DOME, a term applied to a covering of the whole or part of a building. The word dome is strictly applied to the external part of the spherical or polygonal roof, and cupola to the internal part.
The most magnificent dome of antiquity is that of the Pantheon, supposed to be a cham ber of the great baths of Agrippa. Internally it is divided into five rows of square compart ments, which are supposed to have been deco rated with plates of silver. The external part of the dome appears also to have been deco rated with bands of bronze. The base of the dome externally consists of a large plinth with six smaller plinths or steps above it ; and in the curve of the dome a flight of steps is formed which leads to the opening at the top. The dome is constructed of bricks and rubble. The thickness is about 17 feet at the base, 5 feet La inches at the top of the highest step, and 4 feet 7 inches at the top of the dome. The circular wall which supports the dome is 20 feet thick.
The Thermm, or baths, of Caracalla, Titus, Constantine, and Diocletian, were all sur mounted by domes. Near Pozzuoli there is a very perfect circular building with a dome 96 feet in diameter, built of volcanic tufo and pumice stone. The temple of Minerva Me dica, without the walls of Rome, had a dome of ten sides built of brick and pumice stone. The dome of Santa Sophia, at Constantinople, built in the reign of Justinian, rests on the square formed at the intersection of the arms of a Greek cross. The dome is supported by four corbellings placed in the angles of the square, surmounted by a kind of cornice which supports a circular gallery. Externally the dome is divided, by projecting ribs, rounded and covered with lead. The top is surmounted by a lantern or finishing like a baluster, on which is a cross. The dome of San Vitale at Ravenna, is curiously constructed. The lower part of the plan is a regular octagon, which is supported by eight piers placed at the angles of the dome ; in the spaces between the piers are seven niches, divided into two stories. The wall above the niches sustains a hemispherical dome, the plan being a circle described within a regular octagon. The dome is built of a double row of pipes, hollow at one end and pointed at the other, the point of one being placed in the hollow of the preceding ; it is covered with mortar both within and with out. The church of San Marco at Venice, built in the tenth century, is decorated with live domes. One of these, placed in the centre of the church, is much larger than the others. Each dome is enclosed within four pieces of semi-cylindrical vaulting, forming together a square, in the angles of which are four corbels, which gather inthe circular base of each dome. The dome of Santa Maria del Fiore, at Florence, built by Bruneleschi, stands upon an octagon tower 175 feet high ; it is double, being the first of the kind that is known. The internal dome is connected only at the angles to the external one, and forms a species of gothic vault. The first modern dome constructed in Rome was that of the church of Our Lady of Loretto, built in 1507; it is double, and circular in plan, and is con structed on double consoles.
The dome of St. Peter's at Rome, as planned by Michael Angelo, and executed by himself and succeeding architects, is thus constructed. It stands upon four piers 62 feet high. From the arches spring corbellings, which are finished by an entablature ; upon this is a plinth, octagonal within and circular without ; upon this is a circular stylobate, 12 feet high. Above the stylobate is the drum of the dome, built of rubble and fragments of brick, and pierced by sixteen lofty windows : the height of the drum is about 52 feet. On this is placed a circular attic story, 19 feet high, and then comes the double dome. The space between the two domes varies from three to ten feet in width. The thickness of the inner dome is about six feet ; the outer dome is of less thick ness : the two are joined together by sixteen strong walls or spurs. Above the dome are lofty lantern and cross : the dome is about 102 feet high above the drum, and the lantern and cross 90 feet above the dome.
The dome of St. Paul's Cathedral, London, is placed over the intersection of the four naves. The ground plan is a regular octagon : four of the sides are formed by the four great arches of the naves ; the other four sides are formed by false arches of the same size. The corbellings gather in a circle, and are sur mounted by a complete entablature decorated with consoles. The drum, which surmounts the entablature, is 52 feet in height ; and from the summit of this rises the double dome. The inner dome is much less lofty than the outer : the outer one is constructed of wood, covered with lead.
The dome of the Invalides at Paris is raised on the centre of a Greek cross, on an octa gonal base with four large and four small sides A circular entablature is placed over the cor • bellings, and on the entablature is raised the drum of the dome. The dome, which is double, rises from a springing common to both. The lower or internal dome, constructed with masonry,is spherical ; whereas the outer dome is of a spheroidal form, and constructed of stone at the base, and of brick above. The dome of the Pantheon at Paris is constructed entirely of stone, and is placed in the centre of a Greek cross. It is supported by four tri angular piers, pierced above with arched open ings, and between the piers with the openings are large arches. Between these arches rise the corbellings, which are gathered in to form I the circular plan of the drum. The arches and the corbellings are crowned with a large entablature about 13 feet high, and above this is the circular drum, 55 feet high. There are three domes one within another; one forming i the interior vault or roof, a second forming the exterior, and a third shaped like the small end of an egg, and intermediate between the other two, for supporting the lantern.
The following admeasurements of most of the principal domes of Europe are from Mr. Ware's ' Tracts on Vaults and Bridges'