Tim rule given by Vitrurius (b. v. e. 7.) for placing one order above smother, is, to make the columns of the superior or der a fourth part less in height than those of the inferior.
Scamozzi's rule is, to make the diame ter at the bottom of the shaft of the supe rior order equal to the upper diameter of the inferior order.
Let us now suppose that the Ionic of nine diameters is to be raised upon the Doric ofeight diameters, as in the Roman Doric; according to the rule given by Vi troyius, the bottom diameter of the Ionic will be two-thirds of that of the Doric, a quantity much less than is to be found in any ancient or modern examples of the diminution of the Doric shalt; which di minution is the lower diameter of the su perior order, by Scamozzi's rule, In insulated columns, when the diminu tion of the superior order is very great, the intercolumn becomes so wide, and the entablature so small, and consequently weaker, that it is in danger of breaking ; and if a third range is added, this defect must be increased. The Vitruvian rule is therefore not so applicable as the Scamoz zissi, which, for the above reasons, is uni versally esteemed the best, and is the same as if the several shafts had been cut out from one long tapering tree on the other hand, when the diminution of the inferior diameter of the superior order is too little, or nothing, the columns will not only be too high forthe windows, but the lower order will be loaded with unneces sary weight. Let the stronger order be made the superior; for example, let the Doric be placed upon the Ionic, and al lowing the shaft of it to diminish five sixths of its bottom diameter, the height of the Doric column will be only 6 2-3ds diameters of the Ionic below : this would not only make a complete Attic of the Doric, but would render the application of the orders in this inverted way useless, as they could not be made to accommodate the stories of the builslingotor could the upper ranges support their own entabla tures, which must be the consequence in insulated columns.
When the front of a building is to have two or more orders in the altitude,the sue cession ought to be complete, otherwise the harmony will be destroyed by the vio lent contrast of the parts. When columns are attached, a recedure of the superior order will not offend the eye in any great degree, nor will the solidity of the strue tore be impaired ; this is to be seen in the theatre of Marcellus ; but when the sto ries of orders are insulated, it is necessary that the axis of the superior and inferior columns should be in the same vertical lines. If the upper order only insists in the middle of that below in two equidis tant parts from the middle, the portions of the entablature of the lower order, in which there is no superior order, are generally finished with a ballustrade, level with the sills of the windows. In England we have few examples of more than two ranges of columns in the same front ; for when there are three, it is difficult to preserve the character of each order in the intercolumnial decorations, without some striking defects. The first and second orders should stand upon a plinth, and the third also, when there is one, the point of view regulating the two upper plinths. In this case pedestals should be omitted in the upper orders, and if there is one, or a ballustrade. under the windows, the base and cornice should have but a small projection, and should be continued to profile upon the sides of the columns. In raising the stories of ar cades upon each other, with orders deco rating the piers, the inferior columns should be placed upon a plinth,andthesu perior ones upon a pedestal, in order that the arches may obtain a just proportion. Pedimenta. A pediment is a part of a building having a horizontal cornice be low, and two equally inclined ones, or an arched cornice, above, joined at the ex tremities of the horizontal one ; the cor nices including a plane surface within, called the tympanum, which is therefore either a triangle or the segment of a circle.
This definition does not comprehend every species of pediments which have been absurdly introduced ; but it may he said to be the only genuine one, as pedi ments represent the ends of roofs, and were originally intended to discharge the ram from the middle of the building, by compelling it to descend and fall over the flanks or extremes, and not over the front, which must be the case with every other figure that can be introduced, except those of a polygonal form, which present their interior angles to the horizontal cornice, or the exterior ones upwards. To find the pitch of the pediment Vitru vius directs as follows : divide between the extremities of the cymatium of the corona into nine equal parts, and one makes the height of the tympanum; but this rule is not correct, as the tympanum will vary its angles according as there are more orless mouldings of the inclined cornices within the extremities of the cy matium of the corona; for since the mid dle part by this rule is invariable, and the broader the parts are of the inclined cor nices within each extremity of the cyma tium of the corona, or rather within the under edge of the fillet of the syma upon each inclined cornice, the less is the base of the tympanum, and consequentlythe ver tical angle less obtuse, and the base angles less acute ; but if this height extended to the meeting of the two under sides of the fillets of the syma, or crowning moulding, then the figure of the tympanum would be invariable. The Vitruvian rule has been thought by many to be too low; but it is to be recollected, that that of the Parthenon at Athens, which has an octostyle portico, is nearly of this proportion ; that of the temple of Theseus, which has an hesa style portico, is about one-eighth ; that of the Ionic temple on the Ilyssus, and of the Doric portico, which are both tetra style, are about one-seventh ; the tympa num of the pediment of the door on the Tower of the Winds is about one-fifth of the span. The edifices here mentioned are all Athenian buildings. From this comparison it would appear, that a kind of reciprocal ratio subsists between the extension of the base of the tympanum and its height. Indeed, if a fixed ratio were applied to windows, the pediment would frequently consist of a cornice, without the tympanum. It is therefore with great reason that we often make the pitch of pediments of than those which crown porticos, or the fronts of buildings. The plinths by which pedi ments are sometimes decorated are call ed acroterions, or acroters : the two which present triangular faces at the ex tremes have their heights, according to Vitruvius, half of that of the tympanum, and the middle one saddled on the sum mit is one-eighth part higher than those at the extremes. Pediments owe their origin most probably to the inclined roofs of primitive huts. Among the Romans they were only used as coverinTs to their sacred buildings, till Cmsar obtainedleave to cover his house with a pointed roof, after the manner of temples. In Gre cian antiquity we meet only with tri angular pediments, and in Roman build ings we meet with both the triangular and circular. In rows of openings, or niches, both kinds of pediments were employed in the same range, and disposed in alter nate succession. The horizontal cornices of pediments should never be discontinu ed, as may be seen in many of the street houses of London, in order to give room for a fan light, and to lessen the expenses of the frontispiece, by introducing short er columns and a less massy entablature : for since the horizontal cornice represents the tie-bearn, and the inclined ones the rafters, the columns will appear to have a. tottering effect, bv spreading them out at the top beyond ale extremities of their bases.