extensive trade, and is celebrated for its figs, which are grown in great abundance in the beautiful orchards between the town and the river, and form an important article of export. The streets of the town, overshadowed by trees, and having numerous well-frequented bazaars, present a very picturesque appearance. Among the inhabitants are considerable numbers of Greeks, Armenians, and Jews; and there are several churches and synagogues in addition to the fine Turkish mosques. Population, 30,000.

AIDS (Auxilia), a pecuniary tribute under the feudal system, paid by a vassal to his lord on particular occasions; originally a voluntary grant which in process of time became exigible as a right. The aids of this kind were chiefly three, viz.:-1st, When the lord made his eldest son a knight; 2d, To provide a dower when he gave his eldest daughter in marriage; 3d, To ransom the person of the lord when taken prisoner. The amount of the first two was definitely fixed by 3 Ed. I., c. 36, but that of the last was of course uncertain. The right of levying aids was abolished by 12 Car. II., c. 24.

AIKIN, JOHN, M.D. (1747–1822), was born at KibworthHarcourt, received his elementary education at the dissenting academy of Warrington, where his father was tutor, and prosecuted his medical studies in the university of Edinburgh, and in London under the celebrated Dr William Hunter. He commenced his professional career as a surgeon at Chester; but being unsuccessful, he removed to Warrington. Finally, he went to Leyden, took the degree of M.D. in that university (1780), and attempted to establish himself as a physician in London. His success in this new field does not seem to have been considerable; chiefly, no doubt, because he concerned himself more with the advocacy of liberty of conscience than with his professional duties. He therefore began at an early period to devote himself to literary pursuits. Dr Aikin's reputation chiefly rests on his endeavour to popularise scientific inquiries. In conjunction with his sister, Mrs Barbauld, he commenced the publication of a series of volumes on this principle, entitled Evenings at Home (6 vols., 1792-5), a

popular and interesting work, chiefly commendable for the purity of the principles it inculcates, and the pleasing views it gives of human nature. It has been translated into almost every European language. His love of nature, and his power in delineating its features, are well illustrated in The Natural History of the Year, as well as in his miscellaneous Essays. In 1798 Dr Aikin retired from professional life, and devoted himself with great industry to literary undertakings of numerous and varied kinds, among which his valuable Biographical Dictionary (10 vols. 1799-1815) holds a conspicuous place. Besides these, he published Biog. Memoirs of Medicine (1780); Lives of John Selden and Archbishop Usher; Memoirs of Huet, Bishop of Avranches; Geographical Delineations of All Nations, &c. He edited the Monthly Magazine from 1796 to 1807, and started the Athenaeum in 1807. The latter was discontinued, however, in 1809.

AIKIN, LUCY, daughter of the preceding, a well-known historical writer, was born at Warrington on 6th Nov. 1781. After rendering valuable assistance to her father in several of his later works, she commenced her own career as an authoress by the publication of several books for the young, the most important of which were the Adventures of Rolando, a translation, and Lorimer, a tale. In 1818 she published her Memoirs of the Court of Queen Elizabeth, the first and best of the series of historical works on which her reputation rests. It was very popular, and passed through several editions. The Memoirs of the Court of King James I. (1822) was highly commended in the Edinburgh Review, which pronounced it "a work very nearly as entertaining as a novel, and far more instructive than most histories." Her Memoirs of the Court of Charles I. (1833) showed a falling off; and her latest work, the Life of Addison (1843), was declared disappointing by Macaulay in the Edinburgh Review, vol. lxxviii. Aikin died at Hampstead, where she had resided for forty years, on the 29th Jan. 1864. A Life by P. H. le Breton appeared in a volume entitled Memoirs, Miscellanies, and Letters of Lucy Aikin (Lond. 1864).

