On the Magnet/VI-4

by William Gilbert

CHAP. IIII.

That the Earth moves circularly.

f then the philosophers of the common sort, with an unspeakable absurdity, imagine the whole heaven and the vast extent of the universe to rotate in a whirl, it yet remains that the earth performs a diurnal change. For in no third way can the apparent revolutions be explained. This day, then, which is called natural, is a revolution of some meridian of the Earth from Sun to Sun. It revolves indeed in an entire course, from a fixed star round to that star again. Those bodies which in nature are moved with a circular, æquable and constant motion, are furnished, in their parts, with various boundaries. But the Earth is not a Chaos nor disordered mass; but by reason of its astral virtue, it has boundaries which subserve the circular motion, poles not mathematical, an æquator not devised by imagination, meridians also and parallels; all of which we find permanent, certain and natural in the Earth: which by numerous experiments the whole magnetick philosophy sets forth. For in the earth there are poles set in fixed bounds, and at them the verticity mounts up on either side from the plane of the Earth's æquator, with forces which are mightier and præpotent from the common action of the whole; and with these poles the diurnal revolution is in agreement. But in no turnings-about of bodies, in none of the motions of the planets are there to be recognized, beheld, or assured to us by any reasoning any sensible or natural poles in the firmament, or in any Primum Mobile; but those are the conception of an unsettled imagination. Wherefore we, following an evident, sensible and tested cause, do know that the earth moves on its own poles, which are apparent to us by many magnetick demonstrations. For not only on the ground of its constancy, and its sure and permanent position, is the Earth endowed with poles and verticity: for it might be directed toward other parts of the universe, toward East or West or some other region. By the wondrous wisdom then of the Builder forces, primarily animate, have been implanted in the Earth, that with determinate constancy the Earth may take its direction, and the poles have been placed truly opposite^[246], that about them as the termini, as it were, of some axis, the motion of diurnal turning might be performed. But the constancy of the poles is regulated by the primary soul. Wherefore, for the Earth's good, the collimations of her verticities do not continually regard a definite point of the firmament and of the visible heaven. For changes of the æquinoxes take place from a certain deflection of the Earth's axis; yet in regard to that deflection, the Earth has a constancy of motion

derived from her own forces. The Earth, that she may turn herself about in a diurnal revolution, leans on her poles. For since at A and B there is constant verticity, and the axis is straight; at C and D (the æquinoctial line) the parts are free, the whole forces on either side being spread out from the plane of the æquator toward the poles, in æther which is free from renitency, or else in a void; and A and B remaining constant, C revolves toward D both from innate conformity and aptitude, and for necessary good, and the avoidance of evil; but being chiefly moved forward by the diffusion of the solar orbes of virtues, and by their lights. And 'tis borne around, not upon a new and strange course, but (with the tendency common to the rest of the planets) it tends from West to East. For all planets have a like motion Eastward according to the succession of the signs, whether Mercury and Venus revolve beneath the Sun, or around the Sun. That the Earth is capable of and fitted for moving circularly its parts show, which when separated from the whole are not only borne along with the

straight movement taught by the Peripateticks, but rotate also. A loadstone fixed in a wooden vessel is placed on water so as to swim freely, turn itself, and float about. If the pole B of the loadstone be set contrary to nature toward the South, F, the Terrella is turned about its own centre with a circular motion in the plane of the Horizon, toward the North, E, where it rests, not at C or D. So does a small stone if only of four ounces; it has the same motion also and just as quick, if it were a strong magnet of one hundred pounds. The largest magnetical mountain will possess the same turning-power also, if launched in a wide river or deep sea: and yet a magnetick body is much more hindered by water than the whole Earth is by the æther. The whole Earth would do the same, if the Boreal pole were to be diverted from its true direction; for the Boreal pole would run back with the circular motion of the whole around the centre toward the Cynosure. But this motion by which the parts naturally settle themselves in their own resting-places is no other than circular. The whole Earth regards the Cynosure with her pole according to a steadfast law of her nature: and thus each true part of it seeks a like resting-place in the world, and is moved circularly toward that position. The natural movements of the whole and of the parts are alike: wherefore when the parts are moved in a circle, the whole also has the potency of

moving circularly. A sphærical loadstone placed in a vessel on water moves circularly around its centre (as is manifest) in the plane of the Horizon, into conformity^[247] with the earth.

