In columns of equal diameters, the altitudes increase from the Tuscan, by the gradations of the Doric and Ionic to the Corinthian. In this progression, we per ceive that the Tuscan is stronger than the Doric, the Do ric than the Ionic, and the Ionic than the Corinthian; con sequently, if the Doric be the lowest order, the succeed ing one must be the Ionic; if a third be added, it must be the Corinthian.
Vitruvius, book v. chap. vii. directs the height of the columns of the superior order, to be a fourth less than those of the inferior; but Scammozzi's rule, which is esteemed the best, is, that the lower diameter of the su perior column should be equal to the upper diameter of the inferior, so that the different shafts bear the appear ance of one long tapering tree cut into so many pieces ; disposition appropriately corresponding to the origin of columns.
According to the rule of Vitruvius, if an Ionic column of nine diameters were to be raised upon a Roman Doric of eight, the lower diameter of the Ionic would be two thirds of that of the Doric, which is a much less quantity than is to he found in any ancient or modern example of the diminution of the Doric shaft.
In insulated columns, when the diminution of the su perior order is very considerable, the intercolumn be comes so wide, and the entablature so small and weak, as to be in danger of breaking, which defect must be still increased, if a third range be added ; whence we may again infer, that the theory of Scammozzi is preferable to that of Vitruvius.
For the sake of example, we shall suppose the stronger order to be made the superior, as the Doric on the Ionic. Here let the shaft of the Doric diminish five-sixths of its lower diameter, and its height will be but six diameters and two-thirds of the Ionic below, which would make a complete attic of the Doric, and render the application of the orders in this inverted manner useless, as they could not be made to accommodate the stories of the building, neither would the upper ranges support their own entablature.
When the front of a building is to have two or more orders in the altitude, the succession must be complete, or the symmetry will be destroyed by the abrupt contrast of the parts. In attached columns, the superior order may be permitted to recede, without danger either of greatly offending the eye, or of impairing the strength of the structure, as may be seen in the theatre of Marcellus.
(See Plate CLXXXVI.) But when the stories of orders arc insulated, the axis of the superior and inferior co lumns must be kept in the same vertical lines.
In cases where the upper order only consists in the centre of that below, in two equidistant parts from the middle, the portions of the entablature of the lower or der, in which there is no superior order, are generally finished with a balustrade, level with the sills of the win dows.
We have but few examples in England, of more than two ranges of columns in the smile front. Indeed, when there are three, it is difficult to preserve the character of each, without some striking inconsistencies and defects in the intercolunmiations.
The first and second orders should stand on plinths, as likewise the third, when there is one; the point of view regulating those of the upper stories. In this case, pedestals should be omitted in the upper orders ; but if there be one, or a balustrade under the windows, the base and cornice should have but a small projection, and be continued to profile upon the sides of the columns.
When stories of arcades are raised one upon another, and the piers decorated with orders, the inferior columns should stand on plinths, and those of the upper stories on pedestals, that the arches may receive a due propor tion.
In some cases, instead of employing several orders one above another, the ground floor of a building is made in the form of a basement, on which is placed the order by which the principal story is decorated.
In Plate CLXXXVI. examples will be found of or ders upon orders, with and without arcades and pedes tals. They are taken from the inside of the temple of Pxstuni,—the outside of the Coliseum,—the theatre of Marcellus at Rome, and Whitehall in London.
4rcades.—When an aperture in a wall is too wide to be lintelled, it is arched over, and receives the appella tion of an arcade, which term, in the plural number, indi cates a continued range of such apertures. They are not so magnificent as colonnades, but are stronger, more so lid, and less expensive.