Miss

AIKMAN, WILLIAM, a celebrated portrait-painter, born at Cairney, Forfarshire, on the 24th Oct. 1682. He was intended by his father for the bar, but followed his natural bent by becoming a pupil under Sir John Medina, the leading painter of the day in Scotland. In 1707 he went to Italy, resided in Rome for three years, afterwards travelled to Constantinople and Smyrna, and in 1712 returned home. In Edinburgh, where he practised as a portrait-painter for some years, he enjoyed the patronage of the Duke of Argyll; and on his removal to London in 1723 he soon obtained many important commissions. Perhaps his most successful work was the portrait of the poet Gay. He also painted portraits of himself, Fletcher of Saltoun, William Carstairs, and Thomson the poet. The likenesses were generally truthful, and the style was modelled very closely upon that of Sir Godfrey Kneller. Aikman held a good position in literary society; and counted among his personal friends Swift, Pope, Thomson, Allan Ramsay, Somerville, and Mallet. He died in June 1731, leaving unfinished a large picture of the royal family.

AILRED, EALRED, ETHELREDUS, ALUREDUS, an English ecclesiastic and histerian, born at Hexham in 1109. He was educated at the Scotch court with Henry the son of King David. The king is said to have offered him a bishopric, which he refused, preferring to become a monk in the Cistercian abbey of Rievaulx, Yorkshire. In 1146 he was chosen abbot, and he held that position till his death in 1166,—the accounts which state that he was transferred to Revesby in Lincolnshire being probably founded on a confusion of names. Leland says that he had seen his tomb at Rievaulx adorned with gold and silver ornaments.

Ailred was the author of a large number of historical and theological works. The former are of little value, owing to his credulity, except for the occasional glimpses they give of contemporary life and manners. His theological works, including a volume of homilies, a treatise on charity, and a treatise on friendship, are somewhat in the style of St Bernard. (For a full account of the historical writings see Sir T. D. Hardy's Descriptive Catalogue.)

AILSA CRAIG, a remarkable island-rock at the mouth of the Firth of Clyde, off the coast of Ayrshire, Scotland. It is of a conoidal form, with an irregular elliptic base, and rises abruptly from the sea to the height of 1139 feet. The only side from which the rock can be ascended is the east; the other sides being for the most part perpendicular, and generally presenting lofty columnar forms, though not so regular as those of Staffa. The rock is a greenstone or syenite, with a basis of grayish compact felspar traversed by numerous trap veins. A columnar cave exists towards the north side, and on the eastern are the remains of a tower, with several vaulted rooms. Two springs occur on the island, and some scanty grass affords subsistence to numerous rabbits. The precipitous parts of the rock are frequented by large flocks of solan geese and other aquatic wild fowl. It is situated in 55° 15′ N. lat., 5° 7′ W. long. AIN, a department on the eastern frontier of France, bounded on the N. by the departments of Jura and Saôneet-Loire, on the W. by Saône-et-Loire and Rhône, on the S. by Isère, and on the E. by the departments of Savoie and Haute Savoie and the Swiss cantons Geneva and Vaud. It extends at the widest points 52 miles from N. to S., and about the same distance from E. to W., with an area of 2241 square miles. The east of the department is very mountainous, being traversed by the southern portion of the Jura range, but in the north-west the surface is comparatively level, and in the south-west flat and marshy. Ain is wholly within the basin of the Rhône, that river itself being the boundary on the east and south, while it receives the Ain, which passes southward through the centre, and the Saône, which forms the western boundary of the department. The climate is usually cold, but on the whole healthy, except in the damp marshy districts on the west. The soil in the valleys and plains of the department is fertile, producing wheat, barley, maize, rye, and fruits of various kinds, as well as wine of excellent quality; the tops of many of the mountains are covered with forests of fir and oak, and the lower slopes yield excellent pasture for sheep and cattle. The chief mineral product is asphalt, besides which potter's clay, iron, building-stone, and the best lithographic stone in France, are produced in the department. There are many corn and saw mills on the mountain streams; and cotton, linen, and silk fabrics, coarse woollen cloth, paper, and clocks, are manufactured to a limited extent. Ain, which formed a part of the ancient province of Burgundy, is divided into five arrondissements-Bourg and Trevoux in the west, and Gex, Nantua, and Belley in the east; containing in all 36 cantons and 452 communes. Bourg is the capital, and Belley is the seat of a bishop. Population of Ain in 1872, 363,290, of whom 185,074 were males, and 178,216 were females. Of the total population, 115,407 could neither read nor write, and 46,450 more could not write. AINAD, a town of Arabia, in the province of Hadramaut, about 207 miles N.E. of Aden. Near it is the tomb of a Moslem prophet much frequented by pilgrims, at which a great annual fair is also held. The population is said to be about 10,000.

