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The basilica of St Peter's in the Vatican was commissioned by Pope Julius II and started by the architect Donato Bramante in 1506. The works were continued by Raphael and San Gallo and in 1547 by Michalengelo Buonarroti who designed an audacious dome with a diameter of 42 meters.
The dome was completed after Michelangelo's death, and was taller than he had intended. The height of the dome is given as 123m externally and 29m internally, the inside dome being a false ceiling similar to that afterwards used at St Paul's in London.
In 1742 Pope Benedict XIV, concerned with the state of the dome of St Peter's, requested three men, Le Seur, Jaquier and Boscowich to carry out a structural survey to determine the causes of distress and to devise remedial measures. The report, published the following year, was prefaced by an apology that said they had assessed it with theoretical mathematical reflection only because the building was unique. Then followed a detailed survey of the dimensions and a discussion on possible explanations for the damage named the yielding of the tie rings at the circumference as the cause. But the interesting part of this report was the second part because an attempt was made to calculate the horizontal thrust and to prove that the two rings built in at the time of erection were no longer able to carry this thrust.
The report caused a furore. One comment stated: 'If it were possible to design and built St Peter's dome without mathematics and especially without the new-fangled mathematics of our time, it will also be possible to restore it without the aid of mathematicians and mathematics … Michelangelo knew no mathematics and yet was able to build the dome … Heaven forbid that the calculation is correct. For, in that case, not a minute would have passed before the entire structure would have collapsed.' Certainly the analysis contained some errors. But in spite of disagreements as to the causes of the damage most people were agreed on the measures to be taken, and in 1743 five additional iron rings were built in the cupola.
The importance of this event was that, contrary to tradition, the assessment of a structure's stability had been based not on empirical rules and opinions but on a detailed survey and mathematical analysis.