divorce, superseding the unity and indissolubility of the marriage bond. The Encyclical points to the consequences of that departure in the breaking up of family life, and its evil effects on society at large. It points out as a consequence, that the Church, in asserting its autliority over the marriage contract, has shown itself not the enemy but the best friend of the civil power and the guardian of civil society. In conclusion, the Encyclical commissions all bishops to oppose civil marriage, and it warns the faithful against the dangers of mixed marriages.
Acta Sancia Sedis (Rome. 1880). XII, 385-405, tr.; Wynne. Great Encyclicals of Leo XIII (New York, 1903), 58-82; and Eyre. The Pope and Oie People (London, 1896), 176-206. An excellent commentary is that of Mgr, James Corcoran, in Am. Cath. Quar. Review (Philadelphia. ISSO), V, 302-32. M. O'RiORD.VN.
Arch. — A structure composed of separate pieces, such as stone or briclis, having the shape of truncated wedges, arranged on a cur^-ed 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 Greelc 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 liorizontal. 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 suppo.ts which af- ford resting and resisting points to the arch are called piers and abutments. The upper part of the pier or abutment where the arch rests — techni- cally, 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.
FouM.s OF Arch. — In Rome and Western Europe, the oldest and normal type of arch is the semi- circular. 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 le.ss than a semicircle (a segment of a circle), with its centre below the diame- ter, 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 witli 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 cen- tres are still on the line of the diameter, but inside. The third class consists of arches struck from three centres. This is llu; three-centred or "basket- handle" arcli. 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 difTorent ways, and the long curves some- times rej>la(f(l by straight lines witli a sliort curve at the juncture. Another variety of arch struck from three or four centras is the ogee arcii. 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 tre- foliated 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 multifoiled 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 sliglitly 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 or- ders the faces are in the same plane. But as a rule the orders are successively recessed, i. e. the inner- most sub-arch, or order, is narrow, the next above it broader, the next is broader still, and so on. Semicircul.vr Arch. — This arch is specially char- acteristic 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 le^•el as a broad arch, so that the crowns may be on the same level. Seg- mental 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. t>. 540. In Eastern ■work the horseshoe arch is frequently not round- headed, but acutely pointed. This facilitates con- struction, as the upper or more difficult portion of the arch or dome can then be constructed by corbell- ing 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 fiat 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 \ault througliout with stone a clcrestoried church, Basilican in plan, without the aid of the pointed arch, yet the employment of the pointed arcli greatly facilitated building construction. Next to tTie 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 loss lateral thrust than a semicircular one. It was of the greatest use in vaulting.