609 



DOME. 



DOME. 



(10 



attic rises the double dome, the internal diameter of which, at the base, 

 is 138 feet 5 inches. The curve externally is an arc of a circle whose 

 radius is 84 feet 1'62 inches. To the height of 27 feet 8 inches from 

 the attic the dome is solid. At the base the thickness is 9 feet 

 7 inches ; and as the external dome is raised higher than the internal 

 dome, the thickness is increased as the curve ascends, so that where 

 the dome is divided the thickness is 11 feet 4 inches. The circular 

 space which divides the two domes is 3 feet 2$ inches wide ; the in- 

 ternal dome is 6 feet 4 inches thick ; and the height from the attic to 

 the opening of the lantern is 83 feet 10 inches. The diameter of the 

 lantern is 24 feet 10 inches. The external dome is 2 feet 104 inches 

 thick where it separates itself from the internal dome; and it is 

 strengthened externally by 16 projecting bands of the same thickness. 

 The dome is pierced with three rows of small windows. As the curves 

 of the dome are not concentric, the space between them becomes wider 

 as it rises; so that at the opening of the lantern the space is 10 feet 

 wide. These domes are joined together by 16 walls or spurs, diminishing 

 in thickness as they ascend to the lantern ; at the base they are 8 feet 

 thick, and at the summit 3 feet. The base of the lantern is arched, 

 and pierced with small windows. Above the two domes is a circular 

 platform, surrounded with an iron gallery. In the centre rises the 

 lantern, on a stylobate broken into 16 parts, forming projecting 

 pedestals, above which are buttresses similar to the buttresses of the 

 drum, decorated externally with coupled Ionic columns, 174 inches in 

 diameter. The space between the buttresses is filled with arched 

 openings, which give light to the lantern. The external diameter of 

 the lantern is 39 feet ; the internal diameter 25 feet 11 inches ; and 

 the height from the platform to the top of the cross is 89 feet 74 



inches. The whole height, from the external plinth of the dome to 

 the cross, is 263 feet. The total height from the pavement is 437 feet 

 5 inches. The total height internally, to the top of the dome of the 

 lantern, is 387 feet. 



Sixtus V. covered the external dome with lead, and the bands with 

 bronze gilt. One hundred thousand large pieces of wood were used in 

 making the centering of the domes, which was so admirably constructed 

 that it appeared suspended in the air. (See the drawings in the work 

 by Fontana, on the construction of this dome.) This centering was 

 more for the purpose of a scaffolding for the materials and workmen, 

 than to sustain the weight of the double dome. During the construc- 

 tion of the dome it is believed that only two circles of iron were placed 

 round the masonry, one of which was placed on the outside of the 

 internal dome, at about 36 feet from its springing, and one foot above 

 the division of the domes. The bands of iron of which this circle is 

 composed are 3 inches wide by 1 j inch thick. A similar circle is placed 

 about the middle of the solid part of the dome, at about 17 feet 6 inches 

 above the springing of the internal dome. Near the top of the internal 

 dome there are several holes, at the bottom of which upright iron bars 

 appear. These bars are said to be the connecting rods which keep 

 together other circles of iron placed at different heights within the 

 masonry, which are finally terminated by a circle round the eye of the 

 dome. 



The domes were constructed with such haste, that sufficient time 

 was not allowed to the work to take a solid bed as it was carried xip, 

 in consequence of which a great number of vertical settlements have 

 occurred, and the circle of iron round the internal dome was fractured. 

 To obviate the danger arising from these settlements, six circles of iron 



SS1S3S3. . Si: 



ZNGLISH FEET. 



Parllel Section of the four principal Domes of Europe, to the same scale ; by Joseph Gwilt. 



were placed round the external dome at different heights, and the 

 broken circle of the internal dome was repaired. The first circle was 

 placed above the cornice of the external stylobate, or continuous 

 plinth, on which the buttresses stand ; the second circle was placed 

 above the cornice of the buttresses, the third above the attic at the 

 springing of the external dome, the fourth half way up the external 

 dome, and the fifth under the base of the lantern. A sixth was shortly 

 after placed at one foot below where the dome divides itself. The iron 

 bands are flat, from 16 to 17 feet long, 34 inches wide, and 2 inches 

 thick. At one end of the pieces of iron a hole is made ; the other end 

 i turned up and passed through the eye of the next band. The whole 

 ABT8 JLXD SCI. DIV. VOL. HI. 



of these bands are fixed with iron wedges, driven into the rubble with 

 mallets. Sheets of lead are placed under the iron circles. In the 

 ' Encyclopedic Mdthodique ' there is a detailed account of the various 

 fractures of the dome, and the means employed to repair them, 

 (' Encyclopedic Mfthodique,' Architecture, article Coupole.) 



The dome of St. Paul's Cathedral, London, is placed over the inter- 

 section of the arms of the cross. The ground plan is a regular octagon, 

 each face of which is 44 feet 8f inches wide ; four of these sides are 

 formed by the four great arches of the naves ; the other four sides are 

 formed by false arches of the same size ; in each of these arches there 

 is a great niche, the base of which is pierced with two arches. By this 



