Galileo Discusses Magnetism
Galileo's Dialogue, Day 3, Part 5
As I wrote in a recent post, I intend to write posts for each of the four days in the Dialogue Comparing the Ptolemaic and Copernican Systems1, the book that got Galileo in trouble with Rome. This is the fifth post about Day Three in the Dialogue. Here are the previous posts:
Day One: Galileo Dismantles Aristotle's Separation of Earth from the Heavens.
Day Two, Part 1: Galileo Attacks Aristotle’s Followers.
Day Two, Part 2: Galileo Insists the Earth is Spinning on its Axis.
Day Two, Part 3: Galileo on the Acceleration of Falling Bodies.
Day Two, Part 4: Galileo Channels Plato.
Day Three, Part 1: Galileo Calls Out Chiaramonti’s Manipulation of Data.
Day Three, Part 2: Galileo Praises Aristarchus and Copernicus.
Day Three, Part 3: Why the Apparent Motion of the Sunspots Supports the Copernican Model.
Day Three, Part 4: Galileo Discusses the Size of the Universe and the Seasons.
I finished the previous post with a teaser, namely with speculation by Salviati that the fixed tilt of the axis might be the result of a magnetic attraction of the earth to a particular part of the firmament:
But what will Simplicio say if, to this independence of any coöperating cause, we add a remarkable force inhering in the terrestrial globe and making it point with definite parts of itself toward definite parts of the firmament? I am speaking of magnetic force, in which every piece of lodestone constantly participates. And if every tiny particle of such stone has in it such a force, who can doubt that the same force resides to a still higher degree within the whole of this terrene globe, which abounds in this material? Or that perhaps the globe itself is, as to its internal and primary substance, nothing but an immense mass of lodestone2? [pp.399-400]
In the following discussion, Salviati will wax eloquent about William Gilbert, who published in 1600 De Magnete (On the Magnet3), which is considered to be the first scientific treatise about magnetism, and which will be the topic of future posts. But the main target is not about magnetism per say, but to demolish once and for all the Aristotelian idea that a simple body can only participate in a single motion. In fact, the above quote of Salviati is part of a lengthy response to Simplicio:
Salv: But setting all this aside, please tell me what absurdities or excessive subtleties make this Copernican arrangement the less plausible so far as you are concerned.
Simp: As a matter of fact, I did not completely understand it, perhaps because I am not very well versed either in the way the same effects are produced by Ptolemy — I mean these planetary stoppings, retrograde movements, approaches and retreats, lengthenings and shortenings of the day, alterations of the seasons, etc. But passing over the consequences which stem from the basic assumptions, I feel no small difficulties to exist in these assumptions themselves, and if the assumptions fall to the ground then they bring the whole structure into ruin. Now since the whole framework of Copernicus seems to me to be built upon a weak foundation (being supported upon the mobility of the earth), then if this were removed, there would be no room for further argument. And to remove it, Aristotle's axiom that to a simple body only one simple motion can be natural appears to be sufficient. Here three movements, if not four, are assigned to the earth, a simple body; and all of them are quite different from one another. For besides the straight motion toward the center, which cannot be denied to it as a heavy body, there are ascribed to it a circular motion in a great circle around the sun in one year, and a whirling upon itself every twenty-four hours, and (what is most extreme, and possibly for that reason you have remained silent about this) another whirling about its own center, completed in a year, and opposite the previously mentioned twenty-four-hour motion. My mind feels a great repugnance to this. [pp.397-398, emphasis mine]
So what is Simplicio stating here? Yes, it’s true that the Copernican model seems to elegantly explain a whole variety of observations, including the apparent motion of the outer planets and the seasonal changes on earth, but I’m not going to take the trouble to study these carefully, because I know the model to be wrong. How do I know the model to be wrong? Because according to Aristotle, the earth is a simple body and a simple body can only partake of a simple motion. Since the Copernican model requires the earth to participate in four different motions, it must be wrong. There, I’m done.
