DOME, a term applied to a covering of the whole or part of a building. The Germans call it Dom, and the Italians Duomo, and apply the word to the principal church of a city, although the building may not have any spherical or poly gonal dome. From this and other circumstances we may infer the term to he derived from the Latin Domes, house.
A dome is an arched or vaulted roof. springing from a polygonal, circular, or elliptic plan ; presenting a convex sur thee on the outside, or a concavity within, so as that every horizonal section may be of a similar figure, and have a common vertical axis. According to the plan from which they spring, domes are either circular, elliptiv al. or polygonal ; of these. the circular may be spherical spheroidal, ellipsoidal, hyperboloidal, paraboloidal, &c. The word dome is applied to the external part of the spherical or polygonal roof, and cupola to the internal part. Cupola is derived from the Italian cupo, deep, whence also our word cup. But cupola and dome are often used synonymously, although perhaps incorrectly. Such as rise higher than the radius of the base, are denominated surmounted domes ;. those that are of a less height than the radius, arc called diminished or surbased; and such as have circular bases, are termed cupolas.
The remains of ancient domes are generally spherical in their term,, or built of stone or terfo. Ruins of numerous ones still exist in the neighbourhood of Rome and Naples. They were frequently used among the Romans, after the accession of Augustus, in whose reign, the use of the arch, and conse quently of domes, first became common. The arch, indeed, is of Grecian origin, though in all the ancient edifices of that country, we do not meet with a single instance of a built dome: that which covers the monument of l,ysierates, being only a single stone, can only he looked upon as a lintel : and the invention of this species of vault seems justly attributed to the Romans, or Etrurians.
Of the ruins of domes in and near Rome, the principal are the Pantheon, and the temples of Bacchus, Vesta, Romulus, Hercules, Cybele, Neptune, and Venus, mid also some of the chambers of the Thermic. The most magnificent dome of
antiquity is that of the Pantheon, at Rome, built in the reign of Augustus, and supposed to be a chamber of the great baths of Agrippa. It is still entire, and consists of a hemi spherical concavity, enriched with eoffers. and terminating upwards in an aperture, called the eye. The exterior rises from several degrees, in a sloping direction, nearly tangent the truncated segment of a sphere, considerably less than a hemisphere. The diameter of the dome internally is 1.12 feet S inches ; the circular opening at the top in the centre 28 feet 0 inches in diameter ; the height from the top of the attic 70 feet 8 inches. The interior of the dome is orna mented with five rows of square compartments, and as these converge towards the top, each row is considerably larger than that immediately above it. Each of the large squares con tains fi)ur smaller ones sunk one within the other. It is sup posed that these were decorated with plates of silver. 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 of the dome. From the drawings of Serlio, it appears that similar flights (ili steps were formed at intervals all round the dome, but these are now covered with lead. The dome is constructed of bricks and rubble. The thick tess at the base is about 17 feet ; at the top of the highest step, 5 feet 11 inches; and at the top of the dome, 4 feet 7 inches. The circular wall which supports the dome is 20 feet thick, but is divided by several large openings, and has discharging arches of brick. The dome of the Pantheon is incombustible, and is perhaps the cheapest as well as the most durable and uncon