Superimposed Orders
When arcades are used one above the other, the lower order is usually mounted on a plinth, and the upper furnished with a pedestal. The height of the pedestal is determined by the balustrading or balcony, the height of which is governed by its use.
If the Doric is taken as the lower order the centres of the columns are six and a quarter diameters apart, which gives a frieze of five metopes with intervening triglyphs. The plinth on which the order stands is three-quarters of a diameter high. The pilaster supporting the archivolt projects half a diameter, and the height of the arch is determined by the impost, which is two-thirds the height of the column inclusive of the plinth. The base of the pilaster may be moulded, but the top line should coincide with the top of the plinth.
Above the Doric an Ionic order might be placed, and the die and plinth of the superimposed order should be kept as narrow as possible so as to reduce the impression of weight. The pilasters carrying the arch rest on the plinth of the pedestal, and the plinth mouldings are carried round the bases. The plinth and rail of the balustrading should not project but be kept between the pilasters.
The centres of the arches of both tiers are in a line with the tops of the imposts, and the outer edges of the archivolts may nearly reach the lower lines of the architraves.
When the Ionic is used as the lower order it may be surmounted by the Corinthian. The distance between the centres of the lower columns should then be six and a half diameters. The other proportions can be obtained in the same way as the preceding.
If a subsidiary order is employed the columns of the principal order are placed further apart. In the case of the Doric the distance is seven and a half diameters, and the other orders are increased in proportion.