arch1 (arch)

[ ML. archia), mod. F. arc he, an arch, fem. form (prob. by confusion with OF. and F. arc he, ark, lowest vou oirs are termed springers, and the uppermost or central one, when a single stone occupies this position, is called the keystone. The under or concave face of the VVVVV Ex In. Rise Ex In. Spian Extra dosed Arch. sa IMG:content-1066.png:[ocr errors] a, abutments; 2, vou oirs; s, springers; 1, imposts; In, intrados;, piers; é, keystone; Ex, extrados. a embled vou oirs is called the intrados, and the upper or convex face the extrados, of the arch. When the curves of the intrados and extrados are concentric or parallel, the arch is said to be extra dosed. The supports which afford resting and resisting points to the arch are piers or pillars, which receive the vertical pre ure of the arch, and abutments, which resist its lateral thrust, and which are properly portions of the wall or other structure above the springing and abreast of the shoulder of the arch. The upper part of the pier upon which the arch rests (technically, the point from which it springs) is the im post. The span of an arch is the distance between its opposite imposts. The rise of an arch is the height of the highest point of its intrados above the line of the imposts; this point is sometimes called the under side of the crown, the highest point of the extrados being the The thrust of an arch is the pre ure which it IMG:content-1067.png:[blocks in formation] Milton , P. L., vi. 759. crown . exerts outward. This pre ure is practically collected, so far as it is manifested as an active force, at a point which cannot be exactly determined theoretically, but is at about one third of the height of the rise of the arch. The thrust must be counteracted by abutments or buttre es. Arches are designated in two ways: First, in a general manner, according to their properties, Skew Arch. their uses, their position in a building, or their exclusive employment in a particular style of architecture. Thus, Segmental Arch. Semicircular Arch. there are arches of equilibration , equipollent arches, arches of discharge, skew and reversed arches, Roman, Pointed , and Saracenic arches . Second, they are named specifically, according to the curve the intrados a umes, when that curve is the section of any of the geometrical solids, as segmental, semicircular , cycloidal , elliptical , parabolical , Cycloidal Arch. Elliptical Arch. hyperbolical , or catenarian arches ; or from the resemblance of the whole contour of the curve to some familiar object, as lancet arch and horseshoe arch; or from the Lancet Arch. Horseshoe Arch. method used in describing the curve, as equilateral , three centered, four-centered, ogee , etc. When an arch has one of its imposts higher than the other, it is said to be ram Arch of Discharge. 5. In mining , a portion of a lode left standing, either as being too poor for profitable working or because it is needed to support the adjacent rock.-6. The roofing of the fire-chamber of a furnace, as a reverberatory or a gla -fur- nace; hence, sometimes, the fire-chamber itself. - Alveolar arch, aortic arch. See the adjectives. - Arch of discharge, an extra dosed arch built in the ma- sonry of a wall, over a doorway or any other open or weak place, to transfer pre ure from above to points of a ured sta- bility on either side. An arch of discharge is generally dist in- guishable to the eye from the wall in which it is built mere- ly by the position of its stones, or at most by a slight projec- tion beyond the wall-surface.- Arch of the fauces. See fauces . Axillary arches. See axillary . - Back of an arch. See back1.- Backing of an arch. See backing . - Basket-handle arch, an el- liptical arch, or a three-centered low-crowned arch. - Blind arch, an arch of which the opening is walled up, often used as an arch of discharge . See cut under arcature . - Bran- chial arch. See branchial . - Clustered arch, a number of arched ribs springing from one impost, a form usual in medieval pointed vaulting. See cut under ( clustered ) col- umn .-Court of Arches. See court. - Crural or in- guinal arch. See crural. Flat arch, an arch of which the intrados is straight, the vou oirs being wedge-shaped and a embled in a horizon- tal line: used especially in brickwork, where the charge to support is not great. Hemal arch, hydrostatic arch. See the adjectives. Laminated arch, a beam in the form of an arch, constructed of several thickne es of planking bent to shape and Flat Arch. (From Viollet-le-Duc's "Dict. de l'Architecture.") bolted together: a form of arched beam. - Mandibular arch, mural arch, neural arch, etc. See the adjectives.-Oblique arch. Same as skew arch (which see, below).-Pectoral arch. Same as pectoral girdle (which see, under girdle ). - Pelvic arch. Same as pelvic girale (which see, under girdle ). - Preoral arches, postoral arches. See the adjectives. - Rece ed arch, one arch within another. Such arches are sometimes called double, triple, etc., arches, and sometimes compound arches. - versed arch, an inverted arch. - Ribbed arch, an arch composed of parallel ribs springing from piers or imposts. - Rough arch, an arch formed of bricks or stones roughly dre ed to the wedge form. – Round arch, a semicircular arch. - Skeletal arches. See visceral arches , under vis ceral . Skew arch, an arch of which the axis is not perpendicular to its abutments. - Stilted arch, an arch of which the true impost is higher than the apparent im Stilted Arches.-Modern Romanesque. post, or of which the piers are in fact continued above the apparent impost, so that a portion of the intrados on either side is vertical. - Surmounted arch, a stilted semicircular arch; a semicircular arch of which the rise is greater than the radius.- Triumphal arch, a monumental arch in honor of an individual, or in commemoration of an event. Such arches were first erected under Triumphal Arch.- Arch of Const an tine, Rome. archæan the Roman emperors, and were originally temporary structures, festooned and otherwise decorated, standing at the entrance of a city, or in a street, that a victorious general and his army might pa under them in triumph. At a later period the triumphal arch became a richly sculptured, ma ive, and permanent structure, having an archway pa ing through it, and often a smaller arch on either side. The name is at the present day often given to an arch, generally of wood decorated with flowers, evergreens, banners, etc., erected on the occasion of some public celebration or rejoicing. The great arch in a church which gives acce to the choir-the chancel arch-is sometimes so called. In early Christian churches, a representation of the Glory or Triumph of Christ sometimes occupied a wall-space above this arch. Statues, and trophies, and triumphal arcs, Gardens, and groves, presented to his eyes. Milton , P. R., iv. 37. Twyer arch, an arched opening in a smelting-furnace to admit the blast-pipes. - Tymp arch, the arch above the tymp in a blast-furnace. See tymp. - Vascular arches. See visceral arches, under visceral. - Visceral arches. See visceral.