[43] 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. [44] Michael Angelo’s model, on a large scale and finished in every detail, is preserved in an apartment of St. Peter’s. [45] Michael Angelo’s remark, quoted by Fontana (_Tempio Vaticano_, vol. 2, p. 315): “Imitando l’antico del Pantheon, e la moderna di Santa Maria del Fiore, corresse i difetti dell’uno, e dell’altro,” shows that he regarded as a defect the lowness of the Pantheon dome, which in point of construction is its capital merit, and that what he proposed to correct in the dome of Florence was its octagonal form, which is essential to its peculiar structural system. [46] A consistent exterior for such a vault would not, of course, be an unbroken drum, though a perfectly Gothic circular vault might be thus enclosed within a drum. A consistent external form would require salient buttresses against the lines of thrust, and the intervals between these buttresses would be open, as in a Gothic apse. [47] The outside of this vault is figured in my _Development and Character of Gothic Architecture_, 2d edition. New York and London. The Macmillan Co., 1900, p. 287. [48] Cf. my _Development and Character of Gothic Architecture_, p. 70 _et seq._ [49] The turrets, built upon the supporting piers of the interior, give the outside of the drum the aspect of a massive lantern. [50] Cf. Poleni, _Memorie Istoriche delle Gran Cupola del Tempio Vaticano, e de’ Danni di essa, e de’ Ristoramenti loro_ (Padua, 1768), p. 29. [51] Milizia, _op. cit._, vol. 2, p. 325. [52] _Parere di tre Mathematici sopra i danni che si sono trovato nella cupola di S. Pietro sul fine dell’Anno MDCCXLII. Dato per Ordine di nostro Signore Papa Benedetto XIV_, Rome, 1742. [53] See Appendix. [54] “Cominciato forsi poco dopo terminata la fabrica.” _Op. cit._, p. 13. [55] _Ibid._, p. 14. [56] _Ibid._, p. 15. [57] _Ibid._, p. 19. [58] _Op. cit._, vol. 2, pp. 327–328.