There are other domes not differing materially from those already
described, but which it would extend my lecture unduly to dwell upon.
One called the Torre dei Schiavi, in the Via Prænestina, is rather like
the Pantheon on a very small scale, though lighted by round clerestory
windows instead of a central eye. The Temples of Vesta, both at Rome and
at Tivoli, consist of circular walls surrounded externally by a
peristyle. The cell of each is supposed to have been covered by a dome,
though roofed over. A parallel case, but in a more complex form, exists
in what is called the Temple of Jupiter in Diocletian’s Palace at
Spalatro (Fig. 403). In this case the exterior of the cell, with its
peristyle, is octagonal, but the interior, with the dome, _round_. The
latter has a complicated construction of fan-shaped arches throughout,
scarcely any part being constructed of horizontal curves. Next, perhaps,
in date, yet at once displaying similarity of idea with a significant
change in the carrying out, is the tomb of St. Constantia, the daughter
of Constantine (Fig. 404).
[Illustration: Fig. 404.--Tomb of St. Constantia, Rome. (_From
Fergusson._)]
A Christian church, in its early form, has been familiarly described as
the pagan temple turned inside out. To convert the ideal temple into the
ideal church, the wall and the colonnade must change places. So
completely is this the case that some of the earlier commentators on
Vitruvius were completely puzzled between the wall of the temple and the
peristyle. They assumed that the latter _must_ be _within_ the wall, as
in their own churches, and based their remarks on this supposition.
The comparison between these almost contemporary works, the Temple of
Jupiter in Diocletian’s Palace at Spalatro, and the tomb of St.
Constantia at Rome, exactly illustrates this change. In the one the
solid wall forms the circle and carries the dome, and the colonnade is
external (See Fig. 403), in the other the colonnade forms the inner
circle and carries the dome, while the wall becomes external, an _aisle_
taking the place of the peristyle (See Fig. 404). The colonnade is
_doubled_ to support the massive clerestory whence the dome springs, and
the whole assumes the type of one form of Christian church, which
henceforth became of frequent occurrence. The dome, in this instance,
ceases to be an external feature, being covered over by a conical roof.
I may add that the peristyle is repeated in the old manner as an
additional feature beyond the wall of the aisle. The baptistery at
Nocera is similar in distribution to the tomb of St. Constantia, and may
be of similar date. It is ruder, however, and loses much beauty by the
omission of the clerestory, and the admission of light through the
haunches of the dome. Among the innumerable remains of domes of the
older type, I will only mention one more, before proceeding to the
second branch of my subject, to which it, in fact, properly belongs.
That to which I allude is the baptistery at Ravenna, erected, as it is
supposed, about the year 450 (Fig. 405).
This is a very charming building, octagonal in form, yet covered over
by a hemispherical dome. Though having no surrounding aisle, the design
of its sides seems derived from the aisle and clerestory: indeed, it has
a clerestory, though the arcade below is rather rudimentary than real.
The dome, like that last alluded to, is covered externally by a sloping
roof.
The special feature, however, in this dome is that it rests upon an
octagonal wall, or rather upon eight arches.
We shall presently see how this was effected in subsequent times, and I
will not anticipate that subject, but will content myself with
mentioning that this seems to anticipate the Byzantine domes of the
succeeding century, as had been the case in two other instances to which
I shall have to refer, and as had been _nearly_ the case in the Temple
of Minerva Medica.
[Illustration: Fig. 405.--Baptistery at Ravenna. Plan and Section.]
The domes which we have hitherto considered are exclusively and of
necessity carried by circular or other continuous walls. They are
consequently supported uniformly throughout their entire circumference,
and their use is necessarily limited to the coverings of circular or
_quasi_-circular or polygonal buildings. Had no further development been
attained, it would ever have been felt to be a sad deficiency in the
scope of architectural facilities that the noblest form of covering
should be limited to the least usual and, for most purposes, the least
convenient form of apartment. We are happily _as far as possible_ from
being left in this dilemma. A very simple application of geometrical
thought opened a way by which almost _any_ reasonable form of building
may be covered by a dome, or by a series or group of domes.
