From the empirical engineering to the engineering of the science The assessment of three mathematicians for the “San Pietro” dome (1742)

Studies on the mechanical behaviour of domes, based on the static analysis of arches and vaults, started to be developed in the first half of the 18th century, at the end of 1741, when Pope Benedict XIV commissioned three mathematicians Roger Joseph Boscovich, Francois Jacquier and Thomas Le Seur, from the Repubblica Romana dei dotti to carry out an assessment. Serious concerns had arisen over the static conditions of Saint Peter’s vault in Rome where significant cracks could be observed. A number of interesting studies were drawn up on the subject and accompanied by learned and heated debates, which led to the compilation of treatises that ordered the knowledge on the subject. Historical documents also report the opinion of other experts, for example that of the mathematician from Padua, Giovanni Poleni. Despite this, the three previous mathematicians’ study was different from the others that had been carried out prior to this because it contained significant innovation: the assessment was made, perhaps for the first time, using a scientific criterion to interpret the mechanical behaviour of an architectural structure. Its historical importance lies in the scientific theoretical conceptions that were used to analyze a structural problem, which were completely different from the empirical rules, mostly of a geometric nature that had been used in the past. The principle of virtual work was also applied in the assessment. It was used to define the size of the metal rings for the drum of the vault. Some researchers regard this assessment as the time when the change took place from engineering based on artisan traditions to engineering based on the application of the new scientific theories.