4

OBSERVATIONS ON VAULTS.

polygon, or key of the vault, is generally composed ofmany stones posited domewise, (Fig. 21. and 22.)

Pendent rib vaults are generated similarly to pen-dent vaults. The varieties arise from the curvatureof the vault, and also from the positing of the ribs,(Fig. 23. and 24.)

The Romans used cylindrical, cylindroidal, groined,and domical vaults. The Eastern empire adoptedpendent vaults of hemispheres, crossed by cylindricalor cylindroidal vaults, together with the vaults pre-viously in general use with the Romans. The archi-tects of the middle ages have vaulted their buildingssuccessively with cylindrical, cylindroidal, groined,and domical vaults, with groined rib vaults, vaults ofribs of the same curvature, and rib pendent vaults:and in their spires they have used pyramidal and con-vex pyramidal vaults. The moderns have imitatedthe vaults constructed both in the Western and East-ern empires of the Romans, adopting improvementssuggested from the superior practice of the middle