Let us suppose a single column supporting arches of a single order (Fig.
300), and that we desire to extend the arches to _three_ orders,
retaining the same main bearing-shaft. We may imagine the additional
orders to be super-added on all sides of the original square springer
and additional colonnettes (attached or detached) added round the
original bearing-shaft to receive them (Fig. 301). The same may be
applied to an octagon, placed either angularly or in its usual position
(Figs. 302, 303).
[Illustration: Fig. 303. Fig. 302. Fig. 304. Fig. 306.]
The process may be carried a step farther, and eight colonnettes be set
round the original bearing-shaft (Fig. 304). In St. Mary’s Abbey,[35] at
York (towards the end of the twelfth century), we have an instance of
sixteen colonnettes thus placed round a bearing-shaft (Fig. 305), but
only eight of them carry separate orders; and a little later, in the
cathedral at Genoa (the work, apparently, of a northern French
architect), we have no less than twenty-four colonnettes similarly
ranged round an octagon (Fig. 306); though here, again, only eight are
represented in the plan of the abacus or of the base when it rests upon
the floor, the others being introduced probably for the relief produced
by the varied colours of the marbles of which they are composed.
[Illustration: Fig. 305.--St. Mary’s Abbey, York.]
A little later the colonnettes themselves become grouped in threes and
fours, and their edges often filleted, or “keeled,” that is, decorated
by an arris or edge projecting from their round surface. Thus, at
Lichfield (Fig. 307) in the older portions, groups of three shafts
united into one, and carrying a common abacus, were set on each side of
an octagonal bearing-shaft. At Wells (Fig. 308) similar triple shafts
were set alternately against the faces and in the internal angles of a
cross-formed nucleus, with alternately square and octagonal abaci.
[Illustration: Fig. 307.--Lichfield.]
[Illustration: Fig. 308.--Wells.]
My purpose, however, is not to enumerate all possible varieties of
clustered pier, but to explain its principle, and at the same time to
show how unlimited an artistic element was deducible from an intent
thus founded on the natural conditions of arched construction. To go
much farther would carry us on prematurely into the succeeding styles,
and would be also anticipating another cause, which carried on the
principle to a still further development. I allude to groined vaulting,
of which I shall have to treat in detail when I reach it.
Before, however, I quit the subject of arches and piers, I must say a
few words on the application of their principles to doorways and
windows.
Doorways differ in no degree, as to principle, from archways, excepting
in having, at some point in the thickness of the wall, more or less
recessed at pleasure, what Professor Willis calls the “doorway plane;”
that is to say, one of the arch-orders so formed that the door may be
hinged to it, and may shut against it. The actual opening of the door
may or may not be stopped on this plane to a square heading, the arch
over it being filled in with a tympanum, plain or sculptured; or it may
be altered from the form of the main arch to some shape having less
height. In all other respects the principles already stated apply
equally to doorways as to archways. The interior, however, has to be
varied if the door fills in the arch-form, with a view to facilitating
its free opening; but this is a practical point not needful to be here
gone into. The orders of arch-mouldings in a doorway often continue down
the jambs, as in one of the magnificent doorways at Malmesbury Abbey: or
they may be replaced by colonnettes or pilasters, or these methods may
be united in the same doorway,--just as in another door at Malmesbury,
continuous mouldings alternate with colonnettes,[36]--and the arches,
jambs, and capitals, and even the shafts of the colonnettes, may receive
any degree of sculptured enrichment.
The doorway being a point on which much architectural character was
concentrated, and great depth being necessary to give the required
effect, it was customary to thicken the walls at the doorways by various
expedients, so as to obtain depth enough to give several orders of
arch-mouldings; this increased thickness was covered over by gables, and
by other means.
The width, too, of the jambs of doorways is often increased, and more
space gained for enrichment, by giving to each order in the jamb a
larger space than would otherwise be necessary of square face between
the shafts (Fig. 309).
[Illustration: Fig. 309. Fig. 310. Fig. 311.]
In some cases, also (as in the doorway in the Castle at Durham), there
is a small arch-order which continues down the jambs between the
principal orders, and adds much richness to the effect.
