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Note: The Leaning Tower
Tag(s): Galileo, GalileoPIsa, LeaningTower
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In the autumn of 1589 Galileo became a professor of mathematics at the University of Pisa. He had taken the first step on the road to a professional career and to financial security. But there should be no illusions about his situation: his annual salary was sixty florins, significantly less than the income of a good stonemason. 1 It was not possible to live as a gentleman on such an income. There were professors of philosophy in prestigious institutions who were earning fifteen times as much; Galileo's own friend Jacopo Mazzoni was earning seven hundred florins at Pisa. 2 And this was not just because Galileo was starting out; it was also because mathematics was not a prestigious subject.

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 929-935). Yale University Press. Kindle Edition. 
As far as other professors were concerned it had two main functions: to introduce philosophy students to basic geometry, and thus to the idea of a demonstration; and to provide medical students with the skills required to make astrological charts. Mathematics was regarded as intellectually uninteresting, a purely technical training, and the salary of mathematicians reflected this.

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 935-938). Yale University Press. Kindle Edition. 

appointment. Most of the professors, his new colleagues, had been teaching when he was a student. He would have had old friends, as well as relatives, to make him feel at home. And it is likely that he found himself picking up on intellectual discussions that had interested him as a student. Pisan intellectual life was dominated by two philosophers: Girolamo Borro, a follower of Averroes (1126– 98), the great Arab commentator on Aristotle, famous for denying the immortality of the soul; and Francesco Buonamici, who followed the Greek commentators (the one thing on which the two were supposed to be agreed was the foolishness of revealed religion).

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 939-943). Yale University Press. Kindle Edition. 
In 1575 Borro had published a book on the movement of heavy and light bodies: Galileo owned a copy. 4 In 1589 Buonamici had on his desk an enormous manuscript on the same subject; although the book (over a thousand large pages) did not appear in print until 1591, Galileo had quite probably attended Buonamici's lectures as a student, so the book's main arguments would have been familiar to him even before he could read it.

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 944-947). Yale University Press. Kindle Edition. 

part. First, Aristotelians drew a sharp distinction between natural and forced movement. If a ball is thrown, that is a forced movement; if dropped, it moves to its natural place of rest, and this is a natural movement. If it is thrown upwards, the ball decelerates as it rises, and this is because forced movements tend to peter out; if it is dropped, it accelerates, and this is because natural movements tend to speed up. If a ball is thrown, there is an initial forced movement, and then the ball drops downwards. Aristotelians believed that only one type of movement could take place at a time, so they were committed to the view that the ball travelled in a straight line, and then dropped vertically.
Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 953-957). Yale University Press. Kindle Edition. 
They also held the view that any object subjected to a forced movement must be being moved by another object (or, as in the case of the hand, by the will of a sentient creature).

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 958-959). Yale University Press. Kindle Edition. 
Later theorists argued that the ball had acquired some sort of internal energy or impressed force, just as it might acquire heat from the hand and stay hot even after it had been let go. Galileo would have found an account of impressed force in the textbook of the Jesuit philosopher Benedict Pereira, which he used while working on his first study of falling bodies,

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 961-963). Yale University Press. Kindle Edition. 

Aristotelians believed, however, that the universe consisted not only of weight, but also of lightness. If balls naturally move downwards, flames naturally move upwards. Borro and Buonamici were thus debating the natural movement of bodies downwards and upwards. Both movements, they held, continued until the body was obstructed or reached its natural place of rest: in the case of a heavy body this would be the centre of the earth; in the case of a light body it would be an invisible frontier between the earth and the moon – beyond this frontier the only natural movement was circular movement, and there was no coming into being or going out of existence. In the heavens what looked like change was simply, if one waited long enough, repetition.

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 966-971). Yale University Press. Kindle Edition. 
The standard view was that weight was to be understood in the normal sense: a two-pound lead weight would fall twice as fast as a one-pound lead weight. An alternative view was defended by Buonamici: heaviness should be understood as density or specific gravity so that lead weights of different sizes would all fall at the same speed, and they would all fall faster than wooden balls. Borro held a third view. He thought that weight was situationally specific, and related to the composition of the material. Aristotelians held that all earthly objects are made from four elements: earth and water, which are heavy; fire which is light; and air, which can be either heavy or light, depending on its location.

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 973-978). Yale University Press. Kindle Edition. 
Borro had put this theory to the test. He had gathered a group of philosophers at his house and they had tossed comparable lead weights and chunks of wood out of an upstairs window. The wood had consistently reached the ground ahead of the lead. 7 Buonamici was not persuaded: you would first need to weigh the objects and make sure that their weight was identical. There was also an obvious problem: the shorter the drop the harder it was to see what was going on. The solution was to drop objects from a greater height, and it is known that by 1612 Giorgio Coresio had tried dropping objects of different weights but the same material from the top of the Leaning Tower of Pisa. The result, he claimed, vindicated Aristotle, for speed of fall proved proportional to weight. 8

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 981-987). Yale University Press. Kindle Edition. 
According to Viviani, Galileo, when teaching at Pisa (thus between 1589 and 1592), had already carried out this test of dropping objects of different weights but the same material from the top of the Leaning Tower, and had shown that they reached the bottom simultaneously. 9 This is like Viviani's story of the pendulum: he wants to present Galileo as an experimental scientist, and to claim that he was so precocious that he founded modern physics almost effortlessly. But in this case there is a manuscript on natural motion, apparently written in the period 1589– 92, in which Galileo confidently claims that he knows what will happen in such a test: at first, he tells us, the lighter object moves faster, but then it is overtaken by the heavier object, which reaches the bottom ahead of it. He has, he tells us, repeated this experiment many times. 10

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 988-994). Yale University Press. Kindle Edition. 

Wootton, David (2010-10-26). Galileo: Watcher of the Skies (Kindle Locations 987-988). Yale University Press. Kindle Edition.