Although the Romans employed arches in the construction of their edifices, to a very great extent, yet they always confined them to one form, namely the semicircular. It is to the architects of the middle ages we are indebted for the great variety of figure employed in this kind of construction ; among others we may especially notice the pointed arch,—but for further information on this subject, we beg to refer the reader to that particular style of architecture.
Of the forms, cf•c. of arches. Arches are named according to the curve assumed by them, as circular, elliptical, cycloidal, parabolical, hyperbolical, catenarian, &c. : circular arches are again subdivided according to the quantity of the circum ference described by them, such as semicircular, segmental or surbased, containing less than the semicircumference, surmounted, horse-shoe or Moorish, containing more than the semicircumference. Arches are also denominated accord ing to the method adopted in describing the curve, as two, three, or four-centred arches ; also by the nature of the angle formed at the apex, thus, pointed arches are distinguished by the appellation of lancet, equilateral, and depressed. Further, there are arches of equilibration and of discharge ; askew and reversed arches.
The separate masses or stones, of which the arch is com posed, are called voussoirs or arch stones, the central or uppermost of which is called the key-stone, the lowermost, or those nearest the supports, springers. The highest
point in an arch is termed the 'vertex, or crown, the lowest line the springing line, and the spaces between the crown and springing line on either side, the haunches, or flanks.
The under or concave surface is denominated the intrados, the upper or convex the extrados. The supports of an arch are called piers, abutments, springing walls or reins. Piers are distinguished from abutments, the former term being applied to a support to resist a vertical pressure, the latter an horizontal thrust. The upper parts of the supports on which the arch rests, or from which it is said to spring, are named imposts. The span of an arch is the width between the points, where the intrados meets the imposts on either side, which in the case of circular arches coincides with the chord of the arc : the rise is the height of the highest point in the intrados above the springing or Arches which have the curves of both intrados and extrados concentric or parallel, are said to be extradossed ; and such as rise from supports at unequal heights, are called rampant arches. There are other kinds of arches, but these are more applicable to VAULTING, under which head they will be treated of In order to avoid farther extending this article, we must refer the reader for the THEORY of the ARCH, to STONE