Apple-tree, a wood generally hard and close, and of reddish-brown tints, used commonly in Tunbridge turnery, etc.

Apricot-tree, a native wood of Arme

nia, used by the French in turnery. Apron, the sill or lower part of a window; a platform or flooring of plank raised at the entrance of a dock: in naval architecture, a piece of curved timber fixed behind the lower part of the stern of a ship.

Apsis, the east end of a church or chancel; sometimes applied to a canopy over an altar; also to a circle about a star or planet. Apsis gradata, a bishop's throne in cathedral churches.

Aqua fortis, in chemistry, nitric acid diluted: the more concentrated is named spirit of nitre. Aquamale, a holy-water basin. | Aqua regia, nitro-muriatic acid; a compound of two parts nitric acid and one part muriatic acid. Aquatinta, in the arts, engraving which resembles drawings in Indian ink.

Aqueduct, a conduit for water: a construction of stone or timber, built on uneven ground, to preserve the level of water, and convey it by a canal from one place to another. Aquemola, a water-mill. Aquila, a reading-desk, so called from

its shape being that of an eagle with extended wings, supported by a pedestal.

Arabesque, generally applied to a style of ornament for pilasters, friezes, etc., as those painted by Raffaelle ¦ in the Vatican.

Arabo-tedesco, a term applied to the Moorish style of buildings in Spain,

etc.

Ara dignitatis, an altar at which none but the highest ecclesiastics perform divine rites.

Arcostyle, in architecture, the great

est interval or distance which can be made between columns, that is, eight modules or four diameters; also a species of temple which has its columns placed widely asunder.

Arbor, a spindle or axis upon which a ring or wheel is turned in a lathe. Arbor Dianæ, in chemistry, crystals formed by the combination of silver and mercury.

Arbores, brass branches for lights suspended from ceilings.

Arboretum, a grove of trees in a park, pleasure-ground, or garden.

Arbor vitæ, a tree which attains to a height of from 40 to 50 feet; its wood is of a reddish colour, very light, soft, and fine-grained, and is much used in house carpentry. Arc, in geometry, part of the circum

ference of a circle, or any curve lying between two points; a bow, vault, or arch.

Arca, a place in a vaulted chamber for sepulchral purposes; an excavation before the basement story of a house; an enclosed space; a chest in which the Romans deposited their money: the word is also used to signify a beam of wood which has a groove or channel hollowed in it from one end to the other.

Arcade, a series of recesses with arched ceilings or soffits; a covered passage in modern appliances, a vaulted avenue, now much in vogue, more particularly in Paris.-Arcades, though less magnificent than colonnades, are of extraordinary beauty when well contrived, affording shade from the sun and shelter from the rain. Though not so magnificent as colonnades, they are stronger, more solid, and less expensive. They are proper for triumphal entrances, gates of cities, of palaces, of gardens, and of parks; for public squares, markets, or large courts in general, and for all apertures that require an extraordinary width.

THEIR ORNAMENTS.—The piers of arcades may be decorated with columns, pilasters, niches, and apertures of different forms. The arch itself may be turned either with rockworked or plain rustic archstones or voussoirs, or with an

archivolt properly moulded. The keystone is generally carved in the form of a console, or sculptured with some head, or the like. The archivolt springs from an impost or plat-band, or sometimes from columns; but this is not to be practised except in cases of the most urgent nature, for its makes neither substantial nor beautiful work.

In arches that are of large dimensions, the keystone should never be omitted; its carving, however, may be dispensed with, if expense be an object. When the piers are decorated with disengaged columns, the entablature must break round over the columns; and the columns, whether engaged or not, should stand either on a pedestal or high plinth, by which means they will not only be kept dry, but their bases will likewise be protected from accidental damage.-Arches must always rise from an impost or a plat-band; and if there be no keystone to the archivolt, its summit should be kept down from the under side of the architrave of the accompanying order, at least half the distance that it would be, were a keystone employed, in order that the disagreeable appearance of the acute angle which it would otherwise form with the architrave may be avoided.

THEIR PROPORTIONS. - The height of arches to the under side of their crowns should not exceed twice their clear width, nor should it be much less; the piers not less than one-third the breadth of the arch, nor more than two-thirds; but the piers at the angles should be wider than the other piers by one-half, or one-fourth at least. Arce, in Roman architecture, the gutters of the cavedium. Arc-boutant, a kind of arched but

tress formed of a flat arch, or part of an arch, and abutting against the feet or sides of another arch or vault, to support them, and prevent them from bursting or giving way.

Arcella, in medieval architecture, a cheese-room.