I will endeavour, as simply as I am able, to explain this important
development.
It is a property of the sphere that every possible plane section of it
is a _circle_. It follows that every vertical section of a hemispherical
or segmental dome assumes the form of a semicircular or segmental arch.
If, therefore, a square be inscribed in the base of a dome, and walls be
built on that square, and continued up till they meet the dome, they
will intersect with it in four semicircles (Fig. 406). If, instead of
_walls_, you build _arches_ on the sides of that square, these arches
will coincide with the curve of the dome where they meet it, and, if
strong enough, will carry the portion of the dome remaining between
them. If, again, instead of arches, you suppose the dome intersected on
the lines of the inscribed square by _vaults_ at right angles to those
sides, the result will be the same.
[Illustration: Fig. 406.]
In the first case we have a dome, or a portion of one, covering a square
apartment; in the second we have the same covering standing on arches
open towards the exterior; in the third, we have a dome covering the
intersection of two barrel-vaults, just as is more usually done by
groining.
The process, however, is not limited to a _square_; it is equally
applicable to the octagon or any other polygon--indeed, to any figure
which can be inscribed in a circle.
The following diagrams (Figs. 407, 408, and 409) will tend better to
explain this.
Nor is it necessary that the inscribed figure should be _complete_, for
remnants of the circle may equally well be left between the arches or
walls.
[Illustration: Fig. 407.]
[Illustration: Fig. 408.]
Thus, a circular space may be intersected by four vaults of less width
than the sides of a square (Fig. 410), leaving portions of the circular
walls remaining between them.
The dome, again, may as well be _segmental_ in section as
_semicircular_, in which case the arches supporting it will also be
segmental (Fig. 411). Again, the figure inscribed need not be
_equilateral_, so that _oblong_ compartments, such as those customary in
the nave of a church, may be domically vaulted.
In all the cases which I have enumerated, I have supposed the result to
be literally a _portion of the original dome_. As it happens, however,
we have but few ancient examples of so strict an adherence to principle;
though in modern works they are more frequent. The purest specimen I
know (if I judge rightly from drawings) is the dome of the little church
of SS. Nasario and Celso, at Ravenna, the Mausoleum of Galla Placidia,
and of her two husbands, the Emperors Honorius and Valentinian II. This
is a dome such as I described as standing between four walls, which
intersect it in the form of arches.
[Illustration: Fig. 409.]
[Illustration: Fig. 410.]
[Illustration: Fig. 411.]
The next step beyond this was a very important one as to beauty of
effect. I refer to the practice of drawing the circle by means of a
moulding on the surface of the dome, touching the crowns of the arches.
This is not only ornamental, but it has the effect of emphasising the
first completed course of stones, and perhaps even of strengthening it,
and it has the further effect of defining the spherical triangles
between the arches which, when thus gifted with a separate existence,
receive the name of “_pendentives_,” whence this entire class of domes
are called “_pendentive_ domes.” The earliest specimen of this is
probably the little tomb in the Via Nomentana near Rome (Fig. 412),
which, though probably of earlier date than the church of SS. Nasario
and Celso, carries out the pendentive principle to its full development,
just as we see it treated at a later period in the double gate (Fig.
413) and the golden gate of the Temple Area at Jerusalem, most probably
erected under Justinian. All these domes are segmental in section.
[Illustration: Fig. 412. Tomb in the Via Nomentana, Rome.]
[Illustration: Fig. 413.--Double Gate, Temple Area at Jerusalem.]