AINMULLER, MAXIMILIAN EMMANUEL, founder of a new school of glass - painting, was born at Munich on the 14th February 1807. He was induced, by the advice of Gärtner, director of the royal porcelain manu

factory, to devote himself to the study of glass-painting, both as a mechanical process and as an art, and he made such progress that in 1828 he was appointed director of the newly-founded royal painted-glass manufactory at Munich. The method which he gradually perfected there was a development of the enamel process adopted in the Renaissance, and consisted in actually painting the design upon the glass, which was subjected, as each colour was laid on, to carefully-adjusted heating. The fault of this new style is its production of transparent pictures seen by transmitted and not by reflected light; but the popular verdict in its favour has been, notwithstanding, proved by the extent to which it has been adopted. The earliest specimens of Ainmüller's work are to be found in the cathedral of Ratisbon. With a few exceptions, all the windows in Glasgow cathedral are from his hand. Specimens may also be seen in St Paul's cathedral and St Peter's College, Cambridge. On the Continent it must suffice to mention Cologne cathedral as containing some of his finest productions. Ainmüller had considerable skill as an oilpainter, especially in interiors; and his pictures of the Chapel Royal at Windsor and of Westminster Abbey have been much admired. He died 9th December 1870.

AINOS, the name of a small but remarkable tribe in Japan, found chiefly in the island of Yesso. They are different in race and character from the ordinary Japanese, and seem to have been the earliest inhabitants of the country. Since the invasion of the islands by the Japanese, however, the Ainos have been gradually supplanted by the invaders, and are now completely subject to them, although they still preserve the appearance of internal self-government, living in societies of from ten to twenty families, under a hereditary chief. Their language is quite distinct from the Japanese, and intercourse between the two peoples is carried on by a sort of mongrel dialect. The Ainos are not tall, averaging a little over 5 feet; but they are wellproportioned and strongly-built, with a type of countenance European rather than Asiatic. They are distinguished by an exuberance of hair on the head and body, a circumstance which has given rise to their name of "Hairy. Kuriles." The women are ugly, and are much addicted to tattooing. The dress of the Ainos consists of a robe of skin or cotton, reaching to the knees and secured by a girdle; their huts are small and uncomfortable, with little or no furniture; and their food is mostly the produce of fishing and hunting, together with rice got by barter from the Japanese. They are probably less than 50,000 in number.

AINSWORTH, HENRY, divine and scholar, was born "about 1560" at Pleasington, near Blackburn, Lancashire, having been second son of Lawrence Ainsworth of Pleasington Hall. Young Henry Ainsworth is believed to have received his education at Queen Elizabeth's Grammar School in Blackburn, of which his father was an original founder. According to tradition, he was a Roman Catholic, and a younger brother, John, a Protestant; and the two brothers, entering into a written controversy, mutually converted each other-Henry having embraced Protestantism, and John, Popery. The subsequent earlier history of Ainsworth is still obscure. No record survives; but various authorities concur in stating that he passed from Blackburn to Cambridge. He associated with the Puritan party in the Church of England, and eventually adopted the platform of the Independents as represented by the Brownists. He was driven from his native country by the state proscription of the sectaries before the year 1593. He is found residentin "ablind lane at Amsterdam" about 1595-6. His exile must have reduced him to extreme poverty. He is stated to have been a porter" to a scholarly bookseller in Amsterdam, who, on discovering his skill in the Hebrew language, made him known to his countrymen. Roger

AIR-ENGINE. Engines which have for their working fluid heated air instead of steam are called "air-engines." The name "caloric engine" has also been applied to them, but is not to be commended, for they have no more right to that title than steam-engines-the useful effect of both machines being due to the transformation of heat into mechanical energy, the air in the one case and the steam in the other being merely convenient media through which to effect that transformation.

