The other form I have omitted is the square or polygonal dome, or that
generated by the intersection of vaults running parallel to the sides
of the base, instead of, as in the groined vault, running at right
angles to them (Figs. 344, 345).
[Illustration: Fig. 344.]
[Illustration: Fig. 345.]
The _square_ dome is, in fact, the exact correlation or complement to
the square groined vault. Like it, it is generated by the intersection
of two barrel vaults of the diameter of the sides of the square; but the
parts of such vaults which are retained in the one, are precisely those
which are omitted in the other. The angular lines are the same, though
in the one case the angles _project_, and in the other they _recede_;
and while the groined vault is reduced in its bearing to four points in
the corners, the square dome demands for its support the whole line of
the walls, which, however, it reduces in height to the level of the
springing line; while the other allows them to rise in their centres to
the full height of the vault. In some cases, as in the vaulting beneath
the tower of Grantham Church, “Welsh” groins are united with the
polygonal dome, a form quite applicable to the vaulting of an apse (Fig.
346).
[Illustration: Fig. 346.]
There is another peculiar feature in the square or other straight-sided
dome, viz., that it may be cut by vertical planes, as is the case with
the spherical dome. Thus, if we inscribe within the base of a square or
triangular dome, another square or triangle whose corners bisect the
sides of the original base, and erect upon the sides of this
newly-formed figure vertical planes, these will intersect the dome in
arched forms, and the parts left will give a new form of vaulting,
rising from the angles of the figure, and terminating in an unaltered
portion of the original dome. This form was not unfrequently used,
especially in vaulting triangular spaces (Figs. 347, 348).
[Illustration: Figs. 347, 348.]
I have now gone through all the normal varieties (of which I can think)
of the round-arched vault, and it is time that I should allude to a
great step which, after perhaps the first quarter of the twelfth century
was introduced into their construction. I allude to the addition of a
moulded rib beneath their intersecting angles. It is clear that these
angular lines are both in reality and in effect, the weak points of
plain groining. I have mentioned that the Romans fortified them in
construction, by using in them stronger material than in the rest of the
vault; and the early Norman builders made a feeble attempt to take off
from the dulness of the intersection, where it approaches its apex, by
artificially sharpening the edge in plastering it; for, without this, it
becomes (in a vault where the courses of stone or brick are concealed)
almost invisible.
The great step in advance which I have now to mention provided both the
constructive and the artistic strength which the line required.
It is also clear that any irregularity of form may render these lines
shapeless and unpleasing, and it is an obvious gain from an artistic
point of view, to adopt a system which will at once render them
_pronounced_ and _regular_. While, then, the introduction of the angular
rib was in many cases a departure from geometrical accuracy, it was a
vast gain both in strength and beauty.
In that form of vaulting, which I have defined as that with the _raised
ridges_, no geometrical inaccuracy would arise, the angles of
intersection being semicircular, and in vertical planes; but in the more
ordinary form of vaulting, where these lines are elliptical, that curve
being unpleasing, two courses offered themselves for choice: the use of
segments of circles for the diagonal ribs, or the bringing down the
springing to a lower level than that of the vault. In either case the
true geometrical figure has to be departed from, and the error has to be
thrown into the vaulting-surfaces--a course which subsequently became so
thoroughly adopted as a principle, that it may be received as an axiom
that in ribbed vaulting, where the ridges are not raised, the ribs are
made of such forms as will satisfy the eye, and the vaulting surfaces
made to fit themselves to them as best they may, apart from geometrical
accuracy,--a principle which, though it may at first sight offend the
mathematical mind, has proved in practice so wonderfully useful, and to
offer so many facilities, as to be a sort of _Magna Charta_ to the art
of vaulting.
This step once taken, round-arched vaulting seems to have completed its
work. Square and oblong spaces were vaulted either with mathematical
accuracy on the raised-ridge principle, or with deliberate departure
from such accuracy on the level-ridge principle. Irregular spaces were
covered over by expedients which satisfied the eye, and met practical
conditions tolerably well, and many beautiful works were the results.
The diagonal ribs, too, became a new source of decoration, not only by
means of their own mouldings or enrichments, and through the bosses now
sometimes placed at their point of intersection, but also because they
were suggestive of additional colonnettes, and thus added more richness
and intricacy to the piers; and sometimes they were carried upon
sculptured corbels, as in the cathedral at Oxford. Among the richest
specimens of this vaulting may be mentioned the gateway and the
Chapter-house of Bristol Cathedral, the chancel of St. Peter’s Church at
Oxford, etc.
We have now arrived at a stage of our investigation when we must pause
for the sake of asking ourselves what need or requirement yet remained
unsatisfied which was essential to the perfecting of our arcuated
developments, and what means remained--hitherto unused--by which such
need might be met.
We have followed out our arched construction, and the process by which
it was rendered at once susceptible and productive of artistic beauty,
till we might fancy it to need nothing but the gradual additions of
refined art to render it a perfect style; and it would be both an
interesting and a profitable field of speculation to take up the style
at such a point, and to study how best to clothe it with the charms of
the highest art, irrespective of our knowledge of its historical
destiny; how, in fact, to perfect our round-arched style to the highest
and most refined artistic status; and I feel that any one who could
fulfil such a task would be a benefactor to our art.
