196 DOME ance of a truncated segment of a sphere, con- siderably less than a hemisphere, and has a circular opening in the top 28 ft. 6 in. in di- ameter. The base consists of a large plinth, with six smaller ones above it. It appears that originally there were flights of steps at intervals all around the dome leading up to the eye, but only one such is now visible, the others having been covered with lead. The interior is a hemisphere of about 71 ft. radius, and the distance from the floor to the top of the dome is equal to the diameter. The thick- ness is 17 ft. at the base, 5 ft. H in- at the top of the highest plinth, and 4 ft. 7 in. at the eye. The ceiling is ornamented with five rows of quadrilateral compartments converging toward the top, each large compartment having four smaller ones sunk one within another, which were probably once ornamented with plates of silver or covered with bronze. The dome is built of brick and rubble, and rests on a circu- lar wall 20 ft. thick. The baths of ancient Eome afford many examples of this kind of roof: those of Diocletian had three domes, two of which remain ; and those of Titus are crowned by two, each 84 ft. in diameter. Near Pozzuoli is an ancient circular building with a dome of volcanic tufa and pumice stone, and the temple of Minerva Medica had a polygonal dome of ten sides, constructed of pumice stone and brick. That of the church of St. Sophia at Constantinople was built in the reign of Justinian, with the professed de- sign of rivalling the glory of the Pantheon. The plan of the church was a cross, and at the angles of the square where the transepts cut the nave, the architect placed four columns at a distance of about 115 ft. apart, and over them threw arches. The triangular spaces at the corners were then filled up to a level with the extradoses of the arches, and on the ring thus formed the dome was built. In ignorance of the principle of hooping, the builder resort- ed to various expedients to resist the lateral pressure of the superstructure, and, after it had twice fallen in, was obliged to fill up the large arcades on the N". and S. sides with three tiers of small arches. This dome was destroyed by an earthquake a few years after its completion. The present one is of nearly the same diameter (107 ft.), 46 ft. high, supported by corbellings at the angles of the square, and encircled by a row of windows with exterior columns. It is surmounted by a lantern. The church of St. Mark at Venice, begun in 977, has five domes ; the central one, which is much larger than the others, was hooped with iron in 1523. The dome of San Vitale at Eavenna consists of a hemisphere resting on an octagon with eight p'u-rs at its angles, and a window on each face. Tin- irivat dome of the church of Santa Maria del Kiore, the cathedral of Florence, was be- gun by Arnolfo di Lapo or Arnolfo di Cambio da Calle about 1298, but after the death of the original architect (about 1300) no one could be frund for upward of a century to finish it ; it was finally undertaken by Filippo Brunelleschi, who brought it nearly to completion. He im- proved upon the original design by carrying up perpendicular walls in the shape of an oc- tagon to a height of 175 ft., and upon these placing two concentric domes, the internal one being 138 ft. 6 in. in diameter and 133 ft. 6 in. high from the top of the internal cornice of the supporting walls to the eye of the lantern. This is the first double dome with which we are acquainted. That of St. Peter's at Eome, the grandest in the world after that of the Pantheon, is also double. It stands upon four piers, each 61 ft. 11 in. high and 30 ft. 10 in. thick, from which spring arches supporting corbellings finished by an entablature. The entablature upholds a plinth, circular within and octagonal without, and on the latter rests a circular stylobate 28 ft. 6 in. thick and 12 ft. 4 in. high, divided into three parts by pas- sages, forming flights of steps communicating with four spiral staircases in the thickness of the wall of the drum, which rises immediately from the stylobate. The drum is pierced with 16 windows, between which are a correspond- ing number of solid buttresses 51 ft. 6 in. high. Above it is placed a circular attic 19 ft. 2J in. in height, and on this rests the great double dome, the internal diameter of which at the base is 138 ft. 5 in. and the external 148 ft. To the height of 27 ft. 8 in. the dome is solid. Its curve describes externally the arc of a circle whose radius is a little over 84 ft., and its height from the attic to the top of the in- ternal dome is 83 ft. 10 in. It is pierced out- wardly by three rows of small windows and strengthened by 16 projecting vertical bands. The whole is crowned by a lantern resting on a platform surrounded by an iron railing and having a cross on the top, the height from the external plinth of the dome to the cross being 263 ft. The top of the cross is 430 ft. above the ground line. This great work was planned by Michel Angelo, who died before its com- pletion, and was finished under the pontificate of Sixtus V., who caused the exterior to be covered with lead, and the bands with bronze gilt. Owing to the haste with which the work was pushed forward the domes settled verti- cally in many places, and the band of iron around the inner dome was broken. Six iron circles were consequently placed around the outer dome, secured by iron wedges, and the fractured hoop was repaired. The dome of St. Paul's, London, built by Sir Christopher Wren, is double, and rests on an attic and a drum placed on four great arches over the in- tersection of the four naves. The external dome is of wood, covered with lead, and or- namented with panels formed by projecting ribs. It is surmounted by a lantern supported on a conical tower terminated by a spherical dome. The height of the tower is 86 ft. 9 in., and that of the whole structure from the ground line is 365 ft. The diameter of the dome is 145 ft., and its internal height from
