level which it occupies in the Pantheon, to the upper part of the drum, in order to provide a solid foundation resting on the pendentives; and this compelled him to eliminate the attic story of the Pantheon scheme. The most radical change was that of substituting the open colonnade for the solid wall on the outside of the drum. It is doubtful, I think, whether the drum thus lightened would have had enough strength to withstand the enormous thrusts of such a dome.
Like the dome of the Pantheon, Bramante's dome was to be hemispherical and to have an opening at its crown. Over this he was to set the lantern which in outline recalls that of Brunelleschi, though it is of lower proportions, in keeping with the less elevated form of his dome, and has a small hemispherical dome instead of a conical roof. The shape of the lantern accords well with the composition as a whole, and contributes much to the aspiring expression which was now demanded, without wholly contradicting the classical spirit that the architect was striving to maintain.
The structural merit of this scheme lies in what it has derived from the forms and adjustments of the Pantheon. Its weakness consists in the increased elevation, lifting the dome away from its abutment to such an extent that it may be questioned whether it could have been made safe without chains. The thrusts of a hemispherical dome are vastly more powerful than those of a vault of pointed outline, like the dome of Florence, but if properly abutted, as in the Pantheon,[1] it is perfectly safe, and makes a better ceiling than a pointed vault. In reducing the efficiency of his abutment by raising the springing of the dome so high, the architect ought to have diminished the force of its thrust in a corresponding degree by giving it a
- ↑ Some writers have supposed (cf. Middleton, Ancient Rome, Edinburgh, 1885, pp. 338-339) that the dome of the Pantheon is entirely of concrete, and without thrusts. We have no means of knowing its exact internal character, but there is reason to believe that it has some sort of an embedded skeleton of ribs and arches, with concrete filling the intervals. But if it were wholly of concrete, as Middleton affirms, it would not be safe without abutment; for, even supposing that a concrete vault may be entirely free from thrust in a state of integrity, there is always a chance of ruptures arising from unequal settlement, which might at once create powerful thrusts. However this may be, the fact is that the builders of the Pantheon took care to fortify it with enormous abutment, which would seem to show that they did not consider it free from thrust.
