Polygonal forms of plan were sometimes employed, of which there is an instance in what la called the temple of 3linerva Medics at Rome, which is circular on the exterior, but internally decagonal, with Dino of Its aides occupied by as many recesses, and the other by the door way—a remarkable peculiarity, it being very unusual to enclose a polygon within a cylindrical structure, although not the contrary, nor to erect a cylinder upon a square or polygonal basement, Octagonal plena were by no means uncommon : such form was frequently made use of for the saloons of public baths; and there is an instance of an octagonal temple, aupponed to have been dedicated to Jupiter, in one of the courts of Diocletian's palace at Spalatro. Of hexagonal etrue turea we are acquainted with no example, but a court with six auks occurs in the remains of the temple of Baalbec, not however a regular hexagon, but of elongated figure, two of the sides being 110, and the remaining four SS feet each. In the later periods of Roman architecture, circular and polygonal structures became more frequent, and those of the first-mentioned kind deviated considerably from the original simple rotundas and circular temples. An inner peristyle of columns was Intexhiced so as to make a spacious circular or ring-shaped ambulatory around the centre, which was much loftier than the colonnade, being covered by a dome raised upon a cylin drical wall over the columns. What is now called San Stefano Rotunda, at Rome, supposed by some to have been originally a temple dedicated first to Faunus, and afterwards to the emperor Claudius, and by others to have been a public market, is a structure planned accord ing to the arrangement just mentioned, with a circular Ionic colonnade of twenty columns and two piers. The church of Santa Costanza supposed to have been erected by Constantine as a baptistery, and afterwards converted by him into a funeral chapel to his daughter Constantia, is a remarkable example, owing to the columns being not only coupled, but unusually disposed, and to there being arches spring ing from their entablature, that is, there are twenty-four columns (with composite capitals) placed in pairs, on the radii of tho plan, or one behind the other, forming twelve inter-columns and as many arches ; and as far as the mere arrangement goes, this interior is strik ingly picturesque.
The circular form was a favourite one with the Romans for their sepulchral structures of a more pretending class than ordinary : following in this the example of the Etruscans. It will be sufficient here merely to mention those in honour of Augustus and Hadrian, an account of which has been given under MAUSOLEUM. The tomb of Cecilia Metella is a ,low cylinder, the height being only 62 feet, while the diameter is 90 ; and it may be considered as nearly solid, the chamber or cella being no more than 19 feet in diameter. This cylindrical mass is raised upon a square substructure; which com bination of the two forms is productive of agreeable contrast ; and it was accordingly frequently resorted to. The tomb of Plautius Sylvanus near Tivoli consists also of a short cylindrical superstructure on a square basement, but is otherwise of peculiar design, one side of that stereobate being carried up so as to form a sort of low screen or frontispiece, decorated with she halbcolumns, and five upright tablets with inscriptions, between them. The tomb of Nunatius Plancus, at Gaeta, is a simple circular structure, of low proportions, the height not exceeding the diameter, and therefore hardly to be called a tower, notwithetanding that it is now popularly called Roland's or Orlando's Tower. Of quite different character and design from any of the preceding, is the ancient Roman sepulchral monument at St. Remui,
which consists of three stages; the first a square stereobate raised on gradini, and entirely covered on each side with sculptures in relief ; the next is also square with an attached fluted Corinthian angle, and an open arch on each side; and the uppermost is a Corinthian rotunda, forming an open or monopteral temple (that is, without any cella), the centre of which is occupied by two statues.
As instances of other combinations, we may refer to what is called the Tomb of Virgil, near Naples, consisting of a square substructure surmounted by a conical one; to the Roman monument at Constantine, in Africa, conjectured to have been a cenotaph iu honour of Constan tine, the lower portion of which is a cylindrical structure surrounded by a peristyle of twenty-four Doric columns, and carried up as a lofty cone, in receding courses or gradini, leaving at its summit a platform for an equestrian statue.
These notices may serve to convey some idea of the variety aimed at by the Romans in the distribution of the plans and general masses of their edifices, independently of decoration. Their Therince, or public bAlis, a class of structures remarkable for their vast extent and magnificence, are most interesting studies of combinations of plan, as they were not merely baths, but places of public resort and amuse ment, and consisted of an assemblage of courts, porticos, libraries, and spacious saloons and galleries, most of which presented somo pecu liarity of form and distribution. [Baena.] If therefore we estimate Roman architecture by the manifold resources which it opened to the art, rather than by its debasement of what it borrowed from that of Greece, we shall find much in it both to admire and to imitate, as well as to censure and to avoid. Its Greek rival has nothing that will bear a parallel with it in this respect. Judging from its remains, we can see little in it that answers to the title of interior architecture ; whereas some of the Roman temples were striking on account both of the size and the magnificence of their interiors. That of the Pantheon has been already referred to ; a very different example is the Temple of Peace, erected by Vespasian. What was its external design is now altogether doubtful, as only the ruins remain, but its interior is very remarkable, the plan being divided in its breadth into three nearly equal portions, the centre one of which formed a spacious nave, termi nating iu a large semicircular tribune, or apsis, covered by a semi dome. This nave was disposed in three compartments, presenting as many arches of exceedingly wide proportions, opening into as many divisions of the lateral portions of the plan, which did not constitute continuous aisles along the nave, but small chapels or recesses. Of these the centre one on each side terminated, like the nave, in a semicircular tribune, of the same dimensions as that apsis, so as to form a transept, and give the whole a marked cruciform appearance. The side divisions were covered by semicircular vaults, concentric with the arches opening into the nave ; and this latter had a ranked roof, in three groins or compartments, the ribs of which sprung from eight Corinthian columns, placed against the piers of the arches. Besides other peculiarities, we have here an instance of the effect resulting from the application of the semicircular form to plans in interiors, and of further varieties of design arising out of it, for the semidomes of the tribunes exhibit a rich specimen of coffering, being composed of octagons and squares.