So also it would move in any other great circle if it could be free; as in the declination instrument, a circular motion takes place in the meridian (if there were no variation), or, if there should be some variation, in a great circle drawn from the Zenith through the point of variation on the horizon. And that circular motion of the magnet to its own just and natural position shows that the whole Earth is fitted and adapted, and is sufficiently furnished with peculiar forces for diurnal circular motion. I omit what Peter Peregrinus^[248] constantly affirms, that a terrella suspended above its poles on a meridian moves circularly, making an entire revolution in 24 hours: which, however, it has not happened to ourselves as yet to see; and we even doubt this motion on account of the weight of the stone itself, as well as because the whole Earth, as she is moved of herself, so also is she propelled by other stars: and this does not happen in proportion (as it does in the terrella) in every part. The Earth is moved by her own primary form and natural desire, for the conservation, perfection, and ordering of its parts, toward things more excellent: and this is more likely than that the fixed stars, those luminous globes, as well as the Wanderers, and the most glorious and divine Sun, which are in no way aided by the Earth, or renewed, or urged by any virtue therein, should circulate aimlessly around the Earth, and that the whole heavenly host should repeat around the Earth courses never ending and of no profit whatever to the stars. The Earth, then, which by some great necessity, even by a virtue innate, evident, and conspicuous, is turned circularly about the Sun, revolves; and by this motion it rejoices in the solar virtues and influences, and is strengthened by its own sure verticity, that it should not rovingly revolve over every region of the heavens. The Sun (the chief agent in nature) as he forwards the courses of the Wanderers, so does he prompt this turning about of the Earth by the diffusion of the virtues of his orbes, and of light. And if the Earth were not made to spin with a diurnal revolution, the Sun would ever hang over some determinate part with constant beams, and by long tarriance would scorch it, and pulverize it, and dissipate it, and the Earth would sustain the deepest wounds; and nothing good would issue forth; it would not vegetate, it would not allow life to animals, and mankind would perish. In other parts, all things would verily be frightful and stark with extreme cold; whence all high places would be very rough, unfruitful, inaccessible, covered with a pall of perpetual shades and eternal night. Since the Earth herself would not choose to endure this so miserable and horrid appearance on both her faces, she, by her magnetick astral genius, revolves in an orbit, that by a perpetual change of light there may be a perpetual alternation of things, heat and cold, risings and settings, day and night, morn and eve, noon and midnight. Thus the Earth seeks and re-seeks the Sun, turns away from him and pursues him, by her own wondrous magnetick virtue. Besides, it is not only from the Sun that evil would impend, if the Earth were to stay still and be deprived of solar benefit; but from the Moon also serious dangers would threaten. For we see how the ocean rises and swells beneath certain known positions of the Moon: And if there were not through the daily rotation of Earth a speedy transit of the Moon, the flowing sea would be driven above its level into certain regions, and many shores would be overwhelmed with huge waves. In order then that Earth may not perish in various ways, and be brought to confusion, she turns herself about by magnetick and primary virtue: and the like motions exist also in the rest of the Wanderers, urged specially by the movement and light of other bodies. For the Moon also turns herself about in a monthly course, to receive in succession the Sun's beams in which she, like the Earth, rejoices, and is refreshed: nor could she endure them for ever on one particular side without great harm and sure destruction. Thus each one of the moving globes is for its own safety borne in an orbit either in some wider circle, or only by a rotation of its body, or by both together. But it is ridiculous for a man a philosopher to suppose that all the fixed stars and the planets and the still higher heavens revolve to no other purpose, save the advantage of the Earth. It is the Earth, then, that revolves, not the whole heaven, and this motion gives opportunity for the growth and decrease of things, and for the generating of things animate, and awakens internal heat for the bringing of them to birth. Whence matter is quickened for receiving forms; and from the primary rotation of the Earth natural bodies have their primary impetus and original activity. The motion then of the whole Earth is primary, astral, circular, around its own poles, whose verticity arises on both sides from the plane of the æquator, and whose vigour is infused into opposite termini, in order that the Earth may be moved by a sure rotation for its good, the Sun also and the stars helping its motion. But the simple straight motion downwards of the Peripateticks is a motion of weight, a motion of the aggregation of disjoined parts, in the ratio of their matter, along straight lines toward the body of the Earth: which lines tend the shortest way toward the centre. The motions of disjoined magnetical parts of the Earth, besides the motion of aggregation, are coition, revolution, and the direction of the parts to the whole, for harmony of form, and concordancy.

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The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.

246 ↑  Page 221, line 10. Page 221, line 11. poli verè oppositi sint.—For verè, the 1628 and 1633 editions read rectæ. All editions read sint, though sunt seems to make better sense.

247 ↑  Page 223, line 7. Page 223, line 8. ad telluris conformitatem.—The word conformitas is unknown in classical Latin.

248 ↑  Page 223, line 16. Page 223, line 17. Omitto quod Petrus Peregrinus constanter affirmat, terrellam super polos suos in meridiano suspensam, moveri circulariter integrâ revolutione 24 horis: Quod tamen nobis adhuc videre non contingit; de quo motu etiam dubitamus.

This statement that a spherical loadstone pivotted freely with its axis parallel to the earth's axis will of itself revolve on its axis once a day under the control of the heavens, thus superseding clocks, is to be found at the end of chap. x. of Peregrinus's Epistola De Magnete (Augsb., 1537).

Gilbert, who doubted this experiment because of the stone's own weight is taken to task by Galileo, in the third of his Dialogues, for his qualified admission.

"I will speak of one particular, to which I could have wished that Gilbert had not lent an ear; I mean that of admitting, that in case a little Sphere of Loadstone might be exactly librated, it would revolve in it self; because there is no reason why it should do so" (p. 376 of Salusbury's Mathematical Collections, London, 1661). The Jesuit Fathers who followed Gilbert, but rejected his Copernican ideas, pounced upon this pseudo-experiment, as though by disproving it they had upset the Copernican theory.