Galileo: Dialogue concerning the two chief world systems: magnetism

I've been reading the "Dialogue concnerning the two chief world systems" by Galileo (see-also: wiki). I've got to the end of day 3 (of 4). Day 4 is apparently about G's (incorrect) theory of the tides, which may well be an interesting thing in itself. Apparently, he was so keen to prove that the earth moved, that he rejected the idea of "influences" from other bodies (the moon) in favour of a theory that (he thought) demonstrated directly what he wanted to prove.

There is a motto in that, of course... but I'll talk about a bit that I have actually read: magnetism. Which comes at the end of the third day. I wanted to point you to an online source, but sadly it the "full" texts I found all omit that bit as peripheral to the Copernican/Ptolemaic dialogue which is the main purpose.

Anyway... they are talking about Gilberts theory, which was that the Earth is a giant lodestone. Which of course we now know to be wrong. Although arguably its close enough. Anyway, the point is, that G points out various things about lodestones, ostensibly from observation. Some of these are true: that the attraction is greater at each end and falls to zero in the middle. That it points N-S (incidentally, I see no evidence that he realised it didn't point to true north). He also notes the fairly new observation of the dip of the lodestone - ie, that if free to pivot vertically, it doesn't stay horizontal but dips, according to latitude. But here he gets a bit weird. He asserts that the southern pole of the magnet is stronger. He asserts this, because he explains the dip by the greater strength of the S pole (isn't this wrong? doesn't a magnet dip with N pole downwards in the NH?). Then a bit later on he says of course, this strength changes, with no difference at the equator and at the pole a magnet stands vertical (this latter "fact" was of course unobserved, despite being presented as an observation). Now it seems pretty weird to write it this way (in terms of changes in strength of the magnet) when an explanation in terms of following-lines-of-force is fairly easy (but did he know about that? He talks of lodestones attracting iron filings, but *doesn't mention putting a lodestone under a sheet of paper and sprinkling filings on top to visualise the lines. Also, (of course?) had he actualy measured the load-bearing strength of each end he would have found it the same.

So... the motto I was going to draw from this was that small flaws in peripheral areas don't invalidate your main argument :-)

1 comment:

Enemy of the Republic said...

Currently I am writing a paper on Dante and Galileo.