The dome of the Pantheon and the dome of St. Sophia, the two greatest domes of former times, had been built on principles that did not admit of much external effect, and the numerous smaller ones of the Middle Ages, in western Europe, had been equally inconspicuous externally, if not entirely hidden from view, in consequence of rising from within a drum which reached far above the springing level. In most cases the whole construction was covered with a timber roof, so that from the outside the existence of a dome would not be suspected. This was a secure mode of construction, and one that for stability could not be improved; but it did not give the imposing external effect that Brunelleschi sought. [Illustration: FIG. 1.--Hagia Theotokos.] [Illustration: FIG. 2.--Aachen.] Attempts to make the dome a conspicuous external feature had indeed been made before Brunelleschi’s time. The later Byzantine builders had raised small domes on drums resting on pendentives, and rising above the main roof of the building, but they had still carried these drums up somewhat above the springing of the dome, and had further fortified them with buttresses built over the supporting piers, as in Hagia Theotokos of Constantinople (Fig. 1). Thus in such designs the dome still remains partly hidden from view, the drum being the most conspicuous part of the composition. Among the early domes of western Europe is that of Aachen (Fig. 2). In this case the drum is carried up far beyond the springing, and is covered with a timber roof which completely hides the dome from external view. The same adjustment of the dome to its drum is, with minor variations of form (the dome being in some cases polygonal on plan, as at Aachen, and in some cases hemispherical) found in most other mediæval domes, and the timber roof over all is likewise common. But in a few cases a different scheme was adopted in which the dome is set on the top of the drum instead of within it. In such cases, however, the drum is low, not rising above the ridge of the timber roof of the nave, and the dome, being unprovided with abutment, is insecure except in so far as it may have a form that is self-sustaining as to thrusts (which removes it from the true dome shape, or may be secured by some kind of binding chain.[7] An example of such a dome occurs on a small scale over the crossing of the cathedral of Pisa (Fig. 3). This dome is not hemispherical, its sides rise steeply, and with such moderate curvature as to render it measurably self-sustaining as to thrust.[8] Another instance of a similar scheme, and on a larger scale, is that which appears to have formed a part of Arnolfo’s design for the cathedral of Florence. This dome was never executed, and our knowledge of it is derived from the well-known fresco in the Spanish chapel of Santa Maria Novella.[9] Here both the dome and the drum are octagonal in conformity with the plan of the part of the building which it covers. The outline (Fig. 4)