As I wrote in the previous post, Salviati quickly dispensed with the first and fourth motions, but that still left the earth participating in two motions: revolution about the sun and rotation about its own axis. And so it is finally time for Salviati to directly address and demolish the completely self-contradictory ideas from Aristotle, and to do this, he will work with his intuition that the earth is a giant lodestone.
So how does Simplicio answer Salviati when the latter asked “But what will Simplicio say if … we add a remarkable force inhering in the terrestrial globe and making it point with definite parts of itself toward definite parts of the firmament?”
Then you are one of those people who adhere to the magnetic philosophy of William Gilbert? [p.400]
Why would Simplicio be so alarmed by Salviati adhering to Gilbert’s ideas? A quick look at the titles of the first four chapters of De Magnete, Book VI, shows us why:
Chap. 1: On the globe of the earth, the great magnet. [p.211]
Chap. 2: The Magnetick axis of the Earth persists invariable. [p.212]
Chap. 3: On the magnetic diurnal revolution of the Earth’s globe, as a probable assertion against the time-honoured opinion of a Primum Mobile. [p.214]
Chap. 4: That the Earth moves circularly. [p.220]
In other words, Salviati is directly channelling Gilbert, who argued in favour of the earth’s diurnal rotation. Salviati’s response to Simplicio is devastating:
Certainly I am, and I believe that I have for company every man who has attentively read his book and carried out his experiments. Nor am I without hope that what has happened to me in this regard may happen to you also, whenever a curiosity similar to mine, and a realization that numberless things in nature remain unknown to the human intellect, frees you from slavery to one particular writer or another on the subject of natural phenomena, thereby slackening the reins on your reasoning and softening your stubborn defiance of your senses, so that some day you will not deny them by giving ear to voices which are heard no more.
Now, the cowardice (if we may be permitted to use this term) of ordinary minds has gone to such lengths that not only do they blindly make a gift — nay, a tribute — of their own assent to everything they find written by those authors who were lauded by their teachers in the first infancy of their studies, but they refuse even to listen to, let alone examine, any new proposition or problem, even when it not only has not been refuted by their authorities, but not so much as examined or considered. One of these problems is the investigation of what is the true, proper, basic, internal, and general matter and substance of this terrestrial globe of ours. Even though neither Aristotle nor anybody else before Gilbert ever took it into his head to consider whether this substance might be lodestone (let alone Aristotle or anybody else having disproved such an opinion), I have met many who have started back at the first hint of this like a horse at his shadow, and avoided discussing such an idea, making it out to be a vain hallucination, or rather a mighty madness. And perhaps Gilbert's book would never have come into my hands if a famous Peripatetic philosopher had not made me a present of it, I think in order to protect his library from its contagion. [p.400]
In the exchange that follows, Salviati explains to Simplicio that the main part of the earth cannot be the earth that is found on the surface, but some kind of rocks, and that among these is lodestone, because of the magnetic properties of the earth:
[U]nder this covering or wrapper of earth, stone, metal, water, etc. there is concealed a huge lodestone. For in regard to this there are recognized, by anyone who observes carefully, all the same events which are perceived to belong to a true and unconcealed sphere of lodestone. [p.404]
Salviati then moves on to describe some experiments which he himself conducted on a lodestone. In particular, he had part of a large lodestone polished, and demonstrated that iron was more strongly attracted by the polished part of this lodestone rather than by the rest of it. As a result, he concludes that lodestone is a kind of porous stone whose cavities are filled with some rock:
Thus was I assured that my idea had been quite correct when I first judged that the substance of the lodestone must be not continuous and compact, but porous. Better yet, spongy; though with this difference: where the cavities and cells of a sponge contain air or water, those of the lodestone are filled with hard and heavy stone, as shown by the high lustre that they take on. [p.409]
It is the understanding that most of the earth is some kind of lodestone, combined with the fact that lodestone is heterogeneous, that leads to the contradictions with Aristotle. Salviati begins:
What I wanted to bring up for consideration was precisely the lodestone, to which three movements are sensibly seen to belong naturally: One toward the center of the earth as a heavy object; a second is the horizontal circular motion by which it restores and conserves its axis in the direction of certain parts of the universe; and third is this one discovered by Gilbert, of dipping its axis in the meridian plane toward the surface of the earth, in greater or less degree proportionate to its distance from the equation (where it remains parallel to the axis of the earth). Besides these three, it is perhaps not improbable that it may have a fourth motion of turning about its own axis, whenever it is balanced and suspended in air or some other fluid and yielding medium and all external and accidental impediments are taken away; Gilbert himself also shows his approval of this idea. So you see, Simplicio, how shaky Aristotle's axiom is. [p.411]
In the exchange that follows, it is Sagredo who confronts Simplicio:
Sagr: Wait a moment, Simplicio. Answer the questions I am going to ask you. You say that the lodestone is not a simple body, but a compound one; now I ask you what are the simple bodies which are mixed in the compounding of lodestone?
