Chap. XXII.
TIMBER DOMES.
1345
novelties of their kind, and admirably fitted for their purpose : although their first construc-tion may be far more costly, they will in all probability supersede timber structures, onaccount of their greater security against fire.
It has been suggested to form roofs of bent timber for the purpose of covering thebuilding docks in our shipyards : when the depth of a piece of timber does not exceedof its length, it may be curved easily, and its elastic force not be destroyed, if its versedsine is not more than an eighth of its span : two pieces may be so bent and laid one overthe other, by first securing one end by a cord or iron strap, and then pulling down thesecond piece to the back of the other; a number of layers may be thus brought downupon each other, and secured by bolts or ties.
Domes and Cupolas _To construct these in timber it is usual either to frame trusses
entirely across the opening, or, where the ties cannot he allowed, to form the ribs, whichcarry the covering like a series of centres, thus dispensing with the ties. The dome of theVal de Grace at Varis is an example of the first kind; it is composed of four principaltrusses, two of which cross the others at right angles, leaving an opening to the cupolain the centre : sixteen half trusses converging to the centre, arranged at equal distancesaround the dome, carry the rafters, which support the covering: such constructions
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Fig. 2167.
as the above admit of application where the exterior effect is alone considered, or where itis merely required to cover in and protect a hemispherical dome of masonry : it is evidentthat the interior cannot be made available either for use or decoration, in consequence ofthe timbers which are introduced.
Dome of the Invalids, at Paris , whose external diameter is 90 feet, is another example,composed of two principal trusses, which cross in the middle at right angles, and unite in adouble upright post; each quarter of the dome, or the space between these principal
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