DOME, in architecture, an ovoidal or hemispherical vault ; also any vault of polygonal plan that approaches the true dome in shape. The origin of the term lies in the fact that such a vault was a common feature of the Italian cathedrals—duomo.
The dome is an obvious type of covering for the primitive circular hut in countries where building materials are only in small units, such as bricks, and where timber is lacking. In the Mesopotamian valley, where these conditions existed, domes, probably of sun-dried brick, were built from an early period ; many representations of them are shown in Assyrian stone bas reliefs of the 7th and 8th centuries B.C. ; in these the domes are of a high elliptical form, and certain authorities find their origin in tents. With the growing use of square and rectangular rooms, this early form of dome, because of the difficulties of building a circular vault over such structures, largely disappeared, although it persisted in tomb design, where circular chambers remained in use. In such primitive tomb chambers, however, the form may be that of a true vault, but the construction is usually by corbel ling, or building the whole covering of horizontal courses, each one of which projects slightly in beyond the one below, until the opening at the top is small enough to be covered by a single slab of stone. Such an arrangement may be seen on a grand scale in the famous so-called Tholos or tomb of Atreus at Mycenae (c. 1200 B.C. ) , and in such Etruscan tombs as that from Volterra (6th century B.c.), now in the Archaeological museum at Florence. A rock-cut tomb at Viterbo, of approximately the same date, shows exactly the form of domical covering that appears on Assyrian reliefs.
Roman.—The Romans transformed this primitive idea into one of the most fruitful architectural forms. It first appeared as a half dome -over niches, and was commonly so used throughout the early empire. In the Tabularium, Rome, 78 B.C., the square bays of the gallery, still existing, are roofed with vaults of ovoid section, rising equally from all four sides—a type of vault known as the cloister vault, which in reality is a square dome. With the development of many buildings of circular plan, during the empire, such as nymphaea, or round garden buildings, and the calidaria, or hot rooms of the baths, the dome naturally came into use. An early example is the scalloped domical vault at the nymphacum of the Garden of Sallust, Rome (c. A.D. 7o-8o). Hadrian, however, was the first to realize the tremendous op portunities of the simple dome form; he applied it on enormous scale to the circular Pantheon, built between A.D. 0 and 125. This dome, 144 ft. in diameter, is constructed of brick, with the lower portion stiffened by a series of relieving arches. It is deco rated on the inside with coffers, or deeply sunk panels, which seem to have been cut into the brick at a later period, and it is lighted by an enormous eye, or circular opening, at the top. Another interesting dome of the same period occurs in the villa of Hadrian at Tivoli, in the vestibule of the so-called Piazza d'Oro. Here, again, the scalloped type is used.
From the time of Hadrian, the dome became more and more common. In the baths of Caracalla, the calidarium, an immense, circular room, 116 ft. in diameter, was roofed with a dome, but still more interesting was the attempt to put a dome over an octagonal room in one of the side buildings. The problem thus presented, of supporting the in-curving parts of the dome, had puzzled the Romans for many years, and all sorts of attempts, by means of corbelling, and diagonal arches and niches, had been made. It was never thoroughly solved until the Byzantine period. Roman domes, except on a very small scale in late tomb chambers, are therefore limited to buildings of polygonal or circular plan. The most remarkable of the late Roman dome-roofs is that of the so-called temple of Minerva Medica, in reality, the nymph aeum of the Licinian gardens (c. 25o), noteworthy because of the daring lightness of its construction and its use of a structural framework of brick ribs.
Byzantine.—The Byzantine perfection of the pendentive (q.v.) made possible the use of the dome over a plan of any shape. This style became pre-eminently a domed style, and its achievements, in building light but substantial domes, coloured not only all Renaissance church architecture, but also almost all Mohammedan building. A possible source for Byzantine skill in dome building was in the facility of the Sassanian vault con structors of Persia, who, in the palace at Firouzabad, probably end of the sth century, used ovoid domes, about 5o ft. wide, supported on diagonal, arched squinches (q.v.).