Arch, the curved part of a building, supported at its extremities only, and concave towards the earth; a vaulted roof, or dome, constructed either with bricks, stone, or other materials: the arch of a bridge is formed of segments of a circle, elliptical or catenarian; in Christian architecture, arches display twentytwo varieties of form. Arches

are used in large intercolumniations of spacious buildings; in porticoes, both within and without temples; in public halls, as ceilings, the courts of palaces, cloisters, theatres, and amphitheatres: they are also used to cover the cellars in the foundations of houses and powder-magazines; also as buttresses and counterforts, to support large walls laid deep in the earth; for triumphal arches, gates, windows, etc.; and, above all, for the foundations of bridges and aqueducts: they are supported by piers, abutments, imposts, etc.-Arches are of several kinds, circular, elliptical, cycloidal, catenarian, etc., according as their curve is in the form of a circle, ellipse, cycloid, catenary, etc. Arches are to be found in the Greek theatres, stadia, and gymnasia, some of them erected probably 400 years before Christ. The most ancient arches of which we have correct data are those of the cloaca at Rome. The Emperor Hadrian threw a bridge over the Cephisus, between the territories of Attica and Eleusis, on the most frequented road of Greece. Arch (theory of the). This important

subject has exercised the talents and ingenuity of some of the greatest mathematicians in modern times, and many different solutions have been given to the various problems connected with it; but, as the greater part of them are founded on suppositions that have no existence whatever either in nature or practice, they have had a tendency

rather to mislead than direct those who are engaged in the operations of bridge-building. Dr. Olinthus Gregory, in the preface to his excellent work on 'Mechanics,' states, that "theoretical and practical men will most effectually promote their mutual interests, not by affecting to despise each other, but by blending their efforts; and further, that an essential service will be done to mechanical science, by endeavouring to make all the scattered rays of light they have separately thrown upon this region of human knowledge converge to one point. Gauthey, speaking of the theory of La Hire, observes that such analy. tical researches are founded on hypotheses which every day's experience contradicts. The following are the principal writers on the equilibrium of the arch. In 1691, the celebrated mathematicians, Leibnitz, Huygens, James and John Bernouilli, solved the problem of the catenary curve: it was soon perceived that this was precisely the curve that should be given to an arch of which the materials were infinitely small and of equal weight, in order that all its parts may be in equilibrium. In the Philosophical Transactions' for the year 1697, it is stated that David Gregory first noticed this identity; but his mode of argument, though sufficiently rigorous, appears not to be so perspicuous as could be desired. In one of the posthumous works of James Bernouilli, two direct solutions of this problem are given, founded on the different modes of viewing the action of the voussoirs : the first is clear, simple, and precise, and easily leads to the equation of the curve, which he shows to be the catenary inverted; the second requires a little correction, which Cramer, the editor of his works, has pointed out. In 1695, La Hire, in his Treatise on Mechanics, laid down, from the theory of the wedge, the proportion accord

ing to which the absolute weight of the materials of masonry ought to be increased from the keystone to the springing in a semicircular arch. The historian of the Academy of Sciences' relates, in the volume for the year 1704, that Parent determined on the same principle, but only by points, the figure of the extrados of an arch, the intrados being a semicircle, and found the force or thrust of a similar arch against the piers. In the Memoirs of the Academy of Sciences' for the year 1712, La Hire gave an investigation of the thrusts in arches under a point of view suggested by his own experiments: he supposed that arches, the piers of which had not solidity enough to resist the thrust, split towards the haunches at an elevation of about 45 degrees above the springings or impost; he consequently regarded the upper part of the arch as a wedge that tends to separate or overturn the abutments, and determined, on the theory of the wedge and the lever, the dimensions which they ought to have to resist this single effort. Couplet, in a memoir composed of two parts, the first of which was printed in the volume of the Academy for 1729, treats of the thrusts of arches and the thickness of the voussoirs, by considering the materials infinitely small, and capable of sliding over each other without any pressure or friction. But, as this hypothesis is not exactly conformable to experiment, the second part of the memoir, printed in the volume for 1730, resumes the question by supposing that the materials have not the power of sliding over each other, but that they can raise themselves and separate by minute rotatory motions. It cannot however be said that Couplet has added materially to the theories of La Hire and Parent, and none of them treated the subject, either in theory and practice, in such a satisfactory manner as was