The pure form, however, of a pendentive dome--that is to say, the form
in which the pendentives and the upper portion are really veritable
parts of one and the same original dome, in the plan or base of which
the rectilinear figure is inscribed--was not long adhered to. It was
soon felt that the disc enclosed by the circular moulding looked _flat_
and _ineffective_; and the idea early suggested itself of converting the
circular moulding into a massive cornice, raising upon it a new dome of
such proportions as should approve themselves to the eye, and allowing
nought but the pendentives to remain of the original dome (Figs. 414,
415).
[Illustration: Fig. 414.]
[Illustration: Fig. 415.]
No bolder idea was ever introduced in constructive architecture; for now
the dome, instead of being, as at the Pantheon, supported firmly by a
solid wall throughout _every_ portion of its circumference, finds its
conditions _absolutely reversed_; for in _no_ portion of its
circumference has it now a solid support, but all floats upon vacuity,
suggesting the poetical similitude to Procopius that the Dome of St.
Sophia appeared as if suspended by a chain from heaven.
Pendentive domes, in neither of their typical forms, seem to have been
frequently or customarily made use of in the more genuine Classic ages,
though in modern times they have both been very wisely adopted into the
revived Classic styles. They were, in fact, the special characteristic
and the great glory of the Byzantine style.
Mr. Freeman, on this subject, remarks:--“The offspring of the arch is
the vault, of the vault the cupola; and this majestic ornament is the
very life and soul of Byzantine architecture, to which every other
feature is subordinate. Its use had hitherto been mainly confined to
circular buildings. To make it the central point of a Christian temple
was a grand and bold idea, and one which involved a complete revolution
in the existing principles of architecture.... And not only did the
grand cupola crown the whole pile, but the smaller portions are often
covered with smaller domes and semi-domes.... The eye, habituated to the
long naves ... of our own great churches, is totally bewildered with so
huge a pile, with apses and semi-domes ‘sprouting out,’ to use the
expression of Mr. Hope, in every direction, and all circling round the
vast central cupola, like tributary rulers encircling an imperial
throne.”
It is thought by some that the Byzantines borrowed the pendentive dome
from Persia, but this seems insusceptible of proof; indeed, it exists of
earlier age in Italy. It is more certain that having once discovered its
wonderful utility, it was communicated by them to every region to which
their influence extended, and that, having been learned from them by the
Mahometans, it became the conspicuous feature of the architecture which
extended continuously from the Bay of Bengal to the Atlantic.
The next development I will mention is the raising of the dome proper
upon a drum or circular wall, elevated upon the pendentives or corbels,
so as to convert it into a species of tower. This seems to have been the
first step by which, in later times, the dome came to be made a
conspicuous external feature, though rather at the sacrifice of internal
beauty (Figs. 416, 417).
[Illustration: Fig. 416.]
[Illustration: Fig. 417.]
It is, in fact, the weakest point in the dome, æsthetically considered,
that the same dome cannot be made artistically perfect both within and
without. If its height be limited to what looks thoroughly well from
within, it is so low in its external aspect as to have little artistic
value; while, if raised so high as to be an important external feature,
it is only seen by a painful effort from within. This is manifest even
in the rotunda, where the dome rises from a circular wall, as in the
Pantheon and the Temple of Minerva Medica; but it becomes much more so
in a pendentive dome, where the angles are externally encumbered with
large masses of masonry. In the earlier Byzantine buildings we
accordingly find the dome to have been viewed almost solely as an
internal feature, and its exterior very much neglected, and in the case
of St. Sophia itself no one would be prepared by its low, heavy,
external aspect for the unrivalled glories of its interior. Many of the
old architects, in fact, gave up the external form altogether, covering
over the dome, as at Parma, etc., by an ordinary sloping roof.
The change I have last chronicled,--the interposition of a circular wall
between the pendentives and the dome,--though by no means in all cases
leading to the result I am referring to, was unquestionably the origin
of the treatment of the dome as an important external feature. It was,
in fact, the elevation of the rotunda upon the top of the pendentives.
