Arch

Collier's New Encyclopedia (1921)

in architecture, 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 even bear a great superincumbent weight. The stones and bricks of a truncated wedge shape used in building arches are called voussoirs. The sides of an arch are called its haunches or flanks. The highest part of the arch is called its crown, or by the old English English authors the scheme or skeen, from the Italian schiena. The lowest voussoirs of an arch are called springers, and the central one which holds the rest together 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 to the arch at its lower part is called its span, and a line drawn at right angles to this chord, and extending upward to its summit, is called its height. The impost of an arch is the portion of the pier or abutment from which the arch springs. If the height of the crown of an arch above the level of its impost is greater than half the span of the arch, the arch is said to be surmounted. If, on the contrary, it is less, then the arch is said to be surbased. The curved arch was known to the Assyrians and the Old Egyptians. The arch was brought into extensive use by the Romans, and everywhere prevailed till the 12th century A. D. when the arch pointed at the apex, and called in 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, 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 plants after which they are modeled. Other arches are the pointed one; the equilateral one, the drop arch, lancet arch, etc. ARCHEAN (ar-kē'an) ROCKS, the oldest rocks of the earth's crust, crystalline in character, and embracing granite, syenite, gneiss, mica-schist, etc., all devoid of fossil remains. These rocks underlie and are distinctly separate from the stratified and fossiliferous formations, which indeed have chiefly taken origin from them. ARCHEOLOGICAL INSTITUTE OF AMERICA, a society founded in Boston in 1879 for archæological research. It has about twenty affiliated societies with headquarters in different cities. It has founded several schools for classical studies, including those in Athens, Rome, Palestine, and the United States. The society has carried on many important researches in European countries and in various parts of the United States and Central America. It has about 2,000 members.