The complete plan of the springer upon a detached pier now takes the
cross form, suggesting the substitution of a cluster of four shafts
round a square, or of a larger column, with a capital broken into the
cross form. Where, however, the weight to be carried was small, as in
crypts whose vaulting supported only the floor above, this enlargement
of the pier was obviated by making the ribs die out at their springing
one into another, and the groin to commence a little higher up; or
sometimes by the awkward expedient of making the outer curve of the rib
_eccentric_ with the inner one.[38]
[Illustration: Fig. 321.--Crypt, Durham Cathedral.]
Where we have already clustered pillars carrying a main arcade, the
presence of vaulting on either side adds a new member to the pier, both
behind and in front; and if, as is usual in churches, the central vault
springs from a higher level, the additional shaft on that side runs up
_through_, or rather _by_, the capital of the pier till it reaches the
higher springing, thus emphasising the division of the bays throughout
their entire height.[39] This multiplication, however, of shafts is by
no means essential, as the ribs may be brought, by a little management,
on to the capital of a single column, which supports the arcades, and on
their other side shafts may be carried up upon corbels to receive the
higher groining.
Having said enough upon this simple case of groined vaulting to show
that it may be made both the source and the vehicle for architectural
treatment of a most reasonable kind; and, as you will readily imagine
that its ribs and their supporting capitals, corbels, and colonnettes,
may receive _any amount_ of sculptured enrichment, and its vaulted
surfaces any degree of decoration in the form of painting or mosaic
work, I will here close my lecture, hoping that, though its
subject-matter may have appeared somewhat dull and its arguments almost
self-evident, it may, nevertheless, have placed simple and familiar
facts before you in a form more systematic than that in which they might
otherwise have presented themselves; and that, like the definitions and
axioms of Euclid, it may be serviceable in preparing the way for more
intricate and less obvious matters of consideration which I shall have
to bring under your notice while following out, in my succeeding
lectures, the principles of vaulting into those more difficult and
ornate forms which became so important an artistic element in the
subsequent developments of Mediæval architecture.