"The quantity of metal employed in the work is as follows:— "The roofing is wholly of iron, covered with copper. The raising of the monolithic shafts of the 24 columns of the exterior peristyle of the dome—each of which weighed nearly 66 tons—to the height of 150 feet, was an operation requiring considerable skill. The first column was raised on the 17th November, 1S37, and in two months the 24 columns were completely fixed.
"The skeleton of the entablature of the peristyle of the dome is of cast and wrought iron, resting on the columns, and affixed to them by wrought-iron pins, which are let a con siderable depth into the shafts, and the f•ame-work is also let into the cylindrical wall of the dome, securely affixed to three templates. The cornice, with its modillions and mouldings, rests on cast-iron corbels; the caissons and rosettes of the inner soffit also rest on cast-iron girders.
"The careful skill with which the architect has fulfilled his part, and the feeling thr decorative art with which he has embellished the church of St. Isaac, render it one of the most striking edifices of the nineteenth century." All the ancient Roman domes are on the convex side a much less portion of a sphere than a hemisphere ; but those, from the completion of the church of Santa Sophia, to the finishing of St. Paul's cupola. are of the surmounted kind, approaching in a greater or less degree to the proportion of towers, or spires. which were so much admired and adopted in the middle ages. The sides of the section of St. Paul's dome are struck with centres in the base line, which, if con tinned, would meet in an angle in the axis of the dome. Since the revival of Grecian architecture, the contour of the old Homan dome has also been revived, especially in cases where other parts of the building are decorated with any of the orders. Exterior domes can never be correctly applied to buildings in the pointed style of architecture.
The following are the admeasu•ements of some of the prin cipal domes in Europe, taken from Mr. Ware's "Tracts on Vaults and Bridges : — Domes of Antiquity.
Feet Height in diameter from the taken externally. ground line.
Dome of the Pantheon 142 143 Minerva Mediea, at Rome 78 . 97 Baths at Caracalla. 112 116 Baths at Diocletian 74 83 Temple of Mercury • 68 " Diana 98 78 " Apollo 120 " Proscrpine and Venus. 87 77 Domes of comparatively Modern Times.
Santa Sophia, at Constantinople 115 201 Mosque of Aehmet, 92 120 Sau. Vitate, at Ravenna 55 91 San. Marco, at Venice 44 Front the time of Branelleschi to the present period.
Santa Maria del Fiore, at Florence 139 310 The Chapel of the Medici. 91 199 Baptistry, at Florence 86 110 Cathedral of St. Peter, at Rome 139 330 Chapel I if the Madonna della Salute, at Venice 70 133 Chapel of the Superga, at Turin 64 128 Invalides, at Paris, 80 173 Val de Grace, Paris 65 133 Sorbonne, Paris. 40 110 Pantheon, or St. Genevii:ve, Paris. 67 190 Cathedral of St, Paul's, Loudon. 112 215 In the reigns of queen Elizabeth, and her successor king James I., square turrets, surmounted with domes resembling a bell in their outline, were much used.
Domes are sometimes made.convex below, and concave above; the former being a much greater portion of the side than the latter : these may be denominated Moresque, Turkish, or I lindoo.
Mr. Bunce invented a dome that requires no centering; in this construction; all the abutting joints are continued in uninterrupted vertical planes ; but the horizontal joints of every two stones break on the middle of the stones on either side ; so that every alternate stone of a course projects upwards, and leaves a recess for the insertion of the stones of the next course. Upon this principle, the intervals. as the building approaches nearer the top, becomes more wedge formed, and, the interior circumference being less them the exterior, the stones can be inserted only on the outside : con sequently, if made so exact as just to fit into their place's, they cannot fall inwardly. This mode of joining stone: may be convenient, as requiring no centering ; hut unless the courses be nicely equilibrated, it is more liable to burst, than when a dome is constructed in the ordinary manner, since every row of stones, from the base to the top, forms an arch inde pendent of the rest.
The equilibrium and pressure of domes is very different from that Id' common arching, though there are sonte proper ties common to both. Thus, in cylindrical and evlindroidal vaulting, of unifbrm thickness, if the tangent to the arch at the bottom be perpendicular to the horizon, the vault cannot stand ; neither can it be built with a concave contour in whole, or in part ; and to bring an arch to an equilibrium, whether its section be circular or elliptical, the intrados being given, both extremitles of the arch must be lea led, ad ivi ndunf, between the extrados of the curve which runs upwards, and the vertical ass% mptote rising f•iun each foot. So, in thin chimes. of equal thickness, if the curved surface rise perpen dicularly from the base, it will burst at the bottom, whatever be the contour.