other ornaments in relief. In seme Inslances many of these
modes are adopted, in others they are all blended together.
Hence arises so great a variety, that Palladio asserts that no
rule can invariably be laid down with respedl to ceilings.
§. Of vaulted ceilings the latin names are not fully aseer-
tained. Arcus, fornix, tejiudo, concha, camera, * are terms
applied without dislindlion to all vaulted cielings whatever.
The two Greek words, hemisphjerium and hemicylin-
DRUM, are susEciently understood, and always imply the mod
perfed kind of arch, that is, the semicircular one.
But vaults are often made not in the sorm of a semicircle,
but with a less degree of inssexion \ or, as Vitruvius expresses
it, “ad circinum delumbata.” See B. vi. Chap, 5. This kind
of vault is not circular, but by the help of a compass origi-
nates srom a semicircle in the following manner.
In Fig. 8. c a b d is a semicircle equally divided at pleasure,
c d the radius drawn at right angles : c e the apsis or height of
the arch you intend to deseribe, which the Italians term frez-
za, we sagitta.f On the center c with the disiance c e deseribe
the semicircle cg eg parallel to the former. Let the radii c f
and the sines f g be drawn ; where the radii cut the peri-
phery of the lesser circle, let right lines be drawn to the sines
parallel to the diameter; through the points where they in-
tersedl them let the equable curve a e b be drawn, which is
the curve required.
Palladio says that the arches of ceilings, less than semicir-
cular, are mod advantageoussy deseribed when they have the
srezza or arrow a third part of the breadth of the room.
Phis he shews by seven plans of as many rooms construdted
by himsels, with arched roofs peculiarly adapted to them.
* Ail signifying an arch,
f Frezza, sagitta, an arrow in Englith, In mathematics the term signisies a
versed line of an arch, standing on the chord like an arrow. See Chambers s