In later examples, two ranges of shafts were often introduced; the outer
ones carrying the orders, and the inner ones having capitals lost under
the main capitals, as if carrying an imaginary order hidden within the
visible mass of the arch. These are, in fact, the parallels of the
supernumerary shafts I have mentioned as often existing in clustered
piers. Thus, in St. Leonard’s Priory, Stamford (a work of the twelfth
century), we find two ranges backed by a plain splayed surface (Fig.
312). In the Galilee at Ely (somewhat later), the second range is backed
by large hollows between salient mouldings (Fig. 310); and again at
Lichfield, the back range is, as at Stamford, placed against a splayed
surface, but relieved by ranges of large toothed ornaments running up
behind each of the front shafts (Fig. 311).
[Illustration: 312.--St. Leonard’s Priory, Stamford.]
The windows also differ from mere arched openings in having a
_functional_ plane, which occupies one order, and is needed to receive
the glazing. The orders are never so numerous in windows as in rich
doorways, rarely exceeding two besides that which receives the glass.
The inner side is usually splayed, to diffuse the light through the
interior. It is not my intention in this lecture to treat in detail
either of doorways or windows; but having stated that a system of
receding arch-orders was originally the origin of window tracery, I will
say a few words in explanation of my statement.
Many early windows and window-like openings--such as those with the
triforium galleries of churches[37]--are divided into two or more
portions by pillars and small arches in the inner plane or order; the
outer order or orders embracing the whole, and the plane of the inner or
functional order forming a second wall-space over the heads of these
subordinate arches. Thus the triforium at St. Bartholomew’s is divided
into _four_ subordinate arches. This window plane, as it may be called,
is often ornamented in different ways, and occasionally even in Norman
work, is pierced. At a later stage this piercing becomes systematic, and
has received the name of “plate tracery,” the _plate_ being the window
plane or order. It is simply the piercing of this plane of the
functional order of the window arch; and as it is clear that this
piercing developed itself into window-tracery, so is it equally manifest
that the plane thus pierced originated in the division of the
window-arch into receding orders; and, consequently, that traceried
windows were a natural result of the conditions of arcuated
architecture. The subject of windows being quite sufficient to occupy a
separate lecture, I leave it for the present to go on with the more
elementary questions resulting from the conditions I laid down at the
outset.
You will have noticed that, having in those prescribed conditions
divided my subject into two great natural heads,--viz. the _arching_
over of openings in walls, and between piers; and the _vaulting_ over of
the spaces enclosed by walls or ranges of piers,--I have hitherto dealt
exclusively with the former; and that, as the forms of piers and
clustered columns are influenced as much by the requirements of the
_vaulting_ as of the _arches_ they have to support, I have been obliged
to leave my description of their forms imperfect; and as it is my wish
to treat of vaulting as systematically as I am able, I must beg you to
allow this incompleteness to remain till it is incidentally filled up as
we proceed with this, the second great elementary division of arcuated
architecture.
It must be clear, even on the most superficial glance, that the vaulting
over of extended areas is a matter of far greater intricacy, and
requiring vastly more thought and contrivance, than the mere arching
over of an opening in a wall; and though its primary elements are
simple, I must beg you to follow me over easy ground,--and ground
already trodden in my previous lectures,--because these early and simple
steps are needful to the due appreciation of the more advanced and
complex ones which we shall presently have to consider.
The simplest elements of vaulting are--first, the covering over of a
rectangular space enclosed between parallel walls by means of a
semi-cylindrical vault, usually known as a “_barrel vault_;” and
secondly, the covering over a space enclosed by a circular wall, by
means of a hemispherical vault or _dome_.
[Illustration: Fig. 313.]
[Illustration: Fig. 314.]
The first is the prolongation of an arch in a direct line at right
angles to its plane (Fig. 313), the second may be conceived as generated
by the revolution of an arch upon its vertical axis (Fig. 314).
I will keep, for the present, to the development of vaulting from the
first of these types. We will first suppose that, while limited by
constructive convenience to some moderate span, we have occasion to
vault over an area of double that width.
The most natural expedient which suggests itself is to divide the space
into two widths by an arcade whose top ranges on a level with the
springing of the vaulting, and on this and the outer walls to place twin
and parallel barrel vaults.
