Fig. 353.—Interior of the Chapter-house, Westminster Abbey.
there is a choice between two methods of effecting it: either by supposing the main vault to span from wall to pillar or from angle to pillar (Figs. [351], [352]).
The former is, on a primâ facie view, the more natural, but it has the disadvantages of breaking the chief side of the vaulting compartment which rises from the corners into a resalient angle, and also of rendering the main ribs from these angles across to the pillar, in one half of their length diagonal ribs, and in the other transverse; and of making one half represent a projecting and the other a receding angle, while the angle ribs of the outer half meet the transverse ribs of the inner half of the vault.
These objections are entirely removed by supposing the main vaults to run directly from the angle to the pillar. In either case the ridge which surrounds that half of the vault which springs from the pillar takes the form of an inner octagon.
In the first case, the sides of this are parallel to the walls, while in the second they take an intermediate direction; the angles of the inner octagon being opposite the centres of the sides of the outer one, and vice versâ.
The vaulting compartments which rise from the angles of the great octagon are precisely similar to the opposite ones which rise from the pillar, and the ribs which rise from the angles to the pillar are throughout transverse ribs, while the angle ribs from each side duly meet one another.
I exhibit a view of the interior of the Chapter-house, Westminster (Fig. [353]), to show the beauty of this form of vaulting. Few forms, in fact, in any style of architecture present such beauties as an octagon vaulted in this manner; and I am happy to think that our London specimen, which has been lost for the last century or more, will now very shortly be restored to its original form and condition.
I have already mentioned that in all these forms of vaulting,—that is to say, those with level ridges,—owing to a geometrical error resulting from the use of circular curves for all the ribs, the filling in of the vaulted spaces must be artificially shaped to fit those curves.
The use, however, of a form of vaulting analogous to that before described as having raised ridges would obviate this inaccuracy.