afterwards done by Coulomb. Subsequently a memoir was published by Bouguer on the curve lines that are most proper for the formation of the arches of domes. He considers that there may be an infinite number of curve lines employed for this purpose, and points out the mode of selecting them. He lays it down uniformly that the voussoirs have their surfaces infinitely smooth, and establishes, on this hypothesis, the conditions of equilibrium in each horizontal course of the dome, but has not given any method of investigating the thrusts of arches of this kind, nor of the forces that act upon the masonry when the generating curve is subjected to given conditions. In 1770, Bossut gave investigations of arches of the different kinds, in two memoirs, which were printed among those of the Academy of Sciences for the years 1774 and 1776 he appears to have been engaged in this in consequence of some disputes concerning the dome of the French Pantheon, begun by the celebrated architect Soufflot, and finished from his designs. In 1772, Dr. Hutton published his principles of bridges, in which he investigated the form of curves for the intrados of an arch, the extrados being given, and vice versa. He set out by developing the properties of the equilibrated polygon, which is extremely useful in the equilibrium of structures. Mr. Attwood has written a dissertation on the construction of arches on the same principles as La Hire. Arch, in architecture, a concave structure raised or turned upon a mould, called the centering, in form of the arc of a curve, and serving as the inward support of some superstructure. Sir Henry Wotton says, " An arch is nothing but a narrow or contracted vault; and a vault is a dilated arch."

Arch, in geometry, a part of any curved line, as of a circle or ellipsis.

Arch, in masonry, a part of a building suspended over a hollow, and concave towards the area of the hollow the top of the wall or walls which receives the first archstones is technically called the abutment or springing.

Arch, in mining, a piece of ground left unworked.

Arch-band, applied by workmen to that portion of an arch or rib which is seen below the general surface of vaulting. Arch-brick, a wedge-shaped brick employed in the construction of arches.

Arch-buttress, a piece of insulated masonry usually named a flyingbuttress, extending from the clerestory of a church and over the roof of its aisle, where it rests on the buttress of the outer wall. Arch of equilibration, that which is in

equilibrium in all its parts, having no tendency to break in one part more than in another. Arch, triumphal, a building of which an arch is the principal feature, usually raised to commemorate some great achievement. Archæology, the study of ancient art, but more particularly that of the middle ages.

Arched, in mining: the roads in a mine, when built with stones or bricks, are generally arched level drifts.

Archeion, a recess in a Grecian temple, for the reception of the treasures of the deity to whom the temple was dedicated. Archeion, in Athens, the office in

which the decrees of the people and other state documents were preserved.

Arches, Norman, semicircular, which

form continued to the latest date of this style, and is frequently intermixed with pointed arches, even when other parts had advanced into the next style, of which the Temple Church is an instance.

There are some Norman arches more than a semicircle,-the horse

shoe, and a few instances of a double arch.

Archetus, a saw for cutting stones:

Muratori used the term for a crane or pulley for raising stones to the upper part of a building. Archimedes screw-propeller, in 1836, was launched by T. P. Smith, patentee. The vessel 232 tons, 125

feet long, 21 feet 10 inches beam,

80-horse power. Archimedean screw, a machine invented by Archimedes for raising water; also now applied to propel vessels through water. Archiepiscopal palace, the dwelling of an archbishop.

Architect, a person skilled in the art of building; one who forms plans and designs for edifices, conducts the work, and directs the secondary artificers employed; and whose emoluments are generally 5 per cent. on the amount of money expended. Architecture, a science applicable to the art of constructing domestic, ecclesiastical, municipal, palatial, or other buildings, and the adornment of the same according to the rules of the several orders, Doric, Ionic, and Corinthian, also the Tuscan and Composite, from Roman models, or other styles, each for its purpose, such as is usually called Gothic architecture, and modes subservient to climate and fashion, or caprice. "Architecture," says Palladio, "being grounded upon rules taken from the imitation of Nature, admits of nothing that is contrary or foreign to that order which Nature has prescribed to all things. An architect is not restrained from departing sometimes from common methods or usage, provided such variation be agreeable and natural.”

The public at large has a claim over the architecture of a country. It is common property, inasmuch as it involves the national taste and character; and no man has a right to pass off his own barbarous in

ventions as the national taste, and to hand down to posterity his own ignorance and disgrace to become a satire and a libel on the knowledge and taste of his age. Architecture, the Orders of.-Among the ancients, the use of the orders was very frequent; many parts of their cities were provided with spacious porticoes, their temples were surrounded with colonnades, and their theatres, baths, basilicæ, triumphal arches, mausolea, bridges, and other public buildings were profusely enriched with columns ; as were likewise the courts, vestibules, and halls of their private villas and houses.

In pure architecture, says A. W. Pugin, the smallest detail should have a meaning or serve a purpose; and even the construction itself should vary with the material employed, and the designs should be adapted to the material in which they are executed.

Strange as it may appear at first sight, it is in pointed architecture alone that these great principles have been carried out: we may be enabled to illustrate them from the vast cathedral to the simplest erection. Moreover, the architects of the middle ages were the first who turned the natural properties of the various materials to their full account, and made their mechanism a vehicle for their art. The wonderful strength and solidity of their buildings are the result, not of quantity or size of the stones employed, but of the art of their disposition.

The two following pages contain a synopsis of the proportions of the Orders, and of various examples of each, compiled by Mr. W. H. Leeds for Pugin's edition of Normand's Parallel of the Orders.'