SECTION V.
OF THE CONSTRUCTION OF DOMES OR CUPOLAS.
195. —A dome or cupola is a roof, the base of which is a circle, an ellipsis, or a poly-gon ; and its vertical section a curve line, concave towards the interior. Hence, domes arecalled circular, elliptical, or polygonal, according to the figure of the base.
The most usual forrii for a dome is the spherical, in which case its plan is a circle, thesection a segment of a circle.
The top of a large dome is often finished with a lantern , which is supported by the fram-ing of the dome.
196. —The interior and exterior forms of a dome are not often alike, and in the spacebetween, a staircase to the lantern is generally made. According to the space left betweenthe external and internal domes, the framing must be designed. Sometimes the framingmay be trussed with ties across the opening; but often the interior dome rises so high thatties cannot be inserted: in the latter case, the observations made on the equilibrium ofdomes in Sect. I. (art. 62—66,) should be attended to.
Accordingly, the construction of domes may be divided into two cases : viz. domeswith horizontal ties, and those not having such ties.
ON THE CONSTRUCTION OP DOMES WHICH ADMIT OF HORIZONTAL TIES.
197- A truss for a dome where horizontal ties can be inserted is shown by fig. 71?Plate XII. In this figure AA is the tie; BB posts, which may be continued to form thelantern; C, C, are continued curbs in two thicknesses, with the joints crossed and boltedtogether; DD, a curved rib to support the rafters. This design is calculated for a span ofabout 60 feet, and may be extended to 120 feet.
Two principal trusses may be placed across the opening, parallel to each other, and at adistance equal to the diameter of the lantern apart, as AB, CD, fig. 72; with a sufficientnumber of half trusses to reduce the bearing of the rafters to a convenient length.
Or, the two principal trusses may cross each other at right angles in the centre of thedome, the one being placed so much higher than the other as to prevent the ties interfering.
p