" On the constructions of Michael Angelo a circular attic was first formed, 19 feet 21 inches bight, and 9 feet 7 inches thick. This attic is externally by 16 projections, 2 feet 11 inches deep, and 6 feet 4+ inches wide, placed over the buttresses of the dome; on the attic rises the double dome, the internal diameter of which at the base, is 138 feet 5 inches. The curve externally is an are of a circle whose radius is 54 feet 1.62 inches. To the height of 27 feet S 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 2-1 inches wide; the internal dome is 6 feet 4 inches thick ; and the height frum the attic to the opening of the lantern is S3 feet 10 inches. The diameter of the lantern is 24 feet 10 inches, The external dome is 2 feet 10,1 inches thick, where it separates itself from the internal dome ; and it is stretTthened externally by 16 projecting bands of the same thickness. The dome is pierced is ith three rows of small windows, as the curves of the dome are not concentric, the space between them becomes 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 S 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 plattbrm, stir rounded 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 columns, 17,1 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 Off inches; and the height the platform to the top of the cross is 59 feet 7.' inches. The height, from the external plinth of the dome to the cross, is 263 feet. 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. During the construction of the dome it is believed that only two circles mt• iron were placed nuntil 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 11 inches 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 he the connecting rods which keep together other circles of iron placed at different heights within the masonry, which are finally ter minated by a circle round the eye of the dome.
"The domes were constructed with such haste, that suffi cient time was not allowed to the work to form solid beds as it was carried up, in consequence of which a great number of vertical settlements took place, and the circle of iron round the internal dome was fractured. To obviate the danger
arising from these settlements, six circles of iron 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, 31 inches wide, and inches thick. At one cud of the pieces of iron a hole is made the other end is turned up, and passed through the eye of the next band. The whole of' these bands are fixed with iron wedges, driven into the rubble with mallets. Sheets of lead are placed under the iron circles." (Coupolc, Encyclop6die Jfothodique; `Architcclurel St. Paul's cathedral, London, the workmanship of the great Sir Christopher Wren, was begun in 1655, and finished in 1710. The dome is placed over the intersection of the four naves. The ground plan is a regular octagon, each face of which is 44 feet 8:1- inches wide: four of these sides are formed by the four great arches of the naves; the other four sides are formed by thlse arches of the smile size ; in each of these arches there is a great niche, the base of' which is pierced w ith two arches. By this means eight supports are obtained instead of tour, and the do not project too much, as in other siunilar constructions. The corbel Hugs gather in a circle. the diameter of which is 101 feet'! inches, the octagon base being 107 feet. The are surmounted by a complete entablature, S feet three inches high, decorated with consoles. The drum is set back 3 feet 2i inches from the 1;lee of the frieze, and this intermediate space is occupied by two steps and a seat. The cornice is 95 feet 91 inches from the pavement. The height of the drum from the top of the seat is 62 feet 61 inches to the springing of the internal dome. The wall forming the drum is inclined internally 4 feet 114 inches, or about the 12th part of its height. This was designed by the architect to increase the resistance of the walls to the united pressure of the large internal vault, and the conical dome which carries the lantern.
The interior of the drum is decorated with a continuous stylobate, on which is an order of Corinthian pilasters. The 32 spaces between the pilasters are tilled with 24 windows and eight large niches. F,xternally the drum is decorated with an order of 32 Corinthian columns engaged, which are united to the wall of the drum by eight solid constructions in masonry. In each space between the constructions there are three intcrcctnnnietions, the columns being joined at their bases by wall pierced with arches. The external colonnade is surmounted by an entablature ; behind this is a terrace, formed by the recessing back. The attic is 22 feet 4+ inches high from the top of the balustrade to the under side of the cornice of the attic. Above the internal order of the drum rises the interior dome, the diameter of which at the spring ing is 102 feet 24 inches by 51 feet in height. The top of the dome has a circular opening 14 feet 101- inches in diameter.