Unhappily, however, it had at once the effect of lifting up the dome
above the level favourable to its internal effect, while, if erected on
four arches only, the weight became so serious as usually to limit its
use to domes of very moderate sizes.
[Illustration: Fig. 418.]
[Illustration: Fig. 419.]
A large number of domes thus raised high above their pendentives or
corbellings are really of a class whose claims to the name of dome are
somewhat ambiguous. I refer to those whose horizontal section is not
circular, but polygonal (Figs. 418, 419). Strictly speaking, this is a
variety of groined vault; it is generated by the intersection of several
barrel-vaults, springing from the horizontal tops of the surrounding
walls. Now, my definition of a dome was a vault generated by the
revolution of an arch on its vertical axis. If this were an exhaustive
definition, it would follow that the vaults under consideration are no
domes at all; yet they look so much like them, and as the number of the
sides of the polygon increases, actually approach so closely to the
genuine dome, that it would be affectation to deny them the name. They
may form the covering of any rectilinear figure at all approaching to
regularity of form; as the triangle, the square, the canted square,
polygons, either regular or elongated, oblongs or parallelograms of any
kind; but the usual form is the octagon or other polygon, and for our
general purpose it may be best to limit them to figures capable of being
inscribed in a circle or an ellipse.
I have introduced this variety of dome as occurring in those which are
raised high above their pendentives or corbels. They occur, however, in
numerous positions. The greatest I know of is that of the Cathedral at
Florence, of which I shall have to speak more in detail in my next
lecture.
[Illustration: Fig. 420. Plan, SS. Sergius and Bacchus. (_From
Fergusson._)]
[Illustration: Fig. 421. Section, SS. Sergius and Bacchus,
Constantinople. (_From Fergusson._)]
But to return to the Pendentive Dome. The Pendentive Dome, though
occasionally used at an earlier period, established itself as the
leading feature of a style about the time of the Emperor Justinian, and
its central seat was Constantinople. The earliest, or _about_ the
earliest, church now remaining in that city, seems to be that of SS.
Sergius and Bacchus (Figs. 420, 421). Its dome is supported by an
octagon. It appears itself to be conchoidal in its horizontal section,
and to be supported on sixteen small pendentives. It bears considerable
resemblance to the Temple of Minerva Medica, but is really less
developed than the baptistery at Ravenna, which dates full half a
century earlier.
Contemporary with this was the Church of the Apostles, also erected by
Justinian, but now destroyed. It showed, however, according to the
description of it by Procopius, an immense advance upon that last named;
for, while in one the dome was carried by an octagonal wall, thus
showing no practical advance upon the antique form of dome, in the
latter a vast cruciform building was covered by five domes, which is
just the advantage which the pendentive system affords; for, when the
base of a dome is cut into a square by four arches, those arches may aid
in the support of other domes beyond, and thus any space may be covered
over by a series or a group of domes. This last church, then, was the
true type of advanced domical structure.
The great glory, however, of this age, and of domical structure of this
class (for it has never again been equalled), is the Church of St.
Sophia, or of Sacred Wisdom, erected by the same Emperor as the
Metropolitan or Patriarchal Cathedral of the Eastern Church. The plan of
this church differs in ideal, and yet more in fact, from the
contemporary church last referred to.
The ideal of each is a cross with a central dome. The difference is that
in the Church of the Apostles the limbs of the cross were each covered
by a complete dome of equal dimensions with the central one (Fig. 422),
while those of St. Sophia are covered each by a semi-dome only of equal
diameter with the arches carrying the central dome (Fig. 423); so that
if we consider the latter to spring from the top of its pendentives,
which it in effect does, its springing is on a level with the crown of
the surrounding semi-domes.
[Illustration: Fig. 422. Church of the Apostles.]
[Illustration: Fig. 423. St. Sophia.]
