The profile of the horse-shoe arch is that of a segment of a circle greater th m a semicircle, or, in usher words, it is a circular segmental arch, which is struck front a centre above the springing Inc.i A modifieation or the saute titian is used. which may be termed the pointed-crescent or borse-shor arch. This !Min consists of two segments meeting in it point at the apex, and is struck from tWo centres, both, as bef'ore, above the springing line.
The sem/cirri/tar arch was borrowed by them, and used occasionally in conjunction with the others. The pointed arch is by some supposed to be invented by the Arabians, and to have been copied from them by the Gothic architects; be this, luy•ever, as it may, it is certain that it was exten sively in use amongst them, and is found in Persia, Egypt, and Arabia, but was most prevalent at 11;e»lad and in the East. It is said to have been introduced by the house of El Abbas, who adopted it in opposition to the crescent form which had been employed by the rival house of the Otmniades ; but even by them the old form was not entirely discarded, being retained in the principal entrances, and also in the form of the domes.
Another variety of arch is the stilled, which is of semi circular form, the centre from which it is struck being taken, as in the case of the horse-shoe arch, soniew hat above the springing line; but instead of the circle being continued downwards, and contracted in width towards the impost line, the ends of the semicircle are carried down straight or per pendieularly, so as to give an appearance of elevation and lightness to the arch. which it would not otherwise possess.
The last fitrm which we shall notice is the cusped or seal toped arch, the outline of which is similar to the poly foil headed compartments in Gothic apertures, Icing produced by three or more intersecting semicircles. There is this diGrence, however, that whereas the Gothic examples are merely ornamental accessories, the Moorish form main parts of the construction ; the scallops are large and form the out ins of their main arches, the extrados as well as the intrados following the same profile.
All the above forms of arches are treated in various ways, both as to their impost and decoration. Sometimes the arch springs directly from the solid wall, at other times it is made to rest upon columns. In the case of the horseshoe When the former arrangement takes place, the whole of the arch rests upon the wall, or, in other words, the will projects as far as the 11106t 1)1'0111111Na part of the arch, so that the arch does not overhang the impost; but in the other arrange.
went, the columns are recessed back so as to range with the extremity of the diameter with which the arch is struck, thus leaving a space between the columns as wide as the diameter or extreme width of the arch; the capital or the column, or an impost moulding above it, projects fitrward so as to sustain the Of the arch. Sometimes, but more especially when stilted, the arch is supported by corbels prejecting from the walls, and serving as imposts. This practice of eorbelli»g is very presalent. 111 some cases the face or the rich, that is. the width between the intrados and extrados, is continued down the impost -jambs, hut in others it steps at the spring'ng,. This face ag min varies cep siderably in breadth, and sometimes is not shown at all on the fare of the is all ; it is generally, however, a wide band or a•chisolt fOLowing the outline of the arch, sometimes plain, showing only the joints, sometimes plain with the addition of a menlillng round the extrados, and at others both moulded and otherwise decorated. The deeeration is usually in compartments fin med by the joints, and ft equently the alternate vttussoirs are contrasted by a differenve of ornamen tation. The depth of the archivolt was sometimes so great as to equal the radius. Cusping in the direction of the out line of the arch is a common mode of decoration, being some times applied in the shape of a moulding round the extrados, and sometimes the intrados or sellit only being so cut or serrated. Occasionally the arches of apertures are left blank, being filled up with walling, and a squa•e-headed aperture underneath ; this is especially the case with the ereseent arch. Arches of whatever kind are generally placed within a square-headed panel or compartment, which is fre quently surrounded by a border or plat-band, somewhat similar to the square hood-mould of the perpendicular period of Gothic architecture. This hand, as well as the spandrels intercepted between it and the arch, are usually covered with ornamentation of various descriptions. Sometimes two plat bands are introduced, with a space between them, whieh is commonly Idled up with inscriptions or other decoration. This last method, how ever, is not common except in large apertures, or in principal entrances.