Galileo Galilei's and Simon Stevin's Tower Experiments
One of the best known stories in the history of science is that one day while professor at the University of Pisa, Galileo Galilei went up to the top of the Leaning Tower of Pisa, i.e., the campanile [bell tower], with a crowd below, in order to drop two objects, of the same nature but of significantly different weights, and thereby demonstrated that the two objects would land at the same time. At the base of the tower, there is a plaque in honour of this event.
There is a good chance that the story is apocryphal, as the only documentation supporting the event is in Racconto istorico della vita del Sigr. Galileo Galilei
[Historical Account of the Life of Galileo Galilei1], the biography of Galileo that his student Vincenzo Viviani published in 1654.
At that time, Galileo seems to have learned that inquiring into natural effects necessarily requires a true knowledge of the nature of motion (given that philosophical and commonly used axiom, Ignorato motu ignoratur natura [The ignorance of motion is the ignorance of nature]) and devoted himself fully to the study of it. In so doing, to the utmost bewilderment of all philosophers, Galileo — by way of experiences and rigorous demonstrations and arguments — showed that very many of Aristotle’s statements about motion, which were regarded as perspicacious and beyond doubt, were false. Among them was the proposition that the speeds of movable objects made of the same kind of matter but differing in weight, and moving through the same medium, are in proportion to their weights, as Aristotle taught; rather, they all move with the same speed. He showed this with repeated experiences from the top of the Tower of Pisa, in the presence of the other lecturers and philosophers, as well as of all their students. He also showed it false that the speed of the same movable object through different media is inversely proportional to the resistance or density of the same media, inferring this from the most obvious absurdities that would follow in consequence, contrary to experience itself. [p.15, my emphasis]
Now it turns out that this experiment was actually done, but not in Pisa, nor by Galileo. Rather, Simon Stevin and Jan Cornets de Groot conducted the experiment from the Niewe Kerk [New Church] in Delft, the Netherlands. Here is the relevant passage in the Appendix to Stevin’s De Beghinselen der Weeghconst [The Art of Weighing],2 published in 1586.
The experience against Aristotle is the following: Let us take (as the very learned Mr. Jan Cornets de Groot, most industrious investigator of the secrets of Nature, and myself have done) two spheres of lead, the one ten times larger and heavier than the other, and drop them together from a height of 30 feet on to a board or something on which they give a perceptible sound. Then it will be found that the lighter will not be ten times longer on its way than the heavier, but that they fall together on to the board so simultaneously that their two sounds seem to be one and the same rap. The same is found also to happen in practice with two equally large bodies whose gravities are in the ratio of one to ten; therefore Aristotle's aforesaid proportion is incorrect. [p.519]
But the importance of this experiment, whether it was done by Galileo, by Stevin and de Groot, or even by Giovanni Battista Benedetti, as Alexandre Koyré wrote (see below), has often been exaggerated by historians of science. In his Études Galiléennes,3 Koyré makes the following crucial comment:
Historians of Galileo, and of physics, commonly confuse two very different propositions: 1) that which Galileo established in Pisa by experiments that he never made — and which he did not need to make — … and which in fact were done by Benedetti, according to which bodies of the same nature fall at the same velocity; and 2) that for which the Two New Sciences gives to us, for the very first time, the proof, according to which bodies, whatever their nature, fall at an equal velocity. [p.248, n.4, my translation]
[Unfortunately, the English translation, Galileo Studies,4 does not include Koyré’s footnotes. Translating in this manner the work of a historian of science whose pages were often more covered by footnotes than principal text, was, in my opinion, simply unprofessional.]
In my next post, I will focus on Galileo’s demonstration in the Two New Sciences of the second point, namely, that all bodies, whatever their nature, fall in a vacuum at an equal velocity.
Vincenzo Viviani. Racconto istorico della vita del Sigr. Galileo Galilei / Historical Account of the Life of Galileo Galilei. In Stefano Gattei, ed., The Life of Galileo: Viviani’s Historical Account & Other Early Biographies. Princeton University Press, 2019, pp.1-94.
Simon Stevin, Appendix to the Art of Weighing. In E.J. Dijksterhuis, ed., The Principal Works of Simon Stevin, Volume I: General Introduction, Mechanics. Amsterdam: C.V. Swets & Zeitlinger, 1955, pp.503-529.
Alexandre Koyré. Études galiléennes. Paris: Hermann, 1966.
Alexandre Koyré. Galileo Studies. Translated from the French by John Mepham. Atlantic Highlands, New Jersey: Humanities Press, 1978.