This was a system at first largely made use of, as we may see, in some
of the covered tanks or piscinæ of the ancients, and in the galleries of
the Colosseum. It is clear, however, that this is an imperfect covering
for a single room or hall, not only from its severing it too much into
two separate areas, but from its placing so much of the covering above
the level of side windows, and thus practically reducing the available
height of the walls; not to mention its heavy effect.
[Illustration: Fig. 315.]
Let us see how these imperfections may be obviated.
The solution of the question may have arisen from a different and
accidental case. Let us suppose two corridors, each covered by a barrel
vault, crossing each other at right angles. It is easy to see that these
vaults must, by their intersection, generate angles running diagonally
from corner to corner of the crossing of the corridors, and that these
angles of intersection would assume curves of an elliptical form (Fig.
315).
This square of intersection would in fact be found to be vaulted on a
system previously unthought of.
Let us next suppose _twin_ corridors, severed only by a wall, crossing
two other such corridors, all similarly covered by barrel vaults.
Instead of the simple intersection of our previous case, we now have a
group of four, or _two pairs_ of such intersecting vaults, meeting in
the centre on a mere frustum of the partition walls reduced to a square
pier, from whose angles spring four of those edges of intersection
before described (Fig. 316).
[Illustration: Fig. 316.]
This, then, contains the solution of the problem under consideration,
for, returning to our first case of vaulting a hall of double width, we
may, by repeating as many as we may need of these pairs of intersecting
or “groined” compartments, such as we have generated by the last
process, effect our object in a perfect manner; the vaults being all of
equal height, and the two widths being practically united into one,
while the walls cease to be stunted of their full height, and room is
left in them for windows reaching nearly to their top.
The same process may be applied to an area of any extent by repeating
the ranges of piers, or limited to a single span or to a single
compartment, at pleasure; and in all these cases it has the advantages
of giving all the internal cubic space, and all the height of wall of
which a vaulted area is capable; while, by concentrating the lateral
pressure upon points at convenient intervals, where it may be readily
resisted by external buttresses, it leaves the intervening wall-spaces
at liberty to be pierced by windows, doors, or archways at pleasure.
The Roman builders usually strengthened their vaulting by narrow strips
of brickwork or cut stonework from pier to pier, constructing the rest
of inferior materials. Their groined vaults were similarly fortified at
the lines of intersection; but, as the whole was usually encrusted with
plaster, these constructive expedients had no effect on the appearance.
Sometimes, however, in their barrel vaults (as in the piscina at Baiæ,
mentioned by Professor Willis, and in the corridors of the Colosseum),
we find these strengthening strips appearing as ribs projecting
downwards from the surface of the vaulting, and supported by projecting
piers.
The application of this to _groined_ vaulting is an obvious step, and
adds vastly both to its strength and beauty. Let us suppose a length of
vaulting so divided; we find at once that we are getting into a very
sightly system, and one susceptible of excellent architectural
treatment. Let us then, before proceeding to more advanced or intricate
developments, apply to what we have reached the same process of
architecturalisation which we have gone through for mere arching.
Now, so far as relates to a barrel vault, it is evident that when
divided by transverse ribs, those may be carried by pilasters or by
colonnettes just as the orders of an ordinary arch (Fig. 317); and if
we further mould or otherwise decorate the ribs and continue the
capitals as an impost along the springing line, we have given a very
fair amount of architectural character to the simplest form which
vaulting can assume (Fig. 318).
[Illustration: Fig. 317.]
[Illustration: Fig. 318.]
To pass on to the simplest form of _intersecting_ or _groined_ vaulting,
it is equally clear that columns may be substituted for the square piers
which are its normal supports.
[Illustration: Fig. 319.--Crypt under the Church of the Holy Trinity at
Caen.]
In my theoretical description of this form of vault, I supposed the
springers which are next to the wall to rise directly from its face
(Fig. 316); but in practice it is better that they should rest upon
projecting piers; and it is obvious that for these pilasters or columns
may be substituted. The crypt under the Church of the Holy Trinity at
Caen is a good example of this class of vaulting (Fig. 319). When we
apply the transverse rib to this vaulting, we give it at once a strictly
architectural character, as every compartment is now distinctly defined
(Figs. 320, 321).
[Illustration: Fig. 320.--Crypt under the Choir. View looking West.