Dome (Gr. building; Lat. domus, a house; mediaeval Lat. doma, a cupola), a concave covering to a building or part of a building. The Italians apply the term il duomo to the principal church of a city, and the Germans call every cathedral church Bom; and it is supposed that the word in its present English sense has crept into use from the circumstance of such buildings being frequently surmounted by a cupola. Some writers on architecture restrict the term dome to the convex surface of the roof, and cupola to its concave part. The dome may be a segment of a sphere, spheroid, ellipse, polygon, or any similar figure, but in all cases every horizontal section should have a common vertical axis; it is called surmounted when it rises higher than the radius of its base, surbased or diminished when its height is less than the radius of its base. The thickness should increase toward the base, where the structure is weakest, and where the spreading force of the superincumbent weight tends to burst the dome outwardly. To counteract this pressure, iron hoops or chains are often employed.
When built of stone the dome is stronger than the arch, as the tendency of its parts to fall inward is resisted not only by the parts above and below it, but also by those on each side. The constituent pieces are formed somewhat like the frustum of a pyramid, so that when placed in their positions their four angles may point toward the axis of the dome. Each course is thus self-supporting, and not only may the whole be constructed without centering, but an aperture, called the eye, is frequently left in the top without damage to the security of the structure. - The dome seems to have been invented by the Romans or Etruscans, and in the time of Augustus was a common feature in Roman architecture. There is no proof that the Greeks or Egyptians had any knowledge of it, nor is it found in any of the early monuments of Hin-dostan, but after the Mohammedan invasion of India it was generally adopted in that country. The grandest dome that has remained to us from antiquity is that of the Pantheon at Rome, which, though 19 centuries have passed over it, still retains all its stability and magnificence. Its exterior presents the appearance of a truncated segment of a sphere, considerably less than a hemisphere, and has a circular opening in the top 28 ft. 0 in. in diameter.
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 1/2 ft. radius, and the distance from the floor to the top of the dome is equal to the diameter. The thickness is 17 ft. at the base, 5 ft. 1 1/2 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 circular wall 20 ft. thick. The baths of ancient Rome 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 design 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 resorted 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 Ravenna consists of a hemisphere resting on an octagon with eight piers at its angles, and a window on each face. The great dome of the church of Santa Maria del Fiore, the cathedral of Florence, was begun by Arnolfo di Lapo or Arnolfo di Cambio da Calle about 1295, but after the death of the original architect (about 1300) no one could be found for upward of a century to finish it; it was finally undertaken by Filippo Brunelleschi, who brought it nearly to completion. He improved upon the original design by canning up perpendicular walls in the shape of an octagon 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. 0 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 wo are acquainted. That of St. Peter's at Rome, the grandest in the world after that of the Pantheon, is also double. It stands upon four piers, each 01 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 1/2 in. thick and 12 ft. 4 1/2 in. high, divided into three parts by passages, 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 corresponding number of solid buttresses 51 ft. 6 in. high. Above it is placed a circular attic 19 ft. 2 1/4 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 internal dome is 83 ft. 10 in. It is pierced outwardly 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 2G3 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 completion, 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 vertically 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 intersection of the four naves. The external dome is of wood, covered with lead, and ornamented 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 the springing 51 ft. The dome of the Pantheon (or St. Genevieve's) at Paris is entirely of stone, and is supported by four triangular piers rising from the centre of a Greek cross.
It is triple, having besides the inner and outer vaults an intermediate structure built to carry the lantern. The internal dome is 60 ft. 8 1/2 in. in diameter at the springing; the external 77 ft. 8|- in. The height of the edifice above the ground line is 190 ft. The reading room of the British museum, opened in May, 1857, is covered by a magnificent dome 140 ft. in diameter and 106 ft. high from the ground. It is built principally of iron, with brick arches between the main ribs supported by 20 iron piers. Between the vaulting and the exterior covering of copper a space is left for the equalization of the temperature, and between the vaulting and the inner decorated ceiling is a similar air chamber for ventilation. There are 20 large windows around the base of the dome, and an eye in the top 40 ft. in diameter. The cast-iron dome of the capitol at Washington has a height of about 55 ft., and an internal diameter of 94 ft. 9 in. The height of its ceiling from the floor of the building is 220 ft. The exterior of the structure presents a peristyle 124 ft. 9 1/2 in. in diameter, with columns 27 ft. high, from which springs an attic 44 ft. high, supporting the great dome of a semi-ellipsoidal form, the top of which is 230 ft. above the pavement.
Above this rises a lantern, 52 ft. high and 17 ft. in diameter, crowned with a bronze statue of Liberty 18 ft. high. In the interior there is a vertical wall raised upon the cornice of the rotunda, with a panel 9 ft. high richly sculptured; above this is a series of attached columns and large windows, and above these springs a dome which, contracting to a diameter of 65 ft., permits a second dome, 73 ft. in diameter, resting also on a colonnade, to be seen through the opening. - Domes are sometimes made convex below and concave above, in which form they take the name of Moresque, Turkish, or Hindoo. In Russia they are very frequently built of a bulbous shape, and many of the churches are surmounted by five, representing Christ and the four evangelists. The Isaac's church at St. Petersburg, founded in 1819 and consecrated in 1858, has a central dome of iron covered with gilded copper, with a small rotunda rising from the centre; it is in the Byzantine form, having a diameter of 87 ft. 4 in. and a height of 275 ft. above the floor, or, including the lantern, of 327 ft. Four smaller domes rest on each corner of the edifice.
The dome of the temple of the Saviour at Moscow, built to com-memorate Russia's triumph over Napoleon, is 84 ft. in diameter; its interior height above the floor is 225 ft. 9 in., and its exterior height, including the cross on its summit, is 343 ft. This dome is supported by four pillars, each 21 ft. in diameter.