By William Gilbert
Book 1, Chapter III:
THE LOADSTONE POSSESSES PARTS DIFFERING IN THEIR NATURAL POWERS, AND HAS POLES CONSPICUOUS FOR THEIR PROPERTIES.
The many qualities exhibited by the loadstone itself, qualities hiterto recognized yet not well investigated, are to be pointed out in the first place, to the end the student may understand the powers of the loadstone and of iron, and not be confused through want of knowledge at the threshold of the arguments and demonstrations. In the heavens, astronomers give to each moving sphere two poles; thus do we find two natural poles of excelling importance even in our terrestrial globe, constant points related to the movement of its daily revolution, to wit, one pole pointing to Arctos (Ursa) and the north; the other looking toward the opposite part of the heavens. In like manner the loadstone has from nature its two poles, a northern and a southern; fixed definite points in the stone, which are the primary termini of the movements and effects, and the limits and regulators of the several actions and properties. It is to be understood, however, that not from a mathematical point does the force of the stone emanate, but from the parts themselves; and all these parts in the whole - while they belong to the whole - the nearer they are to the poles of the stone the stronger virtues do they acquire and pour out on other bodies. These poles look toward the poles of the earth, and move toward them, and are subject to them. The magnetic poles may be found in every loadstone, whether strong and powerful (male, as the term was in antiquity) or faint, weak, and female; whether its shape is due to design or to chance, and whether it be long, or flat, or four-square, or three-cornered, or polished; whether it be rough, broken-off, or unpolished: the loadstone ever has and ever shows its poles. But inasmuch as the spherical form, which, too, is the most perfect, agrees best with the earth, which is a globe, and also is the form best suited for experimental uses, therefore we purpose to give our principal demonstrations with the aid of a globe-shaped loadstone, as being the best and the most fitting. Take then a strong loadstone, solid, of convenient size, uniform, hard, without flaw; on a lathe, such as is used in turning crystals and some precious stones, or on any like instrument (as the nature and toughness of the stone may require, for often it is worked only with difficulty), give the loadstone the form of a ball. The stone thus prepared is a true homogenous off-spring of the earth and is of the same shape, having got from art the orbicular form that nature in the beginning gave to the earth, the common mother; and it is a natural little body endowed with a multitude of properties whereby many abstruse and unheeded truths of philosophy, hid in deplorable darkness, may be more readily brought to the knowledge of mankind. To this round stone we give the name microge, or Terrella (earthkin, little earth).
To find, then, poles answering to the earth's poles, take in your hand the round stone, and lay on it a needle or a piece of iron wire: the ends of the wire move round their middle point, and suddenly come to a standstill. Now, with ochre or with chalk, mark where the wire lies still and sticks. Then move the middle or centre of the wire to another spot, and so to a third and fourth, always marking the stone along the length of the wire where it stands still: the lines so marked will exhibit meridian circles, or circles like meridians on the stone or terrella; and manifestly they will all come together at the poles of the stone. The circles being continued in this way, the poles appear, both the northand the south, and betwixt these, midway, we may draw a large circle for an equator, as is done by the astronomer in the heavens and on his spheres and by the geographer on the terrestrial globe; for the line so drawn on this our terrella is also of much utility in our demonstrations and our magnetic experiments. Poles are also found in the round stone, in a versorium, in a piece of iron touched with a loadstone and resting on a needle or point (attached at its base to the terrella), so that it can freely revolve, as in the figure.
On top of the stone AB is set the versorium in such a way that its pointer may remain in equilibrium: mark with chalk the direction of the pointer when at rest. Then move the instrument to another spot and again mark the direction in which the pointer looks; repeat this many times at many different points and you will, from the convergence of the lines of direction, find one pole at point A, the other at B. A pointer also indicates the true pole if brought near to the stone, for it eagerly faces the stone at right angles, and seeks the pole itself direct and turns on its axis in a right line toward the centre of the stone. Thus the pointer D regards A and F, the pole and the centre, but the pointer E looks not straight either toward the pole A or the centre F. A bit of fine iron wire as long as a barely-corn is laid on the stone and is moved over the zones and the surface of the stole ill it stands perpendicularly erect; for at the poles, whether N. or S., it stands erect; but the farther it is from the poles (toward the equator) the more it inclines. The poles thus found, you are to mark with a sharp file or a gimlet.
