" Above the attic are two steps, from which the external dome springs. The external dome is constructed of wood, covered with lead. and decorated with projecting ribs forming panels, curved at the ends. This dome terminates with a finishing which joins the base of the lantern : the circular gallery formed on the finishing is 274 feet 9 inches above the pavement of the nave. The lantern is supported on a conical tower, terminated by a spherical dome. This tower, which is joined to the internal dome at its base, disengages itself from it at the height of S feet 0 inches above the springing of the same. Time perpendicular height of this tower is SO feet 9 inches, and the walls arc inclined 24 degrees from the perpendicular, the diameter of the base is 100 feet 1 inch measured externally. and 34 feet 1 inch at the springing of the spherical dome which finishes it. The wall of this tower is built of brick, and is 1 foot 7 inches thick, with circular rings of masonry, fiustened with iron bands. The spherical dome at the top of he tower has au opening S feet in diameter at the summit. Iletween the attic and the wall of the tower are 32 walls or buttresses, which also serve to bear the ribs of the wooden external dome."—Penny Cyclopcalia.
About the same time that Wren built the dome of St. Paul's, Hardouin Mansard, a French architect, constructed the dome of' the Invalides at Paris. The plan of this dome is a square, in which is inscribed a Greek cross ; in the angles of the square there are four chapels. The dome is raised in the centre of the Greek cross; the base supporting it is an octagonal figure, with four large and four small sides. The dome, which is double, rises from a springing which is common to both. The lower or internal dome constructed with masonry is spherical. The outer dome is of a spheroidal form, constructed of stone at the base, and of brick above. It is framed of wood and covered with lead, like St. Paul's, London, but the construction is much heavier. The total height to the top of the cross which surmounts the lantern is 330 feet.
The modern Pantheon at Paris, formerly the church of St. Genevieve, was built by J. G. Soufflot, who distinguished himself by his architectural works, in the reign of Louis XV. The dome, which is lofty, is sustained by four pillars, arched over the cross parts. The angular spaces are filled up with pendentives, terminating in a circular ring, on which a cylin drical wall is built, supporting the cupola. ID the latter par ticular it is similar to St. Paul's.
Of wooden domes, that of the Halle du Ble, at Paris, is an excellent example ; it being more than 200 feet in diameter, and only a foot in thickness.
A new material has been lately employed in the construc tion of the dome of the church of St. Isaac, at St. Peters
burg, erected under the direction of the Chevalier de Mont ferrand. An account of the construction is given by Mr. Godwin as follows:—"The walls of the dome are carried up in solid construction of brick, with tiers of stone-bond, and are above 5 feet thick. On the level of the top of the cornice of the circular colonnade which girds the drum, there is a series of twenty-four east-iron ribs, the feet of which rest on a cast-iron plate 7 feet wide, which nuns quite round the circumference. At their head all the ribs are attached to a horizontal plate or curb, 6 feet 3 inches wide, which follows the periphery of the clome. At this height the rib is divided into 2, the one part about 12 feet 6 inches deep, following the sweep of the inner dome for a height of 20 feet, is at its summit bolted to a cast-iron perforated cylinder, 21 feet in diameter, and 7 feet high; this forms the centre aperture at the summit of the inner dome. The other part follows the line of an intermediate cone, with a catenary outline, and similar to the one in St. Paul's ; it is also 21 feet long, and 2 feet 6 inches deep, and perforated to render it lighter. At this height the heads of the ribs are again secured to another horizontal plate or curb, which forms a complete circle, and is 3 feet wide ; and this curb and the ribs are tied to the cylindrical opening of the inner dome, already mentioned, by radiating beams 2 feet 11 inches deep. The conical ribs have then another length of 21 feet. and their heads are again con nected by another horizontal plate, from which spring the circular ribs, about 16 feet long, forming a dome to the inter mediate cone, and their heads are also bolted to a cylinder, 8 feet 6 inches in diameter, and IS inches high. But the upper portions of the ribs diverge at top, so as to form a base fur the octagonal cupolino, which consists of a series of east iron story-posts, ribs, and bracketings, inclusive of the dome of the with its ball and cross at the apex, which last are of brass-gilt. The filling in between the ribs consists of pots, the surfaces of which are subsequently rendered with plaster, and painted with sacred subjects. The external thee of this outer dome is covered with bronze gilt in three thick nesses of leaves of ducat gold. The whole entablature and flat, and the balustrade over the peristyle of the drum of the cupola, likewise consist of cast and wrought iron framing, faced with plates of copper, to form the profiles and mould ings. The 24 pedestals of this balustrade carry winged angels of bronze, above 9 feet high,'each of a single casting.