Dome

Dome (Lat domus, house; Ital. duomo, cathedral), an architectural term, derived from a characteristic feature of Italian cathedrals, correctly applied only to a spherical or spheroidal vault, the horizontal plan of which is always a circle. It may be supported on a circular wall, as in the Pantheon at Rome; or on a drum, as in the later Byzantine churches and generally so in the Renaissance styles; or be carried over a square or polygonal area, in which case the base of the dome is connected to the lines of the main wall by pendentives, squinches, corbels or a series of concentric arches, or two of these combined. Its section may be semicircular, pointed, ovoid or segmental; in the latter case it is usually termed a cupola, although the pendentives which carry it continue, on the diagonal lines, the complete spherical dome, as in the entrance vestibule on the south side of the Sanctuary at Jerusalem, attributed to Herod, or in those crowning the bays of the Golden Gateway by Justinian. The dome may be constructed in horizontal courses, as in the “beehive” tombs at Mycenae, with joints radiating to the centre, or a compromise between the two, in a series of small segments of circles, as in the Temple of Jupiter in Diocletian’s palace at Spalato, or again with the lower portion in horizontal courses and the upper portion with arches, as in the Pantheon at Rome.

The dome is probably one of the earliest forms of covering invented by man, but owing probably to its construction in ephemeral materials, such as the unburnt bricks in Chaldaea, there are no examples existing. But in a bas-relief (see Architecture, fig. 10), brought by Layard from Kuyunjik, are representations of semicircular and ovoid domes, which show that the feature was well known in Assyria, and as they build domes of the same nature down to the present day and without centring of any kind, it suggests that they may have existed from the remotest ages. The most ancient examples in Europe are those of the “beehive” tombs at Mycenae and elsewhere in Greece, ascribed generally to the 11th century b.c. In a sense, they are not true domes, because they are built in horizontal courses of stone, which act like the voussoirs of an arch in resisting the thrust of the earth at the back. This did not exist in the Choragic Monument of Lysicrates or other circular buildings in Greece, because their vertical sections were not portions of circles. For this reason, the conical vault of the Baths in Pompeii is not a dome. The circular Laconicon in the Baths of Titus (a.d. 72) may have been domed, and the great hemicycles in the Thermae must certainly have been roofed with semi-domes.

The earliest Roman domes are those of the great circular halls at Baiae near Naples, described as temples, but really forming part of the immense bathing establishments there, the favourite place of resort of the Romans during the latter part of the Republic. The largest on the east side of the Lake of Avernus, known as the Temple of Apollo, is a circular hall with an internal diameter of 100 ft. Those of Diana, Mercury and Venus at Baiae, were 96, 66 and 60 ft. respectively. The vaults were all built in tufa with horizontal courses in brick and cement. Half of the dome of the Temple of Mercury had fallen down, showing the section to have been nearly that of an equilateral arch. From the fact that there were pierced openings or windows in all these domes, they probably constituted the frigidaria of the baths.

The first example still existing in Rome is that of the Pantheon (a.d. 112), where a circular dome, 142 ft. in diameter, rests on a circular wall, its height being about equal to its diameter. The lower courses of this dome, built in the Roman brick or tile, were, up to the top of the third coffer, all laid in horizontal courses; above that, the construction is not known for certain; externally a series of small arches is shown, but they rested on a shell already built. The so-called Temple of Minerva Medica (now recognized as the Nymphaeum of the Baths of Gallienus, a.d. 366) is the next dated example. The Nymphaeum was decagonal on plan, so that small pendentives were required to carry the brick dome.

The domed Laconicon of the Thermae of Diocletian (a.d. 302) still exists as the vestibule of the church of Santa Maria degli Angeli. Of Constantine’s time there are two small domed examples in the tomb of S. Costanza and the Baptistery of the Lateran, both in Rome, and one in the tomb of Galla Placidia at Ravenna (c. a.d. 450). From these we pass to the Sassanian domes at Serbistan and Firuzabad, of the 4th and 5th centuries respectively. These were built in brick and rested on square pendentives. In section they were ovoid. In Syria, the dome over the octagonal church at Esra, built in stone and dated a.d. 515, is also ovoid, its height being equal to its diameter, i.e. 28 ft. This, as well as the Sassanian domes, was built without centring. The next example is that of the church of Sta Sophia at Constantinople, the finest example existing, both in its conception and execution. It was built by Justinian (537-552) from the designs of Anthemius of Tralles and Isidorus of Miletus. The dome is 104 ft. in diameter, and is carried on pendentives over a square area. The construction is of brick and stone in alternate courses, and the lower part of the dome is pierced with forty windows, which give it an extraordinary lightness. The height from the pavement of the church to the soffit of the dome is 179 ft. No dome of similar dimensions was ever again attempted by the Byzantine architects, and the principal difference in later examples was the raising of the dome on a circular drum pierced with windows.

In order to lighten the dome erected over the church of San Vitale, at Ravenna, it was constructed with hollow cylindrical jars, fitted, the end of one into the mouth of the other; a similar contrivance was adopted in the tomb of the empress Helena (the Torre Pignatiara), the vaults of the Circus of Maxentius on the Via Appia, and the outer aisles of San Stefano, all at Rome, thus dispensing with the buttresses of Sta Sophia.

