William Gilbert States that the Moon Causes the Tides
William Gilbert's De Magnete, Book Two, part 4
This post is part of a series of posts about William Gilbert’s De Magnete (On the Magnet1), which is composed of six books. This is the fourth post on Book Two. Here are the previous posts related to De Magnete.
De Magnete, Nothing Less than the First Ever Work of Experimental Physics
Book One, part 1: William Gilbert Writes about the Loadstone
Book One, part 2: William Gilbert Examines Iron, Calls Aristotle's Earth Element Dead
Book Two, part 1: William Gilbert Compares Electric Bodies to Magnetic Bodies
Book Two, part 2: William Gilbert Discusses Magnetic Bodies
Book Two, part 3: William Gilbert Considers the Internal Structure of the Terrella
In this post, I will describe a number of interesting arrangements of a terrella with iron objects, producing intriguing results, ultimately allowing Gilbert to give a broad explanation of the moon’s influence on the tides. Gilbert begins by stating that, unlike for electric bodies, putting various kinds of material, such as paper, between a magnetic needle and a terrella does not have any affect on the magnetic attraction. However, placing another piece of iron between the magnetic needle and the terrella will have an influence, but in myriad ways, depending on the overall geometry of the various objects.
Nor is the whole of the force suppressed by a plate of iron, but in part diverted. For when the force enters the middle of an iron plate placed within the sphere of magnetic influence or directly over the pole of the loadstone, that force is distributed chiefly to the extremities, so that the rim of a circular plate of suitable size attracts pieces of iron wire at all points. [pp.132-133]
Gilbert then presents the image below, of an iron rod, with center A, placed directly on top of a terrella with north pole E. Then the overall structure will act as a magnet, with A now also acting as a north pole, and both C and D acting as south poles!
The same is seen in a long iron rod rubbed with a loadstone in the middle ; it has the same verticity at both ends. In the figure, CD is a long rod magnetized in the middle by the north pole E; C is a south end or south pole, and D is another south end. [p.133]
Gilbert continues his discussion with the plate:
But here note the singular fact, that a needle magnetized by that pole turns to that pole, though the round plate stands between, the plate not hindering, but the attraction being only weaker; for the force is scattered to the extremities of the plate, and departs from the straight track, but yet the plate in its middle retains the same verticity with the pole when it is nigh it and alongside it: hence does the needle magnetized by the same pole tend to the centre of the plate. [p.133]
But if the plate is removed away from the terrella, then the magnetic needle which had been pointing to the plate will flip around and point away from the plate!
But let the plate be magnetized in the middle by the pole, and then let it be removed beyond the loadstone’s sphere of influence, and you shall see the point of the same needle go in the contrary direction and quit the centre of the plate, which before it sought: for outside of the sphere of influence the plate has the contrary verticity, but near the loadstone it has the same; for near the loadstone the plate is as it were part of the loadstone and has the same pole. [p.133]
Gilbert’s next image shows two situations. To the right is an iron plate A placed against a terrella with north pole C. Then the magnetic needle B will point towards the plate. To the left, the now magnetized plate is taken away from the terrella, and magnetic needle E points away from the plate.
Let A be an iron plate near a pole; B a needle with point tending toward the centre of the plate, which plate has been magnetized by the pole of a loadstone. Now if the same plate be placed outside the sphere of magnetic influence, the point of the needle will not turn to its centre, but only the crotch (the other end) of the same needle. [pp.133-134]
Gilbert then moves on to discussing what happens with an iron sphere. In the next image, first he places the needle F between terrella A and iron sphere B. Then he places the sphere next to the terrella. In both cases, the needle points to the pole C.
Let A be a terrella, B an iron sphere, F a needle between the two bodies, with its point magnetized by the pole C. In the second figure A is the terrella, C a pole, B an iron sphere: the needle tends toward C, the terrella’s pole, through the iron sphere. The needle thus placed between terrella and sphere vibrates more forcibly toward the pole of the terrella, because the loadstone imparts instantaneous verticity to the opposite sphere. [p.134]
From this point, Gilbert makes a direct analogy with the earth:
The earth’s efficiency is the same, produced by the same cause. For if in a thick box made of gold (the densest of metals) or glass, or marble, you put a needle free to revolve, that needle, in spite of the box, will show that its forces are most closely allied to and unified with those of the earth; of its own accord and instantly, regardless of the box that prisons it, it turns to its desiderated points of north and south. And it does the same though it be shut up in iron vaults sufficiently roomy. [pp.134-135]
Once again, Gilbert returns to his idea of the primary form of the earth:
Whatever bodies are produced here on the earth or are manufactured from nature’s products by art, all consist of the matter of the globe: such bodies do not interfere with the prime potencies of nature derived from the primary form; nor can they withstand them, save by contrary forms. [p.135]
This point is crucial for Gilbert, because he explains the moon’s influence on the tides on earth through this mechanism, assuming that the moon has a similar composition to that of the earth:
But coition and movement of the earth and the loadstone, though corporeal hindrances be interposed, are shown also in the efficiencies of other chief bodies that possess the primary form. The moon, more than the rest of the heavenly bodies, is in accord with the inner parts of the earth because of her nearness and her likeness of form. [pp.135-136]
And here we have the irony of Galileo. If you remember my post Galileo Discusses Magnetism, Galileo had nothing but praise for Gilbert’s study of magnetism. Yet, as I wrote in the following post, Galileo and the Tides, Galileo completely rejected the idea that the moon could have any influence on the tides. Yet, here is Gilbert writing exactly on this topic:
The moon causes the movement of the waters and the tides of ocean ; makes the seashore to be covered and again exposed twice between the time she passes a given point of the heavens and reaches it again in the earth’s daily rotation: this movement of the waters is produced and the seas rise and fall no less when the moon is below the horizon and in the nethermost heavens, than when she is high above the horizon. Thus the whole mass of the earth, when the moon is beneath the earth, does not prevent the action of the moon; and thus in certain positions of the heavens, when the moon is beneath the horizon, the seas nearest to our countries are moved, and, being stirred by the lunar power (though not struck by rays nor illumined by light), they rise, approach with great impetus, and recede. [p.136]
Gilbert will only give a full exposition of how he understands the interaction between the moon and the earth in Book Six, but he does make this initial conclusion:
Hence, here on earth, naught can be held aloof from the magnetic control of the earth and the loadstone, and all magnetic bodies are brought into orderly array by the supreme terrene form, and loadstone and iron sympathize with loadstone though solid bodies stand between. [p.136]
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William Gilbert. De Magnete. Dover, New York, 1958. Translation by P. Fleury Mottelay of De Magnete, first published in 1600.
Very interesting. I've always wondered how there are two high tides a day. The moon is only back in the same spot after a full day, and if tide were caused by gravity, surely the tide would be highest underneath the moon. Looking forward to Gilbert's explanation!