Chap. XXIV.

MASONRY.

1459

Conical Vault doubly skewed , and splayed in a Wall having an Inclination. — The voussoirsmay be traced in a similar manner, as in the preceding example. The voussoir in perspec-tive is formed by dressing the vertical face at right angles with the extrados; the mould isthen applied, and the mould of the extrados on the horizontal plan; then with a bevel thesecond face is cut, on which the mould of the sloping head is applied, and the stone dressedaccordingly.

Spherical Domes are formed on the plan and elevation by semicircles; the interior sur-face of this kind of vault presents the effect of a reversed bowl, which has given them theterm cupola, when of large diameter, as those which terminate as domes. The best methodfor setting them out is by horizontal courses, forming concentric crowns. Fig. 2617. showsthe plan of the soffites of the courses of voussoirs of the vault above; it is set out in thesame manner as it would be for a sphere. Supposing the interior surface of each course ofvoussoirs to be formed of a portion of a truncated cone, the sides of which are represented

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Ull

tig. 5614-

2617

/ Spherical / ,r Vault.

Fig. 2618. DEVELOPMENT.

Fig. 2615.

by the cords A a, ab>bc, cd of fig. 2617. Thus tofind the radius of the arc of a circle, showing thedevelopment of each of these parts of a cone, the chordswhich answer to the courses of the voussoirs are pro-longed until they meet the axis.

The interior surfaces of spherical vaults being circular on the plan, as well as on the sec-tion, the moulds for the soffite can only be calculated by approximation, and they also re-quire preparatory surfaces, which are not those on which should be traced the arrises of thejoints of the voussoirs.

To form these voussoirs more accurately, commence by cutting a prism which has forbase its projection on the plan : from this will result a portion of a hollow cylinder, thesurfaces of w'hich will be terminated by the extreme arcs of this projection ; thus the vous-soir represented by fig. 2617. is comprised in a portion of a hollow cylinder, indicated onthe plan by c" 5' 5" c" f . The profile of the mass of this portion of the cylinder is indicatedin the section. It will be found that if the several horizontal lines be traced by a bendingrod on the curved surfaces, they will show the exact position of the arrises which passby these joints. 'There must then be exactly traced on the straight surfaces the arrises in-

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