The domes of the earlier mosques in Cairo were built on the model of Sta Sophia, with windows pierced round the base of the dome and external buttresses between them; these domes were all built in brick coated over with cement or stucco. At a later date, and when built in stone, the upper portion was raised in height and terminated with a point on which a finial was placed. These are the domes inside and outside Cairo, which are carved with an infinity of geometrical patterns interwoven with conventional floral decoration. The upper portion of the dome is very thin, so that there is little weight and comparatively no thrust, and it is to these facts that we probably owe their preservation.

In India, in the “great mosque” of Jama Masjid (a.d. 1560) and the Gol Gumbaz, or tomb of Mahommed Adil Shah (a.d. 1630) at Bijapur, the domes are carried on pendentives consisting of arches crossing one another and projecting inwards, and their weight counteracts any thrust there may be in the dome. It is possibly for a similar reason that in the Jama Masjid of Shah Jahan at Delhi (1632-1638) and the Taj Mahal (a.d. 1630) the domes assume a bulbous form, the increased thickness of the dome below the haunches by its weight served as a counterpoise to any thrust the upper part of the dome might exert. The form is not much to be admired, and when exaggerated, as it is in the churches of Russia, where it was introduced by the Tatars, at times it became monstrous.

From these we pass to the domes of Périgord and La Charente, the earliest of which date from the commencement of the 11th century. Of the western dome of St Étienne at Périgueux (a.d. 14) only the pendentives remain, sufficient, however, with later examples, to show that these French domes were different from the Byzantine both in construction and form. The pendentives are built on horizontal courses of stone, and the voussoirs of the pointed arches which carried them form part of the pendentives; a few feet above the top of the arches is a moulding and a ledge, above which the dome, ovoid in section, is built. The principal examples following St Étienne are those of S. Jean-de-Cole, Cahors, Souillac, Solignac, Angoulême, Fontevrault, and lastly St Front at Périgueux, built about 1150, in imitation of St Mark’s at Venice. The domes of the latter church were introduced into the old basilica about 1063, and were based on the church of the Apostles at Constantinople, which was pulled down in the 15th century, so that we have only the clear description of Procopius to go by. The domes over the north and south transepts and the choir of St Mark’s are smaller than those over the nave and crossing, because they had to be fitted in between more ancient structures. The construction of the domes of St Mark’s is not known, but at St Front the general design only was copied, and they built them in the Périgordian manner. The masons from Périgord are also responsible for the domes of the Crusaders’ churches in Palestine and for some of the early churches still remaining in Cyprus. The domes of San Cyriaco at Ancona and Sant’ Antonio at Padua were based upon those of St Mark’s at Venice.

In central Italy we have the dome (elliptical in plan) of the cathedral of Pisa, and it was a favourite feature over the crossing of the churches throughout Italy, being generally carried on squinch pendentives. The domes of the baptisteries of Florence, Parma, Trieste and Piacenza, are only internal, being enclosed with vertical walls and a sloping roof. In Sicily, on account of the strong Saracenic influence, the squinches are simple versions of the stalactite pendentives described under Architecture: Mahommedan (q.v.), the earliest example being found in the church of San Giovanni-dei-Leprosi (a.d. 1072), all the domes being ovoid in section.

Except in Périgord and La Charente, domes are not found in the churches in France, but in Spain they were introduced over the crossing at Burgos, Tarragona and Salamanca cathedrals, and were made architectural features externally. This is rarely found in Germany, for although in the cathedrals of Worms, Spires and Mainz, and in the churches of St Martin and Sankt Maria im Capitol at Cologne, the crossings are covered by domes, always carried on squinch pendentives, externally they built lanterns round them.

In the Renaissance styles, the dome was at once accepted as the principal characteristic feature, and its erection over the crossing of Santa Maria del Fiore at Florence was the first important work entrusted to Brunelleschi. The dome was begun in 1422, and finished in 1431, with the exception of the lantern, begun the year of his death in 1444, and completed in 1471. The dome, which is octagonal on plan, is 139 ft. in diameter, and is built with an inner and outer casing, concentric one with the other, tied together by ribs between them: the lower portion is stone, the upper part is brick.

The double shell was also employed by Michelangelo in the dome of St Peter’s at Rome, the outer shell being raised higher than the lower and connected by ribs one with the other. The diameter is 140 ft. and the construction in brick, similar to that at Florence, but the ribs are in stone from Tivoli. In both these cases the weight of the lantern was a very important consideration, and is responsible for the repeated repairs required and the introduction of additional ties.

In this respect Sir Christopher Wren solved the difficulty at St Paul’s cathedral, London, in another way: he provided three shells, the lower one with an eye in the centre forming the inner dome as seen from the interior; the middle one of conical form, and the outer one framed in timber and covered with lead. The conical shell carries the lantern, the weight of which is carried direct to the base, bound with iron ties, with such additional strength as may be given by the portico round.

In all these cases these domes are built on lofty drums, so that externally they present quite a different appearance to those of the Pantheon at Rome, or Sta Sophia in Constantinople.

Of other examples, the domes of the Invalides in Paris, by Mansard (1706), and of the Panthéon by Soufflot (1735), have each three shells, the former having a graceful outline. In Spain the dome of the cathedral at Granada (1530) and the Escurial (1563); in Italy those of Sta Maria della Salute at Venice, the small example of Bramante at Todi (1480) and of the Carignano at Genoa, are worth recording, as also the dome of the Suleimanie mosque at Constantinople (1550). See plates illustrating Architecture; and Indian Architecture.

---