A structure composed of separate pieces, such as stone or bricks, having the shape of truncated wedges, arranged on a curved line so as to retain their position by mutual pressure. This method of construction is called arcuated, in contradistinction to the trabeated style used in Greek architecture, where the voids between column and column, or between column and wall, were spanned by lintels.

The separate stones which compose the curve of an arch are called voussoirs, or arch-stones. The lowest voussoirs are called springers. The springers usually have one or both joints horizontal. The upper surface of the springer, against which the first voussoir of the real arch (that is, in which both joints radiate) starts, is said to be skewbacked; the uppermost or central voussoir is called the keystone. The under, or concave, side of the voussoir is called the intrados or soffit, and the upper, or convex, side, the extrados of the arch. The supports which afford resting and resisting points to the arch are called piers and abutments. The upper part of the pier or abutment where the arch rests - technically, where it springs from - is the impost. The span of an arch is, in circular arches, the length of its chord, and generally, the width between the points of its opposite imposts whence it springs. The rise of an arch is the height of the highest point of its intrados above the line of the impost; this point is sometimes called the underside of the crown, the highest point of the extrados being the crown. If an arch be enclosed, or is imagined as being enclosed, in a square, then the spaces between the arch and the square are its spandrels.

FORMS OF ARCH

In Rome and Western Europe, the oldest and normal type of arch is the semicircular. In this the centre is in the middle of the diameter. Where the centre is at a point above the diameter, it is called a stilted arch. When the arch is formed of a curve that is less than a semicircle (a segment of a circle), with its centre below the diameter, it is called a segmental arch. Or if the curve is greater than a semicircle and has its centre above the diameter, it is called the horseshoe arch. All these arches are struck from one centre. The second class is struck from two centres. This arch is the pointed. There are three chief varieties. The first is the equilateral. In this the two centres coincide with the ends of the diameter. The second, more acutely pointed, is the lancet. In this the centres are on the line of the diameter, but outside it. The third is the obtuse, or drop, arch. In this the centres are still on the line of the diameter, but inside. The third class consists of arches struck from three centres. This is the three-Centred or "basket-handle" arch. The fourth class consists of arches struck from four centres. The first variety is the four-centred, or Tudor, arch. The curves can be struck in different ways, and the long curves sometimes replaced by straight lines with a short curve at the juncture. Another variety of arch struck from three or four centres is the ogee arch. In this, one or two of the centres are below, but the other two are above the arch. So the two upper curves of the arch are concave, the two lower convex.

Foiled arches have three or more lobes or leaves. The simplest are the round-headed trefoil; the pointed trefoil; the square-headed trefoil: which goes by the name of the shouldered arch. A trefoliated arch is a trefoiled arch enclosed in a pointed arch. A trefoiled arch is not enclosed in any other arch. Besides the trefoiled, there is the cinquefoil arch, with five lobes or foils, and the multifoile arch with several.

FLAT ARCH

In a flat arch the voussoirs are wedge-shaped, but the extrados and intrados are composed of straight lines. Sometimes, to strengthen a flat or slightly curved arch, the voussoirs are notched or joggled.

COMPOUND ARCHES

If the arch needs to be unusually strong, it is better to construct two independent arches, one on the top of the other. Or it may be constructed in three separate rings. Each of these sub-arches, or rings, of which the whole compound arch is composed, is called an order. It is a safer form of arch than the simple arch. This system of concentric arches was employed by the Romans early in the sixth century B. C., in the Cloaca Maxima at Rome; three occur where it enters the Tiber. In some compound orders the faces are in the same plane. But as a rule the orders are successively recessed, i. e. the innermost sub-arch, or order, is narrow, the next above it broader, the next is broader still, and so on.

SEMICIRCULAR ARCH

This arch is specially characteristic of Romanesque architecture. Gothic semi-circular arches sometimes occur in the architecture of the thirteenth and fourteenth centuries.

STILTED ARCH

By stilting, a narrow semicircular arch can be made to rise to the same level as a broad arch, so that the crowns may be on the same level.

SEGMENTAL ARCH

This arch occurs occasionally in Norman work.

HORSESHOE ARCH

They are not uncommon in Norman ribbed vaults. They occur in the aisled basilica of Diana, near the Euphrates, which has the inscription A. D. 540. In Eastern work the horseshoe arch is frequently not round-headed, but acutely pointed. This facilitates construction, as the upper or more difficult portion of the arch or dome can then be constructed by corbelling and without centering, as in many Indian domes.

POINTED ARCH

Of the antiquity of the pointed arch in the East there can be no question; in many districts it is as much the normal form as is the semicircular in the Romanesque of Europe. But it does not follow that the latter borrowed it. It has probably been invented again and again, as necessity arose. In countries where there was no timber, or no tools to work it, the natives had to build shelters in stone. Frequently the only way known of roofing these was to pile flat stones on one another, i. e. with horizontal bed, not with radiating joints, each course projecting a little further inward as the wall went up. Plainly, these walls would topple in if a semicircular roof had been attempted, but they could be got to stand if the roof was built in the form of a pointed arch - at any rate, if the arch was very acutely pointed.

Although the Romanesque architects had solved the greatest problem of the Middle Ages, viz. how to vault throughout with stone a clerestoried church, Basilican in plan, without the aid of the pointed arch, yet the employment of the pointed arch greatly facilitated building construction. Next to the use of diagonal ribs and flying buttresses it was the greatest improvement introduced into medieval architecture (Francis Bond). The pointed arch is stronger than any other kind of arch; it has a more vertical and a less lateral thrust than a semicircular one. It was of the greatest use in vaulting.

FOUR-CENTRED ARCHES

These arches are parts of four different circles. The position of the centres varies greatly, and with them the beauty of the arch. Perhaps the most usual position is for the upper and lower centres of each side of the arch to be in the same vertical line. The four-centred arch has been considered peculiar to England; but it was common enough in Flanders at the same time it was in England.

OGEE ARCH

As the upper curves of this arch are reversed, it cannot bear a heavy load, and it does not occur in pier arches. In France, the ogee arch does not seem to have come into general use till late in the fourteenth century. In late English Decorated and French Flamboyant the ogee arch is used to the greatest advantage. Its origin is unquestionably Oriental. It is used in India on a vast scale in those domes which are constructed by corbelling. In England it was not used constructionally, but only decoratively. The ogee arch, like the pointed arch, may vary greatly in form, according to the character of the arch whose curve is reversed to give the upper part of the ogee, and according to the length assigned to the upper curve.

FOILED ARCH

Like the ogee, it is of decorative, not of structural, value. The round-headed, trefoiled arch is less common than the pointed. The cinquefoil is usually later than the trefoil arch.

ELLIPTICAL ARCHES

It may be doubted whether any true elliptical arches ever occur otherwise than accidentally. The origin of the arch is not known. It was largely used by the Assyrians, and by the Egyptians as well, at a very early date;. but for some unknown reason they did not introduce it into their greatest works. The practical introduction and use of the arch was due to the Romans. The pointed arch came into use about the twelfth century, and was destined to give birth to a new style of architecture. The pointed arch, whatever its origin, made its appearance almost at the same time in all the civilized countries of Europe. As this was immediately after the first Crusade, it has been conjectured that the Crusaders came to know it in the Holy Land, and introduced it into their respective countries on their return from the East. It was in use among the Saracenic and Mohammedan nations, and was extensively employed in Asia. But exactly with what nation in the East the pointed arch originated, and in what manner, are problems equally difficult to solve.

THOMAS H. POOLE.