ARCH, an architectural term denoting a structural form made up of a series of wedge shaped stones or bricks so arranged over a door or window in an edifice for habitation, or between the piers of a bridge, as to support each other and to carry in addition the weight of the superstructure. These stones and bricks, of a truncated wedge shape, used in building arches, are called voussoirs. The side of an arch between the crown and the springer, or skewback, is called its haunch or flank, and by old English writers of the 16th century, its hanse. The highest part of the arch is called its crown, or by the old English authors, the scheme or skeen, from the Italian schiissa, the back. The lowest voussoirs of an arch are called springers, or skewbacks, and the cen tral one, the keystone. The under or concave side of the voussoirs is called the intrados, and the outer or convex one the extrados of the arch. A chord of the arch at its lower part is called its span, and a line drawn at right angles to this chord and extending upward to the under side of the keystone is called its rise. The impost of an arch is the portion of the pier or abutment whence the arch springs; the thrust of the arch is its outward pressure against the abutments. The voussoirs are also called ring-stones.' The spandrel is the part above the haunches, or, in a bridge, the part between the arch-ring and the roadway. If the height of the crown of an arch above the level of its impost be greater than half the span of the arch, the arch is said to be sur mounted. If, on the contrary, it be less, the arch is said to be surbased. The curved arch was known to the Assyrians and the old Egyp tians. Sir J. G. Wilkinson considers that it existed in brick in the reign of Amenophis I, about 1540 p.c., and in stone in the time of Psammetichus II, 600 B.C. The evidence is de rived from the ruins of actual buildings, but paintings appear to carry the arch back to about 2020 D.C. There is no mention of the genuine arch in Scripture, the term *arches,* in Ezek. xl, 16, being a mistranslation. The round arch was brought into extensive use by the Romans and prevailed everywhere until the 12th century A.D.,. when the arch pointed at the apex, and called consequence the pointed arch—the one so frequently seen in Gothic architecture — appeared in Europe as its rival.
The forms of both curved and pointed arches may be varied. indefinitely. Of the former may be mentioned the horseshoe arch, a name which explains itself, and the foil arch, from Latin folium, a leaf, of which there are the trefoil, the cinquefoil and the multifoil varieties, so named from the plant-forms after which they are modeled. Other arches are the equilateral, in which the centres of the circles whose inter section constitutes the pointed arch coincide with the angular points at the two sides of the base; the lancet, in which the centres of the circles fall beyond these points; the drop arch, where they fall within the base; and the seg mental arch, the sides of which constitute seg ments of circles containing less than 180 de grees. Besides these there are several other varieties of arch distinguished by their respec tive forms. The names applied to arches may be divided into several classes, as referring to geometric or familiar forms, style or position in the building. The following are different geometrical forms: The flat arch, with vous soirs radiating from one centre. Arches with one centre are: semi-circular, segmental, horse shoe. Arches with two centres are: the equilat eral pointed arch, where the centres of the circles coincide with the angular points at the two sides of the base; the drop arch, where they fall within the base; the lancet, where they fall outside of it, and the pointed horse shoe. The common three-centred arch is called basket-handled arch, this being the form generally used instead of an ellipse. Four centred, six-centred and other similar forms are occasionally used. The names horseshoe, lancet, basket-handled, etc., are given because of their resemblance to familiar forms. Gothic, Roman and Moorish arches are names given because these forms were used in those archi tectural styles. Certain names are given with reference to the position of the arch in the building, such as discharging or relieving arch, where the arch is placed over a lintel to carry pressure to the sides.