Book 2, Chapter I:
OF MAGNETIC MOVEMENTS.
Of opinions touching the loadstone and its varieties; of its poles and its recognized faculties (facultatibus); of iron and its properties; of the magnetic substance common to loadstone and iron and the earth itself, - we have treated briefly in the foregoing book. Now remain the magnetic movements and their broader philosophy as developed by experiments and demonstrations. These movements are impulsions of homogeneous parts toward one another or toward the primary conformation of the whole earth. Aristotle admits only two simple movements of his elements - from the centre and toward the centre; light objects upward, heavy objects downward: so that in the earth there is but one motion of all its parts toward the centre of the world, - a wild headlong falling. We, however, will elsewhere consider what this 'light' may be, and will show how erroneously it is inferred by the Peripatetics from the simple motion of the elements; we shall also inquire what 'heavy' means. But now we have to inquire into the causes of the other movements depending on its true form: these we see clearly in all magnetic bodies; these also we find existing in the earth and all its homogenic parts; further, we find that they are in accord with the earth, and are bound up in its forces. Now five movements or differences of movement are perceived by us: COITION (commonly called attraction, an impulsion to magnetic union; DIRECTION toward the earth's poles, and verticity of the earth toward determinate points in the universe, and the standstill there; VARIATION, deflection from the meridian, - this we call a perverted motion; DECLINATION (inclination or dip), a descent of the magnetic pole beneath the horizon; and circular movement, or REVOLUTION. Of each of these we will treat separately, and will show how they all proceed from a congregant nature, or from verticity or from volubility. Jofrancus Offusius distinguishes several magnetic movements, the first to the centre, the second to the pole, traversing 77 degrees, the third to iron, the fourth to a loadstone. The first is not always to the centre, for only at the poles is it in a right line to the centre, if the motion is magnetic, otherwise it is only the movement of matter toward its mass and toward the earth. The second, of 77 degrees to the pole, is no movement, but a direction or a variation to the earth's pole. The third and fourth are magnetic, and are but one movement. Thus this author recognizes no true magnetic movement but coition toward iron or loadstone, commonly known as attraction. There is another movement in the earth as a whole, which does not take place toward the terrella or the parts, i.e., the movement of coacervation and that movement of matter called by philosophers a "right movement:" of that elsewhere.
Book 2, Chapter VI:
HOW MAGNETIZED IRON AND SMALLER LOADSTONES CONFORM TO THE TERRELLA AND TO THE EARTH ITSELF, AND ARE GOVERNED THEREBY.
Coition of bodies that are separate from one another, and that cohere naturally, takes place by another sort of movement, if they be free to move. The terrella sends its force abroad in all directions, according to its energy and its quality. But whenever iron or other magnetic body of suitable size happens within its sphere of influence it is attracted; yet the nearer it is to the loadstone the greater the force with which it is borne toward it. Such bodies tend to the loadstone not as toward a centre nor towards its centre: that they do only at its poles, i.e., when that which is attracted and the pole of the loadstone, as well as its centre, are in a right line. But in the intervals between they tend to it in an oblique line, as seen in the figure below, wherein is shown how the force goes out to the magnetic associate bodies within the sphere. At the poles the line is a right one. The nearer the parts to the equinoctial circle the more obliquely do magnetic bodies attract, but the parts nearer the poles attract more directly; at the poles themselves attraction is in a right line. All loadstones alike, whether spherical or oblong, have the self-same mode of turning to the poles of the world; but it is easiest to experiment with oblong ones. For whatever the shape, verticity is present, and there are poles; but owing to imperfect and irregular shape, loadstones are often subject to drawbacks, and are interfered with in their movements. If the loadstone be oblong, with vertices at the extremities and not at the sides, it attracts best at the vertex; for the parts convey to the poles a greater force in right lines than in oblique. Thus do the loadstone and the earth conform magnetic movements.