The utilisation of the expansion of heated air for driving an engine has for many years been a subject which has exercised the ingenuity of inventors. The history of airengines has, however, been little more hitherto than a history of failures, and they are as far now from superseding steam-engines as they were fifty years ago. This is owing mostly to the fact that the inventors have too often worked empirically, without any real knowledge of the conditions under which, and under which only, the real advantages of the fluid could be attained, and have therefore continually violated these conditions. There are also certain constructive difficulties in the way of making a successful air-engine which have never been fully overcome. It should be distinctly understood that, regarded simply as a medium for transforming heat into work, air possesses no advantage over steam or any other fluid. Its advantage is, that it can be used with safety at much higher temperatures than steam (and therefore a larger proportion of the heat given to it can be transformed into work), and that by employing the gases of combustion in the cylinder much heat can be utilised which with steamengines is necessarily wasted.

Of the air-engines which have actually worked we have (1.) Those in which the changes of temperature take place at a pair of constant volumes; (2.) those in which the changes of temperature take place at a pair of constant pressures; and (3.) those in which heat is received and rejected at a pair of constant pressures. The first two classes, fitted with "economisers," are in theory "perfect" engines; that is, they are theoretically capable of transforming into work the largest fraction the limits of temperature allow of the heat received from the fuel. The third class are not perfect engines, but possess certain practical advantages which will be afterwards mentioned. The well-known engine invented by the Rev. Dr Stirling in 1816, and subsequently improved by him, in conjunction with his brother, Mr James Stirling, C.E., of Edinburgh, belongs to the first class. In this engine the same mass of air is used again and again, and is compressed at starting to a pressure of 7 to 10 atmospheres. A cylindrical air-receiver, in which a plunger can be moved up and down, is placed over the flue of the furnace. The annular space between the plunger and the sides of this receiver is occupied by an immense number of thin sheets of metal, which form the "economiser." In the upper part of the receiver, which communicates freely with one end of a working cylinder of the usual construction, is a "refrigerator," consisting of a coil of tubing through which cold water continually circulates. The plunger is alternately raised and lowered by suitable mechanism, and in its motion causes the great body of air in the machine to occupy alternately the bottom or heating end and the top or cooling end of the receiver. It thus undergoes alternate expansion and contraction, and thereby gives motion to the piston of the working cylinner, and thence to a crank shaft in the usual way. The advantages of this engine were, that the air in the cylinder was always cool, and that the great pressure which could be used rendered the size of the machine for a given power very moderate. It was ultimately abandoned because of the failure of the receiver to stand the destructive action of the heat.

The most familiar example of the second class of air-engines is that invented by Captain Ericsson. It differed from Stirling's in many respects, and does not seem in any one particular to have been an improvement on it. Fresh air was drawn from the atmosphere at every stroke, and a very low pressure used, and what was the receiver in Stirling's engine became the working cylinder of Ericsson's. It was thus excessively bulky in proportion to its power, and all the working parts were exposed to the destructive action of intense heat. It is chiefly interesting on account of the

enormous scale on which its construction was actually carried out. The engines of the steamship "Ericsson" had four working cylinders, of this vessel were conducted in a manner which did not allow any each 14 ft. in diameter, with other parts in proportion. The trials confidence to be placed in the results said to be obtained, and steamengines replaced those of Ericsson within two years.

