NVIten columns arc detached, as in the triumphal arches of Septimius Severus and Constantine, at Rome, it becomes ne cessary to break the entablature, making its projection over the intercolumns the same as if pilasters had been used instead of columns ; or so much as is just suffi cient torelieve it from the nakedness of the wall. This is necessary in all interco lumns of great width, but should be prac tised as little as possible, as it destroys the genuine use of the entablature. When columns are without pedestals,they should stand upon a plinth, in order to keep the bases dry and clean, and prevent them from being broken.
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 arcade ; and the an gular pier should have an addition of a third, or a half, as the nature of the de sign may require. The impost should not be more than one-seventh, nor less than a ninth, of the breadth of the arch ; and the archivolt not more than one eighth, nor less than one-tenth, of that breadth. The breadth of the bottom of the key-stone should be equal to that of the archivolt ; and its length not less than one and a half of its bottom breadth, nor more than double. In groined porticos, the thickness of the piers depends on the width of the portico, and the superincum bent building; but with respect to the beauty of the building, it should not be less than one quarter, nor more than one. third, of the breadth of the arcade. When the arcades form blank recesses, the backs of which are pierced with doors or windows, or recessed with niches, the recesses should be at least so deep, as to keep the most prominent -parts of the dressings entirely within their surface. In the tipper stories of the theatres and amphitheatres of the Romans, the arcades stood upon the podia, or inner-pedestals, of the columns ; perhaps as much for the purpose of proportioning the apertures, as to form a proper parapet for leaning over.
Colonades. A colonade is 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 tercolumniation ; and the whole area be tween every two columns is called an in tercolumn. When the intercolumniation is one diameter and a half, it is called pycnostile, or columns thick set ; when two diameters, systyle ; when two and a quarter, custyle ; when three, diastyle ; and when four, arzostyle, or columns thin set. A colonade is also named according
to the number of columns which support the entablature, or fastigium : when there are four columns, it is called tetrastyle ; when six, hexastyle ; when eight, octo style and when ten, decastyle. The intercolumniations of the Doric order are regulated by the number of triglyphs, placing one over every intermediate co lumn : when there is one triglyph over the interval, it is called monotriglyph ; when there are two, it is called &tri glyph ; and so on, according to the pro gressive order of the Greek numerals. The intercolumniation of the Grecian Do ric is almost constantly the monotriglyph: from this practice there are only two de viations to be met with at Athens, the one in the Doric Portico, and the other in the Propylea ; but these intervals only be long to the middle intercolumniations, which are both &triglyph, and became necessary, on account of their being op posite to the principal entrances. As the character of the Grecian Doric is more massy and dignified than that of the Ro man, the monotri&lyphic succeeds best; buLin the Roman it is not so convenient, for the passage through the intercolumns would be too narrow, particularly in small buiklings, the ciitriglyph is therefore more generally adopted. The arzostyle is only applied to rustic structures of Tuscan intercolumniations, where the columns arc lintelled with wooden archi traves.
When the solid part of the masonry of a range of arcades are decorated with the orders, the intercolumns become neces sarily wide; and the intercolumniation is regulated by the breadth of the arcades, and that of the piers.
It does not appear that coupled, group ed, or clustered columns, ever obtained 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 but as each pair is only placed in the thickness of the wall, and not in the front, they may ra ther be said to be two rows of columns, one almost immediately behind the other. In the baths of Dioclesian, and in the temple of Peace at Rome, we find groin ed ceilings, sustained by single Corinthian columns; a support both meagre and in adequate. Vignola uses the same inter columniation in all his. orders: this prac tice, though condemned by some, is founded upon a good principle ; it pre serves a constant ratio between the co lunins and the intervals.