the roof above it from the
building which it covers. The structure of the vault will be perhaps
most easily understood if it is conceived as a flat roof of stone of the
same height as the top of the clerestory, supported by fan-shaped
brackets springing from a point between the clerestory windows, and
rising and spreading out until they reach the central ridge of the
vault. As the vault is, but for these brackets, in its essence flat,
there must of necessity be a great sacrifice of space between it and the
roof above it. This sacrifice of space is obviated by the barrel roof,
which nearly approaches to the shape of the outside roof, and fits into
it without the loss of space entailed by the vault. But the barrel roof
does not readily submit to a structural division into bays, or a
structural decoration by means of ribs and bosses such as ornament and
emphasise the divisions of the intersecting vault.

Wishing, as has been said, to combine the advantages of both forms, the
designers of the transept roof have given it the shape of a barrel roof,
and have covered it with a network of ribs, some of which converge
between the bays of the building and meet at a point on a level with the
bottom of the clerestory. The roof, therefore, has at first sight the
appearance of a vault, but it remains a barrel roof divided by ribs all
the same; and this will be evident so soon as it is remarked that the
top of the roof is not on a level with the top of the clerestory, but
some way above it. It is, therefore, not to be conceived as a flat roof
supported by brackets, but as an almost circular roof ornamented and
divided by structurally unnecessary ribs. Indeed, it would be altogether
impossible to combine a vault with such a clerestory as is found in
these transepts, for a vault is a roof designed to fit a pointed arch.
Its spreading supports make it impossible to adapt it to any other than
an arched clerestory; and the clerestory of these transepts, consisting
as it does of a row of five lancet windows, is flat at the top. A barrel
roof, on the contrary, will fit any kind of buildings, but,
unfortunately, it is seldom successful, except in round-arched churches.
To some of these--as, for example, in Auvergne--it has been applied with
magnificent effect. It is very rare in England. It is always very
difficult to decorate. The fifteenth century builders having for some
reason or other decided on the form, and being but little accustomed t