Dome (Lat. domus, a house), an architectural term often used synonymously with cupola. Strictly speaking it signifies the external part of a spherical or polygonal covering of a building, of which the Cupola (q.v.) is the inner structure, but in general usage dome means the entire covering. It is also loosely used, as in the German Dom and Italian Duomo, to designate a cathedral, or, at times, to signify some other building of importance. A dome may be of any material, wood, stone, metal, earthenware, or it may be built of a single mass or of a double or even triple series of concentric coverings. The dome is a roof, the base of which is a circle, an ellipsis, or a polygon, and its vertical section a curved line, concave towards the interior. Hence domes are called circular, elliptical or polygonal, according to the figure of the base. The most usual form is the spherical, in which case its plan is a circle, the section a segment of a circle. Domes are sometimes semi-elliptical, pointed, often in curves of contrary flexure, bell-shaped, etc. Except in the earlier period of the development of the dome, the interior and exterior forms were not often alike, and, in the space between, a staircase to the lantern was generally made.
Domes are of two kinds, simple and compound. In the simple dome, the dome and the pendentives are in one, and the height is only a little greater than that of an intersecting vault formed by semicircular arches. The dome over the central part of the tomb of Galla Placidia, at Ravenna, and those over some of the aisles of Saint Sophia, Constantinople, are of this description. In the compound dome two methods were followed. In both methods greater height is obtained, and the compound dome was consequently the one used on all important buildings of the later period. In one, the dome starts directly from the top of the circle formed by the pendentives; in the other, a cylindrical wall or “drum” intervenes between the pendentives and the dome, thus raising the latter considerably. In churches with domes without drums, the windows are in the dome itself immediately above the springing; otherwise, they are in the drum, and the surface of the dome is generally unbroken. At the monastery of St. Luke, Phocis, Greece, are two churches of the eleventh century, side by side, the smaller of which has a drum with windows in it, whereas the larger church has no drum, and the windows are in the dome. The drum is universal in all domed churches of the Renaissance, at which time it received special treatment and became a most important feature. Many of these drums are not circular in plan externally, but are many-sided, and the angles are often enriched by marble shafts, etc. The carrying-up of the walls vertically is a good expedient constructionally, as it provides weight above the haunches of the dome and helps to neutralize its thrusts. In the churches of the second period, at Constantinople, Salonica, Athens, and other parts of Greece, in which the true drum occurs, it is of considerable height and is generally eight-sided. Windows come at each side, and over the windows are arches which cut into the dome itself.
A primitive form of the dome and the barrel vault is of great antiquity. In some districts men were compelled to build in stone or brick or mud, because there was no wood, as in Assyria; in other districts because they had not the tools to work wood. In all such cases some form of dome or tunnel vault had to be devised for shelter. In tracing the growth of the dome in historical times, it has been regarded as an outcome of the architecture of the Eastern Empire, because it was at Constantinople and in the Byzantine provinces that it was first employed in ecclesiastical structures. But it was the Romans who in reality developed the use of the dome, as of all other applications of the semicircular arch. From Rome it was carried to Constantinople and from the same source to different parts of the Western Empire. In Eastern Christendom the dome became the dominant factor in church design; whether a single dome, as at Saint Sophia, Constantinople (built, 532-537), or a central dome encircled by other domes, as at St. Mark’s, Venice, or a row of domes, as at Angouleme. The plan and domes of Angouleme are reproduced in the new Catholic cathedral at Westminster. The Roman dome was a hemisphere supported by a circular wall. Its finest example was the Pantheon, Rome. Equally characteristic, though smaller, examples abound, e.g. at Rome, the temple of Minerva Medica, the tomb of Constantia, now the church of Santa Costanza, etc. Viollet-le-Duc in writing of the dome of the Pantheon says, “This majestic cupola is the widest, the most beautiful, the best constructed, and most stable of all the great domes of the world”. The inside diameter of the dome is 1421 feet. Previous to the building of the Pantheon in its present domical form, during the reign of Hadrian about A.D. 123, the history of the dome is for the most part a blank.
