Ground plan of the Temple of Apollo, Lake Avernus, after Sangallo's drawing in the Barberini Codex (Rome, Vatican, Biblioteca Apostolica Vaticana, Barb. lat. 4424, fol. 8v).The extant Paper Museum contains a total of 54 copies of architectural drawings from the Barberini Codex. The copies were produced by two draughtsmen, identified in dal Pozzo A.IX as the Codex Ursinianus Copyist and the Sangallo Copyist 2. Virtually all the copies are mechanical 1:1 replicas, probably traced. Annotations and measurements are usually omitted. For further, see A.IX, pp. 479-482. 

The drawing is relatively faithful to the Sangallo original, except for making the composition symmetrical by assuming there were two rectangular chambers flanking each side of the central chamber whereas Sangallo has two on one side and only one on the other, because of exigencies of space. The copyist also slightly enlarges the plan of the octagonal structure in the entrance of the larger ‘temple’, so that their walls are contiguous, rather than having a narrow gap each side as in the original.

Sangallo’s larger plan shows a building octagonal externally and circular inside, representing the so-called Temple of Apollo, a thermal pavilion 37m in diameter, the largest surviving ancient Roman dome after the Pantheon. It appears to have been originally erected during the Augustan period and then remodelled in the later second century AD. Internally, it had four rectangular niches alternating with four semicircular ones, whereas Sangallo shows it with six semicircular and one central rectangular niche facing the entrance. Sangallo notes the presence of eight windows in the dome. He identified it as the Temple of the Cumaean Sibyl (as did the contemporary drawing of Francesco di Giorgio – Florence, Uffizi, 329A), which in reality stood in Cumae proper. Giulio Cesare Capaccio in 1607 (Capaccio Pozzuolo, p. 195) still agreed but Loffredo in 1570 (Loffredo Pozzuoli, fol. 9) had seen it as a Temple of Mercury. Both authors were in the dal Pozzo library (see dal Pozzo A.IX/3, Appendix II).

Recent investigations (M. Pagano and J. Rougetet, ‘Le grandi terme detto “Tempio di Apollo” sul Lago di Averno’, Puteoli XIII 1988–9) have confirmed the presence of structures flanking the main pavilion, although not quite as shown by Sangallo: there was a rectangular chamber on the north side, but not aligned on the axis of the pavilion, while on the south side there was a small octagonal chamber, almost on the same axis as the larger octagon. Francesco di Giorgio’s drawing shows two small octagonal chambers in this vicinity.

It is possible that Sangallo’s plan of the small octagonal structure occupying the entrance is a misplaced record of the southern octagon and not, as S. Borsi argues (Giuliano da Sangallo: i disegni di architettura e dell’antico, 1985, p. 80), part of Sangallo’s project for the palace of the king of Naples, which lies contiguous below. The walls of the palace are represented by single lines, probably indicating it was a proposal rather than something already built, whereas the small octagonal plan is shown with the walls represented by double lines with the space between filled with wash, in the same manner as the plan of the Temple of Apollo. Moreover, other copies of the Sangallo drawing – in the Codex Escurialensis, the 'Codex Coner' and by Giorgio Vasari the Younger – all separate the small octagon from the palace (Madrid, El Escorial, Codex Escurialensis 28-II-12, fol.74; London, Sir John Soane's Museum, 'Codex Coner', no.19b; Florence, Uffizi 4839A). 

If the plan does not represent the small southern octagon, it could represent the thermal pavilion at nearby Baia, known as the Temple of Diana, a domed octagon with a diameter of 29.8m, probably erected by the Emperor Alexander Severus (reg. AD 222–35). Like the Temple of Apollo, it had alternating semicircular and rectangular plan alcoves at ground level and eight windows above, five of which survive. The lower parts of the structure are still buried.

Numbering: 30

Text adapted from Ian Campbell, The Paper Museum of Cassiano dal Pozzo: A Catalogue Raisonné. A.IX: Ancient Roman Topography, London 2004, cat. 179.