The only difficulty in the construction of wooden domes is when they arc loaded in the upper part by a heavy lantern, or cupola. Such a dome as has been described would be in danger of being crushed inwardly ; the most effectual method of preventing which, is by making the ribs in the form of trusses, with an inclined timber extending from the base of the dome to the bottom of the lantern, connecting the two extremities of the exterior side, forming as it were the base of a truss in a common roof, and acting contrary to the nature of a tic-beam ; they resist the vertical pressure of the lantern, without having any tendency to burst out the sides by acting entirely longitudinally on the wall-plate. In order to secure the lantern, horizontal braces are fixed from the bottom pf the lantern to the middle of the principal braces under the joggles, so that the whole is resolved into triangles, which are all immoveable at the angles. The wall-plates should be framed as the ribs of a dome constructed as in the last example.
When a dome is to support a heavy cupola of stone, such a construction as that of the cathedral of St. Paul's, London, may be employed. Figure 3, No. 1, exhibits the truss of this dome, taken from accurate measurement. A a a A, a dome of brick, two bricks thick, which, as it rises every live feet, has a course of strong bricks IS inches long, bonding through the whole thickness. abbri is a cone, built with bricks 1 foot 6 inches in thickness, for supporting the heavy cupola above, of Portland stone, which is 21 feet in diameter and near 61 feet high, and also the timber-work of the dome.
The horizontal, or hammer-beams, c c, &c. are curiously tied to the corbels, D, D, D, &e. with iron cramps, bedded into the corbels with lead, and bolted to the hammer-beams. No. 2 shows more particularly the manner of tying the hammer beams to the corbels.
This dome is boarded from the base upwards, and the ribs are therefore fixed horizontally, having their sides in planes tending to the centre of the dome. The contour of the dome is formed of two circular segments, which meet in the axis like a pointed arch. The scantlings of the curve rib of the truss are 10 inches by 11 at the bottom, and 6 inches by 6 at top. It has a very strong double iron chain, linked together at the bottom of the cone, and several other less chains between that and the cupola. This dome was turned upon a centre, supported without standards from below. As every story of the scantling was circular, and the ends of the ledges meeting like so many rings, and truly wrought, it supported itself; and as it was both centering and scaffolding it remained for the use of the painter, there being a space of twelve feet between it and the dome. This machine, it is said, was original of its kind.
A description of a dome constructed mainly of cast-iron girders, over the cathedral of St. Isaac, St. Petersburg, will be found under the article DOME, to which we must refer for further information on the general subject, as well as to