The primitive Eastern dome seems to have been on a very small scale, and to have been used for subordinate purposes only. It was a common architectural feature in ancient Egypt and Mesopotamia. In later times the dome was largely employed in architecture by the Persian Sassanids, Mohammedans, and the Byzantines. From the first domed churches built for Christian worship sprang Byzantine architecture and its offshoots. The builder of the earliest domed church of any magnitude was Constantine; its locality the famous city of Antioch in Syria. The problem of the Christian domed church, so far at least as its interior is concerned, received in Saint Sophia its full solution. The dome is the prevailing conception of Byzantine architecture, and M. Choisy, in his “Art de batir chez lea Byzantins” traces the influence of this domical construction on Greek architecture to show how from their fusion the architecture of the Eastern Empire became possible. Domes were now, from the time of the construction of Saint Sophia, placed over square apartments, their bases being brought to a circle by means of pendentives, whereas, in Roman architecture, domes as a rule were placed over a circular apartment. The grouping of small domes round a large central one was very effective, and one of the peculiarities of Byzantine churches was that the dome had no additional outer covering. The dome was rarely used by medieval builders except when under oriental influence, hence it was practically confined to Spain and Italy. The dome of the cathedral at Pisa, the first model of the Tuscan style of architecture, was begun in the eleventh century, and in the thirteenth was founded the cathedral at Florence. Its dome equals in size that of St. Peter’s at Rome, and was its model. During the Italian Renaissance, domed construction became again of the first importance, possibly on account of its classical precedent, and it is interesting to note that the Pantheon became once more the starting-point of a new development which culminated in the domes of St. Peter’s, Rome, and St. Paul’s, London.
The substructure of the dome of St. Peter’s is a round drum, which serves as a stylobate and lifts it above the surrounding roofs. On this stands the ringwall of the drum, decorated with a Corinthian order and carrying an attic; on this sits the oval mass of the noblest dome in the world. The drum, fifty feet high, is pierced by sixteen square-headed windows. The enormous thickness of the stylobate allows an outside offset to receive the buttresses which are set between the windows, in the shape of spur-walls with engaged columns at the corners, over which the entablature is broken. The curve of the dome is of extraordinary beauty. Between its ribs, corresponding to the buttresses below, are three diminishing tiers of small dormer windows. The lantern above, with an Ionic order, repeats the arrangement of windows and buttresses in the drum below, and is surmounted by a Latin cross rising 448 feet above the pavement. The foremost Renaissance church in Florence is the church of the Annunziata, and is remarkable for a fine dome carried on a drum resting directly on the ground. To the latest time of the Renaissance in Venice belongs the picturesque domed church of Santa Maria della Salute. The two finest domes in France are those of the Hotel des Invalides and the Pantheon (formerly the church of Sainte-Genevieve) at Paris. Domes built in the early part of the twelfth century are to be found at Valencia, Zamora, Salamanca, Clermont, Le Puy, Cahors.
They are also found in Poitou, Perigord, and Auvergne; at Aachen, Cologne, Antwerp, and along the banks of the Rhine; at Aosta, Pavia, Como, Parma, Piacenza, Verona, Milan, etc. There are, besides, the bulbous domes of Russia and the flattened cupolas of the Saracens. The dome became the lantern in English Gothic, and the octagon of Ely cathedral is said to be the only true Gothic dome in existence. The central octagon of the Houses of Parliament, London, is the best specimen of a modern Gothic dome. Arab domes are mostly of the pointed form such as are derived from the rotation of the Gothic arch or bulbous, the section being a horse-shoe arch. Very beautiful examples are seen in the buildings known as the tombs of the caliphs at Cairo. Among the finest examples of domed buildings in the East are the Tombs of Mohammedan sultans in the south of India and at Agra. The largest dome in America is that of the Capitol at Washington. It is built of iron.
THOMAS H. POOLE