The semicircle is unquestionably the typical form for an arch, and one
well suited to the majority of purposes and positions. I therefore wish
well to him who will push a style which claims it as its leading element
to its highest possible pitch of perfection. I should rejoice to see a
round-arched style rendered as perfect, and its accompanying art as
noble, as the Greeks did their trabeated architecture and its
ever-glorious sister arts; nor do I see why such an end should not be
attained, and God speed the man who worthily attempts it!
This task was, in fact, nobly though unconsciously approached by the
artists of the twelfth century; nor can any one examine their works,
particularly from the close of the first quarter of that century,
without being filled with the warmest admiration at their determined
strivings after refinement; their earnest aim to perfect every form, and
to eliminate every footstep of barbaric element; to rid their work of
all rudeness of execution; and in every way within their reach to raise
the architecture they were developing into a really high art.
These earnest and restless strivings, however, had the effect of
rendering apparent to them a defect, both structural and artistic, in
the conditions prescribed by a round-arched style. They had freed
themselves from the trammels of the arbitrary rules of proportion, and
might render their structures lofty or the reverse at pleasure: their
columns might be as short and sturdy as the most archaic Doric, or might
outdo the most elegant Corinthian in the lightness of their proportions;
yet the arch they were condemned to carry was limited in height to
one-half of its own diameter; or, if allowed to exceed this, by means of
stilting, this was evidently but a clumsy expedient, and only suited to
particular positions.
The whole tendency, too, of the onward movement of the art was towards
increased height; and, while walls and pillars might avail themselves to
the full of this upward striving, it was hard that the arch--the very
essence of the style--should be condemned to unalterable stuntiness.
Proportion evidently claimed that the arch should have its fair share in
the increasing height of the buildings, yet the inexorable semicircle
said, “Nay, my proportions are fixed. You may lengthen your straight
lines as you please; but by no law of science can my height exceed
one-half of my width.”
A geometrician might reply that the semicircle might be stretched
upwards into a semi-ellipse with its major axis upright. I do not think
that our Mediæval builders ever tried this dismal experiment, nor do I
know that it was ever attempted, except by the barbarous Parthians, in a
building you will find figured in Mr. Fergusson’s _Handbook_; and so
hideous was the result that one may well suppose it to have been handed
down as a warning to subsequent generations!
Nor was this craving after a loftier arch the result of taste alone.
Constructive motives pointed in the same direction; for it was found
that round arches, when carrying great loads, as those sustaining
towers, etc., were apt to overcome the resistance of their piers; and
many failures were the result. The same was found to result from
_vaulting_ over wide spaces. True it is that the Romans, in the great
halls of their baths, had vaulted over spans of double the width of the
naves of our Norman cathedrals; but this had been effected at the
expense of the utility of their aisles, which were cut up into short
lengths by the ponderous abutments needed to sustain the tremendous
pressure of the central vault. Besides which, the Mediæval builders
aimed at raising the springing of these vaulted naves to a height out of
the reach of the abutment of the aisles. An arch of less lateral
pressure was therefore desired.
Another motive might have led to a similar aim. We have seen what
difficulties and contrivances resulted from the exigencies of vaulting
over irregular spaces where it was desirable that the crowns and
springers of the surrounding arches should range on the same levels,
though their spans might differ to any extent. It was clear, then, that
an arch of more elastic proportions was the grand _desideratum_.
The claims, then, of _proportion_, of construction, and of geometrical
convenience, all took the same direction, and demanded an arch of
variable proportions.
This three-fold demand was met by the introduction of the _Pointed
arch_.
To apply this to our main subject of _vaulting_, we at once see that, in
addition to constructive advantages, the arch could now he proportioned
in height to its supporting piers, and the unequal sides of the vaulted
spaces could now be arched in such a manner as to satisfy the
exigencies of the vaulting without the necessity of resorting to awkward
contrivances; so that an accession was obtained at once of strength,
beauty, and facility of application.
I have called the use of diagonal ribs the _Magna Charta_ of the art of
vaulting; but it must share this honour with the Pointed arch. Let us
now proceed to trace the progress of the art under this double charter
of liberty.
The first introduction of the Pointed arch into vaulting seems to have
been made without a full consciousness of its advantages, and rather
with a view to strength and general beauty than to the convenience of
covering irregular spaces, for in many early specimens--as originally in
the Cathedral of Sens,[46] and in the work of William of Sens at
Canterbury,[47]--the round arch continued to be used in the narrow bay
against the walls, while the pointed arch was used for the wider spans.
In nearly all English specimens, however, full advantage was at once
taken of the newly-attained freedom: thus, at St. Joseph’s Chapel, at
Glastonbury,--a work otherwise purely round-arched,--the groining
assumes throughout the pointed form, the narrow bays being especially
acute.[48] The same is the case at St. Cross, another very early
transitional work,[49] and in the nave and transepts of St. David’s
Cathedral[50] (erected about 1180), though the groining was never
carried out, we have the preparations for it with pointed wall-ribs in
the sides, while the round arch is mainly used beneath. I shall,
therefore, disregard this occasional inconsistency.
[Illustration: Fig. 349.