To the third class of air-engines belong those of Sir George Cayley and several of the older inventors. The best known modern example is, however, the engine of Mr Philander Shaw, which is shown in our engraving, and which was exhibited at the Paris Exhibition of 1867. The most important feature of this type of engine is the use of the products of combustion themselves, instead of merely the air heated by them, to drive the piston. The conwith a trunk on its upper side, which, thus reduced in area, serves struction of the engine is very simple: the working piston is fitted as a compressing pump, and the products of combustion act directly upon its under side, which is protected by a large drum filled with non-conducting material from the heat. The furnace stands beside the cylinder, and is entirely closed up, means being provided for feeding it with fuel without allowing any air to enter. The air compressed by the pump is delivered into the furnace, where it combines with the fuel to form the gases of combustion, and in this way receiving additional heat, expands, and raises the piston of the working cylinder for a portion of its stroke. The admission-valve of heat, until the piston has completed its stroke, and are then disof the latter is then closed, and the gases expand, without addition charged into the atmosphere. By the addition of an "economiser," the efficiency of this type of engine may be very greatly increased; but its principal advantage is that, by actually using the products engines is unavoidably sent up the chimney and lost. of combustion inside the engine, much heat is saved which in other

One of the principal features of all air-engines is the "economiser" (sometimes erroneously called the "regenerator"), an invention of Mr Stirling's. The object of this apparatus is to store up the heat rejected by the fluid when it falls in temperature, and subsequently to raise the temperature of the fluid by re-storing the same heat, so that the only heat which the furnace has to supply is the latent heat of expansion, together with the amount of sensible heat which may be lost through the imperfection of the economiser.

(For a popular explanation of the theory of air-engines, see an admirable paper by the late Professor Rankine in the Edinburgh Philosophical Journal for January 1855; and for a complete account of the same, involving the use of the higher mathematics, see the same author's SteamEngine, pp. 345, et seq. See also Prof. Clerk Maxwell's Theory of Heat, and a series of papers on the subject in Engineering, 1874.) (A. B. W. K.)

AIR-GUN, a weapon like a common gun in shape, in which the force employed to propel the bullet is the elasticity of condensed atmospheric air. It has attached to it, or constructed in it, a strong metal chamber, into which air is forced by a condensing syringe (see PNEUMATICS). In this way a pressure may be obtained of several hundred atmospheres. When a trigger is touched, the condensed air rushes into a space behind the bullet with such force as to propel it from the barrel to a considerable distance. If only a little air be allowed to escape each time, a single charge will propel a number of bullets in succession, with

a constantly diminishing force. Sometimes the weapon is made in the form of a walking-stick, and is then called an air-cane. The air-gun is little else than a scientific toy, and has no practical value. The apparatus is costly, the process of condensation requires considerable labour, and the propulsive force of the air is, at its maximum, less than that of an ordinary charge of gunpowder. The only advantage it can be said to have in any way is the questionable one of its use being unattended by the explosive noise that accompanies the discharge of a common gun. AIR-PUMP, an apparatus by means of which a closed vessel can have the air it contains removed from it. It consists essentially of two parts-a receiver, from which the air is to be exhausted; and a pump, to perform the work of exhaustion. The receiver is in general made of glass, in order that the condition of objects placed within it for the purpose of experiment may be readily seen by the operator. It is open at the bottom, and has its lower edge accurately ground; when in its place in the air-pump it stands upon a smooth brass plate. The pump itself is a brass cylinder, having a piston in it, which can be moved backwards and forwards by means of a rod, in the usual way. At the end of the cylinder nearest the receiver is placed a small valve, in the piston itself is another (or some mechanism which serves the purpose of a valve), and there is frequently a third in the outer end of the cylinder. All these valves open outwards from the receiver. The action of the pump, when arranged in this way, is exactly similar to the action of an ordinary well-pump, with air as the fluid instead of water. The air-pump was invented about 1654 by Otto von Guericke, a magistrate of Magdeburg, and a man who devoted great attention to various problems in pneumatics.1 The first description of his pump was published in 1657 in the Mechanica Hydraulicopneumatica of Gaspar Schottus, professor of mathematics at Wurtemberg. He used a spherical glass receiver, with a pumping syringe attached, and kept the whole of the working parts under water to prevent leakage. His pump

