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. Pantheon, Rome. (_From Fergusson._)] 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.