In the construction of arcades, the piers require the utmost care, to have them of sufficient strength to re sist the pressure of the arches, particularly those at the extremities. In large arches, the key-stones should never be omitted, and they should be carried to the soffit of the architrave, where they will be useful in supporting the centre of the entablature, which would otherwise have too great a bearing. The altitude of arcades should never be much more nor much less than double their breadth. The breadth of the pier should seldom exceed two-thirds, nor be less than one-third of that of the ar cade; and the angular one should have the addition of a third, or even one half more than the rest, according to the nature of the design. The impost should not be less than a ninth, nor more than a seventh of the breadth of the arch ; and the archivolt not less than a tenth, nor more than an eighth of the same breadth. The bottom of the key-stone should be equal in breadth to that of the archivolt; and its length not less than one and a half, nor more than double its bottom breadth. In groincd por ticos, the thickness of the piers depend on the width of the portico and the superincumbent buildings. It should not, however, where beauty and symmetry are consult ed, exceed one-third, nor be less than one-fourth of the breadth of the arcade.
When the arcades form blank recesses, with the backs pierced for doors, or windows, or recessed with niches, the recesses should be at least sufficiently deep, to keep the most prominent parts of the dressings entirely within their surface.
The decorations of arcades may consist of rustics, co lumns, pilasters, cariatides, Persians, or termini, sur mounted with appropriate entablatures; and when the piers arc of sufficient breadth, niches are introduced. The arch is either surrounded with rustic work, or with an archivolt, sometimes interrupted at the summit with a key-stone, in the form of a console, or marsh, or some other appropriate sculpture. In some instances, the ar chivolt rises from a platband, or impost, placed on the top of the piers ; in others, from an entablature, support ed by columns on either side of the arch. Sometimes the arches are entirely supported by single or coupled columns, without the entablature, as in the temple of Vauntis at Rome ; but this is departing from the purest Roman style, and approaching to the Gothic.
In the cases where the columns are detached, as in the arches of S,Ttimius Sevcrus, and of Constantine, it be comes necessary to break the entablatme, and to make its projection over the intercolumns. t •e same that it would have been had pilaw', s been used, or just suffi cient to relieve it from the surface of the wall. Though this be requisite in all interculumniations of great width, it should be practised as little as possible, because it de stroys simplicity and the genuine use of the entablature.
In the upper stories of the theatres and amphitheatres of the Romans, the arcades stood upon the podia, or in terpedestals of the columns, perhaps as much for the purpose of proportioning the apertures, as for the for mation of a parapet. When columns are destitute of pedestals, they should stand upon a plinth, that the bases may be kept dry and clean, as well as to prevent them from being broken.
The Romans employed arcades in their triumphal arches, and in various other buildings. They may with great propriety be adopted for gates of cities, palaces, gardens, or parks, and arc much used in the piazzas or squares of Italian cities. In the latter, they are of great utility for the purposes of shade and shelter in hot or rainy weather, but they darken the apartments of the houses, and, in these northern climates, render them very uncomfortable.
Basements.—This term is applied to the lower story of a building, whether on a level with, or below the sur face of the earth. The height of the basement is deter mined by its being the cellar or the ground story, or as there may be principal rooms in one or both of them. It should never, however, exceed that of the story next above it, for this would invert the order of the edifice, by making the base of more consequence than the body supported by it.
When the basement consists of the cellar story, it should not exceed five, but at most six feet in height above ground, and may be either plain, rusticated, or formed into a continued pedestal : but when it forms the ground story, it is customary to decorate it with rus tics, supported on a base, and surmounted with a crown ing string course. The base itself may be either a plinth simply, or a plinth with mouldings over it; the string course, likewise, may be a plain platband, or a plat band with mouldings under it ; or it may form a cor nice.
The rustics are formed of a rectangular or a triangu lar section, by imagining one of the sides of such sec tion to be a line extended across the front of the joint. The joints of the rustics may be from an eighth to a tenth part of their height. Triangular rtics may have the depth of their joints half their breadth; that is to say, snaking the two planes by which the joint is formed a right angle. The depth of the joints of rectangular rustics, may be from one-fourth to one-third of their breadth.
Both directions of the joints were marked by the an cients; but the moderns frequently make their rustics with the horizontal joints only; in which cases, they seem rather to represent a boarded surface than a stone wall, whose very nature indicates two directions of joints.