132 COMPARATIVE ARCHITECTURE. of an older uncovered piazza, used as a " nympha^um," or place for plants, flowers, and running water, the level of its floor being 8 feet below the present level. In front of this " nympha^um," and facing towards the south, was a decastyle portico, forming a frontispiece to a three-cell temple of the Etruscan type, built by Agrippa during the reign of Augustus, B.C. 27-A.D. 14. The present Rotunda was erected by the Emperor Hadrian, in A.D. 120-124, on the site of the more ancient " nymphseum," the portico to the Etruscan temple being taken down and re-erected at the higher level. As rebuilt this portico was made octastyle instead of decastyle, and was made to face the north instead of the south. The Rotunda (now the Church of S. Maria Rotonda) is a circular structure having an internal diameter of 142 feet 6 inches, which is also its internal height. The walls, of concrete (opus incertum), with a layer of tiles every three feet in height, are 20 feet in thickness, and have eight great recesses, one of which forms the entrance ; three of the remaining seven are semicircular exedrae, the other four being rectangular on plan. Two columns are placed on the front line of these recesses, above which are relieving arches. The eight piers have niches entered from the exterior of the building, formed in three heights, of which the lower are semi- circular on plan, and are 19 feet high to the springing of their hemispherical heads, the second tier have their floor on the same level as the cornice over the inner order, and the third tier are level with and entered from the second cornice of the exterior. In front of the Rotunda is the Corinthian octastyle portico, 1 10 feet wide by 60 feet deep in the centre, the first, third, sixth and ei""hth columns having two others behind them. At the back of the portico are niches, and staircases by which to ascend to the various parts of the edifice. The columns, 42 feet 6 inches high, in front of the recesses in the interior, are believed to be part of the original design of Hadrian's architect. The lower third of these columns is cabled, and the upper portion is fluted (No. 55). The marble facing to the walls between, and the columns, entablature, and pediments of the projecting altars are later additions. The attic or upper story was originally ornamented with porphyry or marble pilasters, with capitals, six of which are in the British Museum, of white marble and panelling of giallo antico, serpentine, and pavonazetto, but in 1747 this marble panelling was removed and the present stucco decoration inserted. The dome or cupola is a hemisphere, having its inner surface coffered in five ranges. The manner in which the sinkings or mouldings are regulated or foreshortened so as to be seen from below is worthy of notice.
