Discourse on Floating Bodies

by Galileo Galilei, Thomas Salusbury (Translator)

This book was converted from its physical edition to the digital format by a community of volunteers. You may find it for free on the web. Purchase of the Kindle edition includes wireless delivery.

Genres: Science, Nonfiction

82 pages, Kindle Edition

First published January 1, 1612

About the author

Galileo Galilei was a Tuscan (Italian) physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism. Galileo has been called the "father of modern observational astronomy", the "father of modern physics", the "father of science", and "the Father of Modern Science." The motion of uniformly accelerated objects, taught in nearly all high school and introductory college physics courses, was studied by Galileo as the subject of kinematics. His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter, named the Galilean moons in his honour, and the observation and analysis of sunspots. Galileo also worked in applied science and technology, improving compass design.

Galileo's championing of Copernicanism was controversial within his lifetime. The geocentric view had been dominant since the time of Aristotle, and the controversy engendered by Galileo's presentation of heliocentrism as proven fact resulted in the Catholic Church's prohibiting its advocacy as empirically proven fact, because it was not empirically proven at the time and was contrary to the literal meaning of Scripture. Galileo was eventually forced to recant his heliocentrism and spent the last years of his life under house arrest on orders of the Roman Inquisition.

Reviews

[Ed Erwin · Rating 4 out of 5]

This work seems shockingly underappreciated! I am not surprised to see no reviews on GoodReads. But even in other places, this isn't given as much consideration as Galileo's work with telescopes and pendulums and falling bodies.

That is sad, because it is important in the history of science. This little book concerns a dispute that turned philosophers against Galileo. Not only did he say they were wrong about something, he did it in a popular little book in common Italian, made for the general public, showing that anyone could do simple experiments themselves to verify his points. The philosophers got mad. Making enemies of philosophers is one of the big reasons why the church eventually turned on him.

This is basically a description of experiments to see what will "stay atop" water, and what will sink. How could that be controversial? Didn't Archimedes already figure out more than 1000 years earlier, during an exciting adventure in a public bath, that it's all about the density? Well, yes, but.... Ice floats on water, so ice must be less dense, right? But Aristotle said that cooling causes condensation, so ice must be condensed water, so it must be more dense than water. Thus if it floats, there must be some other reason, such as it's shape, that allows that to happen, or else Aristotle was wrong.

Ebony and iron will both normally sink. But a thin flat chip of ebony will stay on top of water, as will a carefully placed iron needle. So maybe ice floats because it is thin and flat. (That ice will float no matter what shape you make would seem to quickly disprove this.) Philosophers thought they had caught Galileo in a trap and proved he was wrong with these floating dense things. He did what he always did in response: experiment. This book shows a series of simple experiments and careful observations that show that yes, indeed, thin flat things carefully placed atop water can stay atop water, but even so, they descend down a certain distance, bringing some air along with them, so that the combined volume of air + ebony (or iron, etc.) is less dense than water. And even this won't work if the distance of the descent is more than about 1/8 inch, or the top of the item gets wet.

So, Archimedes remains right: it is all about the density if you take the extra bit of air into account. Galileo could not explain any cause for why the surface of water could be drawn down 1/8 inch. Today we would call that "surface tension", and it can be explained from physical principles concerning attractions between molecules. Galileo had no explanation for it, and that is the crux of the complaints of the philosophers. They felt that you aren't really doing proper philosophy if you can't explain the cause of something, preferably relating back to the causes Aristotle had identified such as "Nature abhors a vacuum" or "every element seeks its natural place at the center of the Earth unless impeded by another object". Galileo's idea was that you can do experiments, test your ideas, and find out how nature works in that way. If you don't understand why something happens, you try to figure it out with other experiments, but ultimately the experiment is what counts, not how well you can explain it.

So this is an important work, right at the beginning of the distinction between science and natural philosophy, written in relatively plain language, involving experiments you can easily reproduce yourself with no fancy equipment. Yet it is mostly forgotten. Everyone talks about the "sun goes around the Earth, or vice-versa" argument instead, though testing that for yourself is much harder. You need a telescope, clear skies, many nights of observation, and a bunch of math before you can see that one of those ideas makes more sense. (I started to say you need "the willingness to believe that gravity can act across a distance", but Galileo didn't believe that, otherwise he would not have so completely misunderstood tides.)

I guess that the experiments with water just seem so obvious today that it is hard to believe there could be such a serious fight over it as recently as 400 years ago. And the text is dry and convoluted and is actually wrong about many things. To understand it, you need a good translation into modern English (or other language) and introduction and notes to explain why it is worth reading about floating chips of wood. All of that is provided in Cause by Stillman Drake which is where I read this text, and I will review that soon.

Stillman Drake was an expert on Galileo and he feels that this fight with philosophers a big part of why the church eventually condemned him. I'll try to address that point when I review Drake's book.

(Five stars for importance; but it is hard to actually enjoy reading this.)

[Avesta · Rating 5 out of 5]

Despite being an incredibly dense (pun unintended) and difficult book to understand in terms of writing and language (it's almost Old English-like and uses old terminology such as 'grave' for kilogram) - it is most certainly quite a solid read.

I'm just really shocked by how Galileo's emphasis of the importance of empirical observation and experimentation was such a heavy contrast to the Aristotelian method, which relied heavily on deductive reasoning and abstract philosophical principles. Really makes you think, doesn't it? Incredibly thought-provoking.

Not to mention how Galileo suggesting that "All Matters, how heavy soever, even to Gold it self, the heaviest of all Bodies, known by us, may float upon the Water" was something revolutionary at the time. Today - it's so intuitive.

My issue, and I do understand it's 411 years old, is that there is shockingly no explicit reference or mention to 'density' affecting floating bodies - more so mass and 'grave' and weight, none of which are particularly close to the concept of density as Galileo does not see the link between mass and volume (ρ = m/v).

That's a bit crazy - how did Galileo not fathom that?

Also rather crazy how Democritus pioneered the atomic theory of the universe and coined the idea of the atom - but yet ALL OF HIS WORK has been disproven. ALL. Nothing is credited to him anymore simply because we have rendered all of it moot. Absolutely mindblowing.[https://en.wikipedia.org/wiki/Democritus]

5/5. That was one splendid read, even though understanding the archaic English is a challenge.

[alexander · Rating 5 out of 5]

Just as interesting as Starry Messenger (Sidereal Message)