I have been reading Galileo's Two New Sciences recently. Galileo explicitly disproved most of the Aristotelian ideas, except for the one regarding vacuum. He seemed to support the idea that nature abhors a vacuum. Here is the detailed description of the experiment that Galileo demonstrated in his book:
To summarize this experiment, Galileo set up a device that contains nothing but water between the cylinder CAB and the stopper EFGH. A limited amount of weight (K) can be applied to the stopper without having the stopper fallen out of the cylinder. My question is: what is this force that holds the stopper back in place? Is it really the “repugnance which nature exhibits towards a vacuum” as Galileo hypothesized?
Here is the detailed procedure of the experiment if you are interested (from the book):
“Let CABD represent the cross section of a cylinder either of metal or, preferably, of glass, hollow inside and accurately turned. Into this is introduced a perfectly fitting cylinder of wood, represented in cross section by EGHF, and capable of up-and-down motion. Through the middle of this cylinder is bored a hole to receive an iron wire, carrying a hook at the end K, while the upper end of the wire, I, is provided with a conical head. The wooden cylinder is countersunk at the top so as to receive, with a perfect fit, the conical head I of the wire, IK, when pulled down by the end K.
Now insert the wooden cylinder EH in the hollow cylinder AD, so as not to touch the upper end of the latter but to leave free a space of two or three finger-breadths; this space is to be filled with water by holding the vessel with the mouth CD upwards, pushing down on the stopper EH, and at the same time keeping the conical head of the wire, I, away from the hollow portion of the wooden cylinder. The air is thus allowed to escape alongside the iron wire (which does not make a close fit) as soon as one presses down on the wooden stopper. The air having been allowed to escape and the iron wire having been drawn back so that it fits snugly against the conical depression in the wood, invert the vessel, bringing it mouth downwards, and hang on the hook K a vessel which can be filled with sand or any heavy material in quantity sufficient to finally separate the upper surface of the stopper, EF, from the lower surface of the water to which it was attached only by the resistance of the vacuum. Next weigh the stopper and wire together with the attached vessel and its contents; we shall then have the force of the vacuum [forza del vacuo].”
Excerpt From: Galileo Galilei. “Dialogues Concerning Two New Sciences.” Apple Books. https://books.apple.com/us/book/dialogues-concerning-two-new-sciences/id451931970
Best Answer
The force is air pressure. A force equal to the weight of all the air above a horizontal area equal to the cross-sectional area of the piston is pushing in all directions except that one which is at a lower pressure.
There’s nothing implicitly wrong with setting pressure at sea level equal to zero and allowing pressures to range from -1 atm to infinity instead of from 0 to infinity, which is how Galileo has set it up in describing the force of the vacuum. Our way just makes the math easier and the conceptual connection to reality less arbitrary. In much the same way, coincidentally, as heliocentrism isn’t strictly more correct than geocentrism, just conducive of easier math and more logical physics.