The greatest of the Byzantine domes was that by Anthemius and Isidorus for the church of S. Sophia at Constantinople, con secrated 561, remarkable for the fact that 4o windows are pierced through its base, not only lessening its weight, but admirably lighting the great interior. This dome is ioo ft. in diameter, with a crown i8o ft. above the floor. S. Mark's at Venice, begun 1°63, and modelled on the church of the Holy Apostles, Constantinople, exerted great influence on northern Italian Romanesque work (S. Antonio, Padua, i3th century). Later Byzantine domes are almost universally raised on drums, pierced with windows, as in the church of S. Theodore, at Athens. Another interesting off shoot of Byzantine dome design is a group of French domed churches, largely in Aquitania. Most of these date from the last half of the II th and the first half of the 12th centuries, and are remarkable for their almost universal use of the pointed arch. These churches are without drums, and in some cases the curve of dome and pendentive is continuous, with, however, a projecting moulding at the dome base. Windows are occasionally pierced in the dome, as in early Byzantine examples. Cahors cathedral, '119, and Angouleme, 1136, are outstanding instances, but the finest of the group is S. Front, Perigueux, begun 1120, whose plan is almost a replica of that of S. Mark's. These French domes are usually pointed in section, and decorated on the outside with stone, carefully cut in scalloped patterns, and crowned with orative finials. Smaller conical dome shapes, with similar details, are used frequently for spire tops and for ornamental pinnacles.
The Mohammedan builders were strongly under Byzantine influence; they adopted the dome as one of their favourite motives, using it in a lavish form, on a small scale, in Spain and northern Africa, but developing it as an important structural feature along Byzantine lines in the later architecture of Egypt, and in Persia, India and especially Turkey. Smaller domes and half domes were usually greatly enriched with fantastic stalactite ornaments, and sometimes elaborate systems of ribs, or trans verse and diagonal arches replaced the pendentive. Particularly noteworthy is the Mohammedan work of India, especially the Djumma Musjid in Delhi (156o), and the Gol Gumbaz at Bi japur (163o). In certain examples, as the exquisite Taj Mahal at Agra (163o), the domes possibly follow Mohammedan Per sian examples in their bulbous or onion form which appeared in Persia in about the 15th century and later, as in the tomb mosque of Tamerlane in Samarkand (1405 ), or the great mosque of Baghdad. Earlier Persian domes are of simple pointed outline, like the tomb at Sultanieh (132o). These domes are frequently double, the exterior surface raised high above the interior dome. The Muslim Turks carried out the Byzantine idea most directly and simply, making of the combination of domes and half domes, which they adopted from S. Sophia, compositions of the most powerful exterior, and airy interior effect. The mosque of Sulei man, designed by the famous Turkish architect Sinan, completed 1556, at Constantinople; that of Selim at Adrianople, finished 1574; and that of Yeni Djami, by passim, 165o, in Constanti nople, are characteristic examples (see MOSQUE).
Renaissance.—The dome of the Pantheon at Rome exercised an enormous influence on the imagination of the Italian Renais sance (see RENAISSANCE ARCHITECTURE). Brunelleschi's so-called dome of the cathedral at Florence, completed 1431, lantern 1471, is, however, not a true dome, but a vast octagonal cloistered vault, with double shells, supported and connected by great stone ribs, which carry the weight of the stone lantern more than 30o ft. in the air. As this vault was supported on a high drum, no buttress ing to withstand its thrust was possible, and although this was minimized by the shape chosen, it was, nevertheless, necessary to build in great bands of wooden beams tied together, in a chain, to prevent the base from spreading. Later designers combined the Byzantine pendentive with the idea of the drum, the dome and the lantern, and this combination became the governing motive of Renaissance dome design. Moreover, in order to get the requisite exterior effect without unduly raising the interior dome, the cus tom grew of building church domes with two or more shells, al though such examples as the church of the Gesu at Rome (i6th century, 57 ft. in diameter) are single. In some cases the exterior dome was merely a curved timber roof above the interior dome of masonry, as in the magnificent dome of S. Maria della Salute, in Venice, designed by Longhena (1631, 8o ft. in diameter). In the dome of S. Peter's, at Rome, designed by Michelangelo, and com pleted in 1J9o, after his death, both shells are of brick, but con nected and strengthened, as in the cathedral at Florence, by stone ribs. The domes of S. Peter's and of the Florence cathedral are nearly of the same size, between 136 and 138 ft. in internal diameter. The support of such a heavy masonry construction as a Renaissance lantern, high in the air, caused great difficulties, tremendously increasing a tendency of the dome to spread. The original construction of the dome of S. Peter's embraced three chains to counteract this, but since its completion it has been necessary to add six more chains. In order to diminish such thrusts Sir Christopher Wren, in the magnificent dome of S. Paul's, in London, used three shells, the inner one almost a hemi sphere (102 ft. in diameter), the middle one a cone shaped struc ture carrying the stone lantern and the outer one merely a pro tecting roof of timber, whose weight is largely carried on the cone, thus helping to diminish its own thrust. In the dome of the chapel of Les Invalides, in Paris, by J. H. Mansard (1706, 92 ft. in diameter) there are also three shells, but in this case the lantern is of timber construction.
See BYZANTINE AND ROMANESQUE ARCHITECTURE, the several articles on RENAISSANCE ARCHITECTURE, and ROMAN ARCHI TECTURE.