In reality, however, this idea is not carried out to completion, as only
two of the semi-domes have been erected, the other two arches of the
central dome being filled in with an arcade in many storeys. This
incompleteness, however, is greatly more than compensated--firstly, by
the vastness of the scale--the central dome, if measured on the
diagonal, being 150 feet in diameter--and, secondly, by other
semi-domical projections branching out from the walls which support the
great semi-domes, three from each, excepting that on the western side,
one is devoted to the entrance, and is not domed. Even these secondary
projections are mostly arcaded, so as to allow the eye to pass onward
into a yet inner chamber (Fig. 424). So that, simple as is the primary
ideal, the actual effect is one of great intricacy and of continuous
gradation of parts, from the arcades last alluded to, up to the
stupendous dome which hangs with little apparent support, like a vast
bubble, over the centre, or, as Procopius, who witnessed its erection,
described it, “as if suspended by a chain from heaven.”
[Illustration: Fig. 424.--Plan, St. Sophia, Constantinople. (_From
Fergusson._)]
The dome is lighted by forty small windows, which pierce it immediately
above the cornice which crowns its pendentives, and which, by
subdividing its lower part into narrow piers, increases the feeling of
its being supported by its own buoyancy.
The interior thus generated, covered almost wholly by domes, or portions
of them, each rising in succession higher and higher towards this
floating hemisphere in the centre, and so arranged that one shall open
out the view towards the others, and that nearly the entire system of
vaulting may be viewed at a single glance, appears to me to be, in some
respects, the noblest which has ever been designed, as it was certainly
the most daring which, up to that time at least, if not absolutely, had
ever been constructed.
Its beauties are of a contrary kind to those of that noblest interior of
antiquity--the Great Hall of Karnac--or to those of later ages--the
Gothic cathedrals. Both of these gain beauty of effect and an increase
of apparent extent through the endless intricacies of their perspective,
and the changes of aspect at every step arising from the multitude of
their columns, and from no possible view showing the whole interior at
once. This, on the contrary, trusts to the very reverse of all this--the
absence of interruptions and the studious distribution of parts, so that
no one conceals another, but that the entire building shall be grasped
at once by the eye (Fig. 425).
I have not seen St. Sophia’s, though I long to do so, if only to view a
form of artistic treatment so different from what I delight in in our
own cathedrals. The internal effect does not, however, trust exclusively
to this panoptical theory. The contrary theory was too well known from
the Christian basilicæ to be lost sight of in this, the greatest of
Christian
[Illustration: Fig. 425.--Longitudinal Section, St. Sophia,
Constantinople. (_From Fergusson._)]
temples. It was, in point of fact, _added_ to the other by means of
arcades, both in the sides of the nave and in its apsidal projections,
opening out mysterious perspectives into the inner recesses of the
temple. This union of the more palpable with the more mysterious, of the
vast unbroken expanse with the intricately broken perspective, must, as
it appears to me, and as I judge from representations, produce an
impression more astounding than that of almost any other building; but,
when we consider the whole as clothed with the richest beauties of
surface; its piers encrusted with inlaid marbles of every hue, its
arcades of marble gorgeously carved, its domes and vaultings resplendent
with gold mosaic interspersed with solemn figures, and its
wide-spreading floors, rich with marble tesselation, over which the
buoyant dome floats, self-supported, and seems to sail over you as you
move--I cannot conceive of anything more astonishing, more solemn, and
more magnificent. Well might its Imperial founder exclaim, when, with
pardonable exultation, he viewed the result of his costly
aspirations--“Glory be to God, who hath thought me worthy to accomplish
so great a work; _I have vanquished thee, O Solomon_!”
I have dwelt longer on my description of this wonderful building because
it is _facile princeps_ among structures on the pendentive domical
principle, just as the Pantheon had been among those with the simple
dome; and as, in after ages, was St. Peter’s among those whose domes
soared upwards as lofty towers.
I must here close my lecture, leaving the continuation of my
descriptive sketch of the history of the Cupola, and such remarks as I
may have to offer on its uses, its practical application, and its future
development, to be followed up in my next.