The dome, as viewed externally, is buried by the wall which rises above
its springing to fully a third of its height, and above this rises a
sort of attic crowned by six gradenæ, burying nearly an equal height,
so that the dome as an external feature is far from conspicuous,
appearing as a mere flattened disc.
Internally, however, it forms a covering of the noblest character. Its
internal surface is deeply coffered by panels of four orders, in depth
dividing the circumference into twenty-eight parts, and its height, up
to about two-thirds of the distance from the springing to the crown,
into five parts, the upper portion being plain. These vast panels or
coffers, the larger range of which exceed twelve feet in diameter, are
curiously arranged as to their sectional recessing, so as to appear
perfectly symmetrical to the eye of a spectator standing beneath the
centre of the dome.
The wall, up to the springing of the dome, is beautifully decorated with
rich architecture in marbles of varied colour, and it cannot be doubted
that the cupola--the very soul of the design--was embellished in a
manner fully proportioned to the beauty of its sustaining wall; indeed,
it is thought to have been coated with gilded bronze.[62] When thus
perfect, it must have formed an interior of surpassing beauty, lighted
as it was solely through the central eye, and the light tempered by the
linen veil stretched across its rich bronze cornice, which still remains
round the opening, and retains vestiges of gilding. The Pantheon can
scarcely be called a _daring_ effort of construction, because its vast
solidity seems to defy all doubts as to its duration. It would be,
however, absurd to suppose it to have been an _early_ effort; for it is,
as I have already said, so artificial in its construction, as clearly
to prove it to be the result of long-continued practice.
The walls, which I have described as being 20 feet thick, are so only in
theory, for practically they are hollowed into innumerable cells, some
of them forming beautiful architectural recesses, and others merely
constructional hollows.
The dome itself is constructed in a manner evincing long-continued
practice; for it differs _toto cælo_ from the normal mode of
construction. It is shown differently by different authors; in fact, it
has probably never been sufficiently exposed to obtain complete
information as to its curiously complicated construction. As far,
however, as I can ascertain, it seems to be in two thicknesses. The
inner thickness consists of the framework of the coffers, which is of
brick, and the filling-in of the coffers, which is of rubble or
concrete. This would form the centering on which the outer shell was
built, which is a curious tissue of arches, each rising from the crowns
of those below it, and so disposed as to concentrate the pressure upon
points in the wall which intervene between the cells. The spaces between
these arches are filled in (so far as I can gather) with rubble or
concrete. The whole was probably covered externally with plates of
bronze or of marble.
The antique Roman dome of the so-called Temple of Minerva Medica--named
from the discovery of a statue of that deity among its ruins, but now
supposed by some to have been, like the Pantheon itself, the great hall
of some public baths--greatly resembles the Pantheon in its general
idea, but differs in this essential particular, that its surrounding
wall is not circular, but decagonal (Fig. 402).
At a later date, as we shall presently see, this peculiarity would have
been seized upon as the suggestion of another type of dome, of which I
shall have subsequently to treat. As a matter of fact, the transition
from the polygonal prism below to the nearly hemispherical dome above,
is got over by “rule of thumb,” rather than on any true system. The
vertical sides of the wall do intersect the dome in arched forms; but
neither are these forms the true sections of a plane with a sphere, nor
have they been used as architectural features, as in later times; but
have been afterwards, so far as I can judge, obliterated by the
incrustation of the dome with plaster, so as to slur over a union of
forms which the architect had fallen into accidentally without
appreciating its true results.
[Illustration: Fig. 402.--Minerva Medica. Plan and Section.]
The dome is surrounded by gradenæ much as in the Pantheon. It is not
lighted by an eye, but by ten windows, surrounding what we should call
the clerestory. Beneath these are ten arches piercing the surrounding
wall; indeed, reducing it to small anglepiers. One of these was devoted
to the entrance, the other nine to semicircular recesses, of which five
seem to have contained basins for water, and four to have opened by
means of colonnades into exedræ or surrounding buildings.[63] It may be
mentioned that this form was in after times extensively imitated. The
span of this dome is about 80 feet. Its date is not known.
[Illustration: Fig. 403.--Diocletian’s Palace at Spalatro. (_From
Fergusson._)