La costruzione della spinapesce nella copertura della sala ottagonale di Simon Mago nella fabbrica di San Pietro

'Herring bone' is a procedure of construction, already known to Brunelleschi and to the Sangallo, which enables the dome to be vaulted without the use of reinforcements. This technique was adopted to cover the octagonal rooms surrounding the dome of St. Peter's. Assisted by analytical photogrammetric techniques and computer simulation of the constructive process, the Authors show that the 'herring bone' construction in these domes is achieved with the simple aid of one main support (namely a spar or a cable fixed at the centre of the dome) and without any other guide tracings. The curve thus described by the bricks of the 'herring bone' on the intrados of the dome is a spiral with equal angles (or rhumb line) similar to the spirals found in nature in mollusc shells.