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.
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.