The dome itself is constructed in a manner evincing long-continued practice; for it differs toto cælo from the normal mode of construction. It is shown differently by different authors; in fact, it has probably never been sufficiently exposed to obtain complete information as to its curiously complicated construction. As far, however, as I can ascertain, it seems to be in two thicknesses. The inner thickness consists of the framework of the coffers, which is of brick, and the filling-in of the coffers, which is of rubble or concrete. This would form the centering on which the outer shell was built, which is a curious tissue of arches, each rising from the crowns of those below it, and so disposed as to concentrate the pressure upon points in the wall which intervene between the cells. The spaces between these arches are filled in (so far as I can gather) with rubble or concrete. The whole was probably covered externally with plates of bronze or of marble.
The antique Roman dome of the so-called Temple of Minerva Medica—named from the discovery of a statue of that deity among its ruins, but now supposed by some to have been, like the Pantheon itself, the great hall of some public baths—greatly resembles the Pantheon in its general idea, but differs in this essential particular, that its surrounding wall is not circular, but decagonal ([Fig. 402]).
At a later date, as we shall presently see, this peculiarity would have been seized upon as the suggestion of another type of dome, of which I shall have subsequently to treat. As a matter of fact, the transition from the polygonal prism below to the nearly hemispherical dome above, is got over by “rule of thumb,” rather than on any true system. The vertical sides of the wall do intersect the dome in arched forms; but neither are these forms the true sections of a plane with a sphere, nor have they been used as architectural features, as in later times; but have been afterwards, so far as I can judge, obliterated by the incrustation of the dome with plaster, so as to slur over a union of forms which the architect had fallen into accidentally without appreciating its true results.
Fig. 402.—Minerva Medica. Plan and Section.
The dome is surrounded by gradenæ much as in the Pantheon. It is not lighted by an eye, but by ten windows, surrounding what we should call the clerestory. Beneath these are ten arches piercing the surrounding wall; indeed, reducing it to small anglepiers. One of these was devoted to the entrance, the other nine to semicircular recesses, of which five seem to have contained basins for water, and four to have opened by means of colonnades into exedræ or surrounding buildings.[63] It may be mentioned that this form was in after times extensively imitated. The span of this dome is about 80 feet. Its date is not known.
Fig. 403.—Diocletian’s Palace at Spalatro. (From Fergusson.)