BOOK 3, CHAPTER I:
In the foregoing books it has been shown that a loadstone has its poles, iron also poles, and rotation, and fixed verticity, and finally that loadstone and iron direct their poles toward the poles of the earth. But now we have to set forth the causes of these things and their wonderful efficiencies known aforetime but not demonstrated. Of these rotations all the writers who went before us have given their opinions with such brevity and indefiniteness that, as it would seem, no one could be persuaded thereby, while the authors themselves could hardly be contented with them. By men of intelligence, all their petty reasoning - as being useless, questionable, and absurd, and based on no proofs or premises - are rejected with the result that magnetic science, neglected more and more and understood by none, has been exiled. The true south pole, and not the north (as before our time all believed), of a loadstone placed on its float in water turns to the north; the south end of a piece of magnetized and of unmagnetized iron also moves to the north. An oblong piece of iron of three or four finger-breadths, properly stroked with a loadstone, quickly turns to north and south. Therefore artificers place such a bar, balanced on a point, in a compass-box or in a sun-dial; or they construct a versorium out of two curved pieces of iron that touch at their extremities so that the movement may be more constant; thus is constructed the mariner's compass, an instrument beneficial, salutary, and fortunate for seamen, showing the way to safety and to port. But it is to be understood at the threshold of their argument, before we proceed farther, that these directions of loadstone or of iron are not ever and always toward the world's true poles, that they do not always seek those fixed and definite points, nor rest on the line of the true meridian, but that at places, more or less far apart, they commonly vary either to the east or to the west; sometimes, too, in certain regions of land or sea, they point to the true poles. This discrepance is known as the variation of the needle and of the loadstone; and as it is produced by other causes and is, as it were, a sort of perturbation and depravation of the true direction, we propose to treat here only of the true direction of the compass and the magnetic needle, which would all over the earth be the same, toward the true poles and in the true meridian, were not hinderances and disturbing causes present to prevent: in the book next following we will treat of its variation and of the cause of perturbation.
They who aforetime wrote of the world and of natural philosophy, in particular those great elementarian philosophers and all their progeny and pupils down to our day; those, I mean, who taught that the earth is ever at rest, and is, as it were, a dead-weight planted in the centre of the universe at equal distance everywhere form the heavens, of simple uncomplex matter possessing only the qualities of dryness and cold - these philosophers were ever seeking the causes of things in the heavens, in the stars, the planets; in fire, air, water, and in the bodies of compounds; but never did they recognize that the terrestrial globe, besides dryness and cold, hath some principal, efficient, predominant potencies that give to it firmness, direction, and movement throughout its entire mass and down to its inmost depths; neither did they make inquiry whether such things were, and, for this reason, the common herd of philosophizers, in search of the causes of magnetic movements, called in causes remote and far away. Martinus Cortesius, who would be content with no cause whatever in the universal world, dreamt of an attractive magnetic point beyond the heavens, acting on iron. Petrus Peregrinus holds that direction has its rise at the celestial poles. Cardan was of the opinion that the rotation of iron is caused by the star in the tail of Ursa Major. The Frenchman Bessard thinks that the magnetic needle turns to the pole of the zodiac. Marsilius Ficinus will have it that the loadstone follows its Arctic pole, and that iron follows the loadstone, and chaff follows amber: as for amber, why, that, mayhaps, follow the Antarctic pole: emptiest of dreams! Others have come down to rocks and I know not what "magnetic mountains"! So has ever been the wont of mankind: homely things are vile; things from abroad and things afar are dear to them and the object of longing. As for us, we are habitants of this very earth, and study it as cause of this mighty effect. Earth, the mother of all, hath these causes shut up in her recesses: all magnetic movements are to be considered with respect to her law, position, constitution, verticity, poles, equator, horizon, meridians, centre, periphery, diameter, and to the form of her whole inward substance. So hath the earth been ordered by the Supreme Artificer and by nature, that it shall have parts unlike in position, terminal points of an entire and absolute body, and such points dignified by distinct functions, whereby it shall itself take a fixed direction. For like as a