Institute and Museum of History of Science, Florence, ITALY

Evangelista Torricelli

3.3 Defense of the Galileian laws of motion
As heir to Galileo, Torricelli was obliged to confront the criticisms put forward against the work of his master, especially by Descartes and Gilles Personne de Roberval. The science of motion put forward in Galileo's Two New Sciences was not accepted by everybody. The proportionality advanced by Galileo between the space traversed and the square of the time in the motion of free-falling bodies, and the parabolic trajectory of projectiles did not convince the two French scientists. Exasperated by their insistence, and especially by the peremptory affirmations of Roberval, according to whom the conclusions advanced by Galileo did not survive experimental tests, Torricelli decided to cut the discussion short. 
Drawing from the letter of
September 1647 to Ranieri
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lettera.gif (10541 byte)

Fragment of Torricelli's letter
on his works on the
Archimedean spiral written to
Michelangelo Ricci on
January 17 1645. It is
published in Opere di
Evangelista Torricelli
, edited
by G. Loria and G. Vassura,
Faenza 1919, p. 257

He remarked to Roberval, in a letter of 7 July 1646, that in one of his works Archimedes had compared the trajectories of projectiles to spirals. When the error was evident, was it necessary to condemn the book? Was it not preferable to read the whole book without any reference to projectiles, by simply adding the word "point", the motion of which follows not a natural law but an imaginary one? In reality, Torricelli explained, geometrical demonstrations do not require assistance - they are self-sufficient. He concludes by suggesting that the words "projectiles", "heavy bodies", "ballistics", and so on, which belong to physics, should be removed from the book of Archimedes, leaving only the abstract propositions which belong to geometry. The provocative tone of the letter to Roberval should not make us forget that in reality Torricelli believed in the validity of the laws of natural motion established by Galileo. In this sense, the content of the letters he exchanged with Giovan Battista Renieri provides significant testimony. His attempt to take refuge behind the abstract aspect of his work on the theory of motion was plausibly a way of avoiding polemics. "Many times, to avoid controversies", Torricelli wrote to Renieri in September 1647, "I have deliberately protested repeatedly and clearly many times in my books on motion that I write for philosophers rather than bombardiers". But this observation does not prevent one from giving an interpretation of the trajectories of projectiles as truly observed, and providing numerical values which are patiently calculated.


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