Galileo Galilei's early work on motion is described in his manuscript De Motu, written in Pisa around 1590. A marginal note in this manuscript mentions Borro's (1575) book on the same topic. (Cf. Drabkin, 1960, p. 106, n. 2.) Galileo's manuscript mentions his own experiments with falling bodies, and records surprisingly refined observations. A leaden sphere and an equally large wooden sphere are dropped simultaneously.

"... wood moves more swiftly than lead in the beginning of its motion; but a little later the motion of the lead is so accelerated that it leaves the wood behind it. And if they are both let fall from a high tower, the lead moves far out in front. This is something I have often tested." (Drabkin, 1960, p. 107.)

The higher speed of the heavier object points to the role of the densities of the falling materials, which Galileo at that time took to be the cause of all spontaneous vertical motion. The slower start of the heavier object is explained by the larger upward force which inhabits it in a state of rest; it takes a little while for the downward pull to outweigh this. (A modern explanation was given much later, when Thomas Settle (1983) found that a human experimenter tends to release the lighter object more quickly, because it is gripped with less force than the heavier object.)

Galileo's experiments with falling bodies achieved a legendary status in the history of science. They are prominently mentioned in a sketch of Galileo's life by his disciple Vincenzo Viviani:

"Aristotle taught that mobile objects of the same material and different weight, when moved through the same medium, receive velocities proportional to their weights. Galileo claimed, however, that they had to move with the same velocity, and demonstrated this with many experiments, which he performed from the Clock-Tower in Pisa in the presence of many professors and students."

If Viviani was correct in reporting the presence of professors and students, Galileo's droppings were not merely scientific experiments in the modern sense of that word. They were also demo's, serving a communicative purpose: persuading colleagues and teaching students. The mythical experiments, as Viviani viewed them, were certainly remarkable. In teaching, they replaced abstract description by vivid demonstration. In empirical science they created a perfect peer review system, which allows researchers direct access to each other's observations. And research and teaching were integrated; students and professors were invited at the same time; the university became one scientific community. Finally, the choice of a monumental and visible site for the experiments created a cultural presence in the city at large, encouraging knowledge dissemination beyond the boundaries of the academic scene.

Taking Viviani's account as a point of departure, later biographies of Galileo have added ever-increasing amounts of made-up detail, and exaggerated the scale and the impact of the events. More careful historians have of course noted this, and have suggested that Viviani's original Galileo-bio may already contain fictional elements, since it was written 60 years after these purported happenings, in a spirit of uncritical admiration. Also, they pointed out that Galileo had no need for actual experimentation to dismiss Aristotle's motion laws: already in De Motu he was happy to show their intrinsic absurdity by reasoning about simple "thought experiments". Many maintain therefore that the Pisa droppings never took place. (Cf. Wohlwill, 1905; Cooper, 1935; Segre, 1989; Martinez, 2011.) De degree of detail in Galileo's observations in De Motu, however, suggests that these authors err on the side of skepticism. We concur therefore with Bertoloni Meli (2006, p. 56): "... in all likelihood Galileo repeatedly dropped balls from high places such as the leaning tower of Pisa; lack of additional contemporary records may be attributed to the fact that such public events and disputations were rather common at Pisa and possibly elsewhere."

The leaning tower at Pisa remained a popular site for gravity experiments. Further droppings at this location were reported by Coresio (1612), who claimed that the Aristotelian motion laws were confirmed, and by Renieri (1641). Renieri dropped balls of the same size and different materials (wood and lead), observing that the heavier one arrived first. He also dropped balls of the same material and different sizes, with the larger one first.

Il Coresio dice di aver fatto l'esperienza “di cima al campanile del Duomo di Pisa, sperimentando vero il detto d'Aristotile [...] che ‘l corpo maggiore si muove più velocemente del minore della medesima materia e nel medesimo modo che cresce la gravità, cresce ancora la velocità.” (Caffarelli, 1992, p. 15.)

In una lettera del 13 marzo 1641 Vincenzio Renieri, che dall'anno prima leggeva le matematiche nello Studio di Pisa, fa sapere a Galileo: "habbiamo qui avuto occasione di far um'esperienza di due gravi cadenti da alto di diversa materia, cioè uno di legno et uno di piombo, ma dell'istessa grandezza; perché un tal Gesuita scrive che scendono nello stesso tempo [...]. Ma finalmente habbiamo trovato il fatto in contrario, perché dalla cima del campanile del Duomo tra la palla di piombo e quella di legno vi corrono tre braccia almeno di differenza. Si fecero anche esperienze di due palle di piombo, una della grandezza eguale a un'ordinaria d'artiglieria e l'altra da moschetto, e si vedeva tra la più grossa e la più piccola, dall'altezza dello stesso campanile, esservi un buon palmo di differenza, del quale la più grossa anticipava la più piccola." (Caffarelli, 1992, p. 15.)



