COLONNADE, (from the Italian colonna,) a range of attached or insulated columns, supporting an entablature. The interval between the columns, measured by the inferior diameter of the column, is called the intercolumniation, and the whole area hetween every two columns is called an inter column. When the intercolumniation is one diameter and a half, it is called pycnostyle, or columns thick set ; when two diameters, systyle; when two and a quarter, eustyle ; when three, diastyle; and when four, arwostyle, or columns thin set. Columns are sometimes set two and two together, having half a diameter for the smaller interval, and three and a halt' diameters for the larger; this disposition is termed aramsystyle. A colonnade is also named according to the number of columns which support the entablature, or fast igium ; as, when there are four columns, it is called tetras/111e; when six, hexastyle ; when eight, octasty ; and when ten, decastyle. The inter coin mniations of the Doric order are regulated by the number of triglyphs. placing one over every intermediate column ; when there is one triulyph over the interval, it is called ni0110 tri g ph ; when there are two, it is called ditriylyph ; and so on, according to the progressive order of the Grecian nume rals. The interco]uniniation of the Grecian Doric is almost constantly the I oh, fin. there are only two deviations from this to be met with at Athens ; the one in the Doric portico, and the other in the portico forming the entrance to the Acropolis. or citadel ; but these intervals only belong to the middle intereolumniations, which are both ditriglyphs, and became on ate mitt of their being opposite to the principal entrances. A, the character of the Grecian Dorki !mire massdve and dipatitied than that of the Roman, tlu monot riglyph succeeds best ; but in the Roman it is not so convenient, for the passage through the intereolumns would be too narrow, particularly in small buildings ; the ditriglyph is therefore more generally adopted. The arico style is only applied to rustic structures of the Tuscan order, where the intereolumns are lintelled over with architraves. \When the solid paits of the masonry of a ratwe of arcades are decorated with the orders, the intereolumns necessarily 1)1a-utile wide. and the intereohnnniation is regulated by the breadth of the arcades and of the piers.
Buildings with a colonnade projecting at one end are termed prostyle ; with a colonnade at both and opposite ends, amphiprostyle ; with the same on all sides of the building, peristyle ; and with a double range of columns, polystyle.
It does not appear that coupled, grouped, or clustered columns ever prevailed in the works of the ancients; though, on many occasions, they would have been much more useful ; we indeed find, in the Temple of Bacchus, at Rome, columns standing as it were in pairs ; but as each pair is only placed in the thickness of the wall, and not in the front, they may rather he said to be two rows of columns, one almost imtne diately behind the other. In the baths of Diocletian, and in the Temple of Peace, at Rome, we find groined ceilings sus tained by single Corinthian columns ; but such a support is both meagre and inadequate. Vignola uses the same inter columniation in all his orders. This practice, though con demned by some, is founded upon a good principle, for it preserves a constant ratio between the columns and the inter vals. Of all the kinds of intercolumniation, the eustyle was in the most general request among the ancients ; and though, in modern architecture, both the eustyle and diastyle are employed, the former is still preferred in most cases : as to the pycnostyle interval, it is frequently rejected for want of room, and the armostyle for want of giving sufficient sup port to the entablature.
The moderns seldom employ more than one row of columns, either in external or internal colonnades, for the back range destroys the perspective regularity of the front range ; the visual rays coming from both ranges produce nothing but indistinct vision to the xpeetator. This confusion, in a cer tain degree also attends pilasters behind a row of insulated columns; but in this the relief is stronger, owing to the rotundity of the column and the flat surfaces of the pilasters. When buildings are executed on a small scale, as is frequently the case in temples, and other designs, used for the ornaments of gardens, it will be found necessary to make the inter columniations, or at least the central one, broader than usual, in proportion to the diameter of the columns ; for when the columns are placed nearer each other than three feet, the space becomes too narrow to admit more than one person conveniently.