Vaulting of spaces of other forms than the mere square—Apsidal aisles, St. John’s Chapel Tower, and St. Bartholomew’s Church, Smithfield—Chapter-house and crypt, Worcester—Round-arched vaulting in its most normal form, as resulting from the barrel vault and its intersections—Short digression on another simple form of vault, the dome—“Domed up” vaults—“Welsh” groining—The square or polygonal dome—The Round-arched style of the twelfth century almost perfect—First introduction of the Pointed arch into vaulting—Names of the parts of groined vaulting—Two specimens in London of the apsidal aisle, one in the Round-arched, the other in the Pointed-arched style—Vaulting a polygon with a central pillar—Ploughshare vaulting—The artistic sentiment and character of early Gothic vaulting.
IN my last lecture I explained the general principles of groined or intersecting vaulting, and just carried on the subject through its simplest case,—the covering of a square space, or any repetition of square spaces, by the intersection of semi-cylindrical vaults; and I just showed how, by emphasising the outlines of the squares so covered by means of transverse ribs or angles, and by placing impost mouldings, pilasters, columns, or colonnettes in the sustaining piers, such a mode of covering a space might be readily made at once susceptible and suggestive of architectural treatment.
Let us now proceed to consider the application of the same principles to the vaulting of spaces of other forms than the mere square.
The next form, perhaps, in point of simplicity is an equal-sided polygon,—say, for example, an octagon ([Fig. 322]). We must here suppose eight cylindrical vaults crossing one another from the opposite sides of the octagon; and it is clear that their intersecting lines will be the diagonals or lines joining the opposite angles of the octagon, which will coincide in position with the transverse ribs. The objection to this form of vaulting is the low proportion of the arches produced by these intersections, which, though more than twice and a half the width of the side arches, only rise to the same height, or about one-fifth of their span,—a defect which will be remedied by a development I shall presently have to describe.[40] Just as the half-dome (as seen in the chapel of the Tower of London)[41] forms a natural covering for an apsidal termination of a barrel vault, so a portion of a polygon, thus vaulted, would appear to be the correlative apsidal termination of a groined vault.[42] A difficulty, however, at once presents itself in the small height of the vault last described, which is not one-half of the height of the semicircular vault which it would have to meet. How, then, is this to be got over? How are the vaults proceeding from the narrow arches of the sides of the octagon to be brought to range in height with the wide vault which spans the whole space (Figs. [323] and [326])?
Fig. 322.