was very imperfectly constructed, but he did eventually succeed in getting a very good vacuum with it. The method of producing the Torricellian vacuum, by filling a vessel with liquid and then removing the liquid without permitting ingress of air, was previously known; but a vacuum produced in this way was obviously useless for experiments with any objects but those which could previously be immersed in the liquid used. Guericke was, however, the first to recognise that, by virtue of its perfect

He was also the inventor of the "Magdeburg hemispheres."

elasticity, or tendency to expand indefinitely, air could be pumped out of a closed space as well as water; and this is the principle of his and all succeeding air-pumps. Although the invention of the air-pump is due to a German, almost all the improvements made in it from time to time have come from Englishmen. Dr Boyle contributed so much to its perfection that for a long time the state of the air in an exhausted receiver was called vacuum Boyleanum, and the air-pump itself machina Boyleana. Dr Hook, Hawkesbee, John Smeaton, and others brought the airpump externally to very much the same form as that in which it is commonly seen at present, and which is shown in the annexed woodcut. The pump here has two cylinders, which are worked by a winch handle, the pump rods having toothed racks on the upper part of their length. Professor Tate is the inventor of a double-action air-pump, now much used where a very perfect vacuum is required. It has two pistons in one barrel, the air being drawn from the receiver at the centre of the barrel, and discharged into the atmosphere at its extremities. Very complete airpumps have two or three barrels, arranged as shown in the woodcut, for rapid exhaustion, until the pressure in the receiver is equal to (say) half-an-inch of mercury; and in addition to these a horizontal Tate's barrel, which can then be put into action to bring the vacuum down to inch of mercury (1-600th of the pressure of the atmosphere), or even less at low temperatures. See PNEUMATICS.

AIR-PUMP, in steam-engines, is the pump which draws the condensed steam, along with the air which is always mixed with it, and also the condensing water (except where a surface condenser is used), away from the condenser, and discharges it into the hot well. See STEAMENGINE. (A. B. W. K.)

AIR, or ASBEN, a country of central Africa, lying between 15° and 19° N. lat. and 6° and 10° E. long. The northern and best known portion of this region is of a very diversified character. It has numerous mountain ranges, some of which rise to a height of 5000 feet, with richlywooded hollows and extensive plains interspersed. The mimosa, the dum-palm, and the date are abundant; and the valleys are covered with the exuberant vegetation of the tropics. Some of the plains afford good pasturage for camels, asses, goats, and cattle; others are desert tablelands. In the less frequented districts wild animals abound, notably the lion and the gazelle. The country generally is of sandstone or granite formation, with occasional trachyte and basaltic ranges. There are no permanent rivers; but during the rainy season, from August to October, very heavy floods convert the water-courses in the hollows of the mountains into broad and rapid streams. Numerous wells supply the wants of the people and their cattle. To the south of this variegated region lies a desert plateau, 2000 feet above the level of the sea, destitute of water, and tenanted only by the wild ox, the ostrich, and the giraffe. Still further south is the district of Damerghu, nominally tributary to Air, undulating and fertile, and yielding rich crops. Notwithstanding the fertility of the valleys in the northern portion of the country, there is little of the soil under cultivation except in the neighbourhood of the villages, where slaves are employed in tillage. Millet, dates, indigo, and senna are the principal productions. The great bulk of the food supplies is brought from Damerghu, and the whole materials for clothing are also imported. Were it not for the traffic in salt between Bilma and the Hansa states of Soudan, the country could scarcely maintain its present limited number of inhabitants. A great caravan annually passes through Air, consisting of several thousand camels, carrying salt from Bilma to Sokoto. Air was called Asben by the native tribes until they were conquered by the Berbers. The present inhabitants are for the most