Simp: I cannot tell you the ingredients or the exact proportions, but it is sufficient that they are elementary bodies.
Sagr: That is enough for me, too. And what are the natural motions of these elemental bodies?
Simp: They are the two simply straight motions, sursum [upward] and deorsum [downward].
Sagr: Next, tell me this: Do you believe that the motion which is natural to such a compound body must be one which could result from the combination of the two simple natural motions of the component simple bodies? Or might it be still another motion, one not possible to compound from those?
Simp: I believe that it will move with that motion resultant from the composition of the motions of the component simple bodies, and that it could not move with any motion impossible to compound from these. [pp.411-412]
It is now time for Sagredo to give the coup de grâce:
But Simplicio, you can never compound one circular motion from the two simple straight motions, and the lodestone has two or three different circular motions. So you see the trouble into which badly founded principles lead — or, rather, badly drawn consequences from good principles. For next you will be forced to say that the lodestone is a compound composed of elemental and celestial substances, if you wish to maintain that straight motion belongs only to the elements, and circular to the heavenly bodies. Therefore if you want to philosophize with assurance, say that the integral bodies of the universe which are naturally movable all move circularly, and that consequently lodestone, as a part of the true primary and integral substances of our globe, partakes of this same nature.
And please note that by your fallacious reasoning you are calling lodestone a compound body, and the terrestrial globe a simple body; yet the latter may be seen to be a hundred thousand times more compounded, since besides containing thousands and thousands of materials quite different from each other, it contains a great abundance of the very thing you call compound; I mean lodestone. This seems to me the same as if someone were to call bread a compound body, and hash a simple body, though into hash there enters no small quantity of bread, besides a hundred different foods which are eaten with bread.
It really seems to me a remarkable thing (among others) that the Peripatetics concede — as indeed they cannot deny — that our terrestrial globe is de facto a compound of infinitely diverse materials; that they next concede that the motions of compound bodies must be compound; that the motions which can be compounded are the straight and the circular, since the two straight motions are incompatible on account of being contrary to one another; they affirm that the pure element of earth is not to be found; and they grant that the earth is never moved with any local motion. Finally they want to place in nature this body which is nowhere to be found, and make it movable with a motion which it has never employed and will never employ; but to this actual body which does exist and has always existed they deny that very motion which they originally conceded must be naturally suited to it! [pp.412-413]
And thus ends Day Three. Having already taken apart in Days Two and Three the anti-Copernican arguments put forward by Johann Georg Locher and by Scipione Chiaramanti, Galileo goes for the jugular, and directly dismantles the Aristotelian arguments put forward against earthly motion.
If you wish to donate to support my work, please use the Buy Me a Coffee app.
Galileo Galilei. Dialogue concerning the two chief world systems — Ptolemaic and Copernican. Translated by Stillman Drake, foreword by Albert Einstein. University of California Press, 2nd ed., 1967.
William Gilbert, On the Magnet. New York: Basic Books, 1958.