In the ancient buildings of Greece, (see Plate CLVII. and CLVIII.), we find only the triangular pediment ; but in those of Rome, both triangular and circular are to be met with ; and in rows of openings, or niches, both kinds are employed in alternate succession. Though they seem to have preferred the triangular form for doors, indows, and gates, and to have applied the circular pe diment to the covering of large or small bodies promis cuously.
Pediments, among the Romans, were exclusively ap propriated to sacred edifices, till Cresar obtained leave of the senate to cover his house with a pointed roof, af ter the manner of the temples.
The ancients introduced but fuw pediments into their buildings; even the Romans usually contenting them, selves with a single one to adorn the middle or principal part. But sonic of the moderns, and the Italians in par-. ticular, have become so immoderately fond of them, that sometimes their buildings can hardly be said to consist of any thing besides.
VitruN Ms has given the following direction for finding the pitch of a pediment : " Divide the space between the extremities of the cymatium of the corona into nine equal parts, and take one for the height of the tympanum." This rule, how ever, is incorrect ; because the tympanum will vary its angle according as there are more or less mouldings of the inclined cornices within the extremities of the cymatium of the corona ; and since, by this rule, the middle part is invariable, and the broader the its are of the inclined cornices within each extremity of the cymatium of the corona, (or rather within the under edge of the fillet of the sima upon each inclined cornice), the less will be the base of the tympanum ; consequently, the three angles will be less acute. But were this height extended to the meeting of the two under sides of the fillets of the sima, or crowning moulding, the figure of the tympanum would be invariably the same.
The foregoing rule of Vitruvius has been considered as too low ; but it must he recollected, that that of the octostyle portico of the Parthenon at Athens, is nearly of the proportion there described ; that of the hexastyle portico of the temple of Theseus, is about an eighth ; that of the Ionic temple on the Ilyssus, and of the Doric portico, which are both tetrastyle, are about one-seventh; and the tympanum of the pediment over the door of the tower of the Winds, is about one-fifth of the span; all which edifices are Athenian.
From this comparison, a kind of reciprocal ratio seems to exist between the extension of the base of the tympa num and its height ; and, indeed, were a fixed ratio ap plied to windows, the pediment would frequently consist of a cornice only, without any tympanum. It is, there fore, with good reason, that the pitch of pediment win dows is frequently made greater than of such as crown porticos or the fronts of buildings.
Pediments are sometimes decorated with plinths, call ed acrotcrions, or acroters; of which, Vitruvius observes, the two presenting triangular faces at the extremities, each in height half that of the tympanum, and the mid dle one, saddled on the summit, is one-eighth higher than the others.
It is an observation of Vitruvius, that " the Greeks never used mutules, modillions, or dentils, in the front, wherein the cud of the roof or fastigium appears ; be cause the ends of the rafters, and of the laths which sup port the tiles, only appear at the caves of the building ; and as mutules and (lentils originated from the project ing ends of the rafters and laths, it would have been ab surd to introduce them into the pediment, where the ex emplars are themselves to be seen." But, just as this argument is, we find in the Grecian remains, that though neither mutulcs, (lentils, nor modillions, are employed in the sloping sides, mutules are constantly used in the horizontal cornice. In edifices of the period of Roman domination in Greece, indeed, we sometimes may ob serve them in the sloping cornices, but they must be considered as innovations. At Rome, we have examples of modillions in the Pantheon, and in the frontispiece of Nero ; and in the temple of Fortune, dentils are used. In the inclined cornices, the sides of the modillions and dentils are planes perpendicular to the horizon and to the front of the edifice, and in the same vertical planes of those of the horizontal cornice.