loadstone, when in a suitable vessel it is floated on water, or when it is suspended in air by a slender thread, does by its native verticity, according to the magnetic laws, conform its poles to the poles of the common mother, - so, were the earth to vary from her natural direction and from her position in the universe, or were her poles to be pulled toward the rising or the setting sun, or other points whatsoever in the visible firmament (were that possible), they would recur again by a magnetic movement to north and south, and halt at the same points where now they stand. But why the terrestrial globe should seem constantly to turn one of its poles toward those points and toward Cynosurea [constellation of the Lesser Bear], or why her poles should vary from the poles of the ecliptic by 23 deg. 29 min., with some variation not yet sufficiently studied by astronomers, - that depends on the magnetic energy. The causes of the precession of the equinoxes and of the progression of the fixed stars, as well as of change in the declinations of the sun and the tropics, are traceable to magnetic forces: hence we have no further need of Thebit Bencora's "movement of trepidation," which is at wide variance with observations. A rotating needle turns to conformity with the situation of the earth, and, though it be shaken oft, returns still to the same points. For in far northern climates, in latitude 70 to 80 deg. (whither in the milder season our seamen are wont to penetrate without injury from the cold), and in the middle regions, in the torrid zone under the equinoctial line, as also in all meritime regions and lands of the southern hemisphere, at the highest latitudes yet known, the magnetic needle ever finds its direction and ever tends in the same way (barring difference of variation) on this side of the equator where we dwell and in the other, the southern part, which, though less the lily of the mariner's compass ever points north. Of this, we are assured by the most illustrious navigators and by many intelligent seamen. The same was pointed out to me and confirmed by our most illustrious Neptune, Francis Drake, and by Thomas Candish [Cavendish], that other world explorer.
Our terrella teaches the same lesson. The proposition is demonstrated on a spherical loadstone. Let A, B be the poles; CD, an iron wire placed on the stone, always tends direct in the meridian to the poles A, B, whether the centre of the wire be in the middle line or equator of the stone, or whether it be in any other region between equator and poles, as H, G, F, E. So the point of a magnetized needle looks north on this side of the equator: on the other side the crotch is directed to the south; but the point or lily does not turn to the south below the equator, as somebody has thought. Some inexperienced persons, however, who, in distant regions below the equator, have at times seen the needle grow sluggish and less prompt, have deemed the distance from the Arctic pole or from the magnetic rocks to be the cause. But they are very much mistaken, for it has the same power and adjusts itself as quickly to the meridian as the point of variation in southern regions as in northern. Yet at times the movement appears to be slower, the point on which the compass needle is poised becoming in time, during a long voyage, rather blunt, or the magnetized needle itself having lost somewhat of its acquired force through age or from rusting. This, too, may be tested experimentally by poising the versorium of a sun-dial on a rather short-pointed needle rising perpendicularly out of the surface of the terrella. The magnetized needle turns to the poles of the terrella, and quits the earth's poles; for a general cause that is remote is overcome by a particular cause that is present and strong. Magnetized bodies incline of their own accord to the earth's position, and they conform to the terrella. Two loadstones of equal weight and force conform to the terrella in accordance with magnetic laws. Iron gets force from the loadstone and is made to conform to the magnetic movements. Therefore true direction is the movement of a magnetized body in the line of the earth's verticity toward the natural position and unition of both, their forms being in accord and supplying the forces. For we have, after many experiments in various ways, found that the disposing and ranging of the magnetized bodies depends on the differences of position, while the force that gives the motion is the one form common to both; also that in all magnetic bodies there is attraction and repulsion. For both the loadstone and the magnetized iron conform themselves, by rotation and by dip, to the common position of nature and the earth. And the earth's energy, with the force inhering in it as a whole, by pulling toward its poles and by repelling, arranges in order all magnetic bodies that are unattached and lying loose. For in all things do all magnetic bodies conform to the globe of earth in accordance with the same laws and in the same ways in which another loadstone or any magnetic body whatsoever conforms to the terrella.