The Dome.
Non-existence of the Dome in our old English architecture--Highly
developed forms in France, Germany, and Italy, contemporary with
our great Mediæval edifices--Suggestions for its introduction into
our revived and redeveloped Neo-mediæval style--So-called Tomb of
Agamemnon at Mycenæ--The Pantheon--Temple of Minerva Medica--Torre
dei Schiavi--Temples of Vesta at Rome and Tivoli--Temple of Jupiter
in Diocletian’s Palace, Spalatro--Tomb of St.
Constantia--Baptistery at Nocera--Baptistery at Ravenna--Important
domical development--“Pendentive Domes”--Early
specimens--Pendentive domes the special characteristic of the
Byzantine style--How this originated--Further domical
developments--Cathedral at Florence--Churches of SS. Sergius and
Bacchus, the Apostles, and St. Sophia, Constantinople.
It has been my lot to deliver my lectures from this chair in groups so
very detached from one another, as to render it impossible for my
hearers to follow them as a continuous series. In spite, however, of
this disadvantage, I purpose to make my present lecture form a natural
sequence to the last which I had the pleasure of delivering three years
ago.
My last course was on arched construction, and my last lectures were on
vaulting. My present one will carry on the same subject into its
culminating development--THE DOME.
Strongly as my tendencies towards our own Mediæval architecture draw me
towards the modes of vaulting which prevail in our own ancient
buildings, and which formed the subject of my later lectures, I am bound
to admit that the noblest of all forms by which a space can be covered
is the dome; and, much more than this, that of all architectural forms
it is the most sublime and the most poetic, and is susceptible of, and
demands, the highest artistic treatment. I deplore, therefore, its
non-existence in our old English architecture.
This regret, however, is diminished by the abundant evidence we possess
that the dome, though absent from English buildings, was by no means
held to be alien from the contemporary architecture of neighbouring
countries, inasmuch as we possess it in highly developed forms over a
large part of France, in Germany, and in Italy, erected at the same
periods with many of our great Mediæval edifices.
If, then, I am departing from the line I had been taking in tracing out
the history of old English architecture, I am not only supplying a
hiatus in that history, but I trust that I may be able to offer
suggestions for a more practical object--the supplying of that hiatus in
our revived and redeveloped Neo-mediæval style.
In a former lecture, after defining a vault as the covering of a
rectilinear space produced by the motion of an arch parallel to itself,
I defined a dome as the covering of a circular space produced by the
revolution of an arch round its central vertical axis. It follows that,
if the arch so revolving is semicircular, the resulting dome is a
hemisphere.
The revolving arch may, however, be of any form which an arch can
assume. It may be elliptical, parabolic, hyperbolic, cycloidal; or it
may be a pointed, a horse-shoe, or an ogee arch. Any one of these, or
other forms of arch revolving on its centre, will generate a dome of
its own sectional form. The _plan_, too, in spite of my definition, need
not be a circle; it may be an ellipse, or of other forms.
I will not at this stage admit of square-planned, polygonal, or other
straight-sided domes, because it may be doubted whether they are genuine
domes at all, or whether they are not figures resulting from the
intersection of a certain number of ordinary vaults. Anyhow, these are
not generated by the revolution of an arch, so that if they are domes,
my definition is at fault.
Simple, however, as is the hemispherical dome, it does not appear to
have been historically the primeval type; for, strange to say, the
earliest known domes assume, in section, the form of a lofty pointed
arch. I refer specially to that of the supposed tomb of Agamemnon at
Mycenæ, and also to some portrayed on the Assyrian sculptures.
These would hardly come within the range of this lecture, were it not so
curious a fact that the earliest form of the dome foreshadows the very
use to which I would now especially desire to accommodate it.
It is easy to perceive why the pointed arch was adopted in these
primeval domes. They were not built with radiating joints, but in
overhanging courses; and it will readily be seen on comparing their
sections, that for this mode of construction the pointed arch is much
more favourable than the circular; while, as soon as ever the radiating
system was adopted, the semicircle at once became the simplest and most
obvious section.
It is, however, strange to observe how little has come down to us of
the history of domes. From the so-called tomb of Agamemnon--some eleven
centuries before the Christian era--we have scarcely any certain
evidences of their history till we arrive at the Pantheon, erected in
the reign of Augustus, or, as Mr. Fergusson thinks, considerably later.
Yet, as that dome is still the widest (built of solid materials) which
exists, and as both in its construction and its architecture it is in a
high degree artificial, and evinces a period of advanced development, it
follows that it must be the representative of a long series of
antecedent domes progressing from the crude idea onwards to this, the
very highest developed form of the simple dome; for we shall presently
see that there are other forms scarcely foreshadowed by even this
magnificent structure.
[Illustration: Fig. 400.--Plan, Pantheon, Rome.
(_From Fergusson._)]
I may here remark that the dome, like the arch and the vault, having
great outward pressure, requires either a vast amount of sustaining wall
carried up considerably above the springing level, or, in the absence of
this, a tie of metal at, or somewhat above, its springing, or perhaps
several of such ties at different heights. Unlike, however, the arch and
the vault, it is independent of a _keystone_, each circular course of
its structure forming a horizontal arch and keying itself. It may,
consequently, be erected without the aid of centering, and may be
discontinued at any level, leaving a central opening or _eye_.
The Pantheon is the great type of Roman domes (Fig. 400). It is a simple
rotunda of 142 feet internal diameter, the wall being some 20 feet
thick. The wall is about 72 feet high to the springing of the dome, and
continues above that level about 28 feet more.
The dome is a semicircle, but has an open eye at its apex of nearly 30
feet in diameter (Fig. 401).
[Illustration: Fig. 401.
Half elevation of Exterior. Half section of Interior.