On the Varying Ratio of Strength, and of the Motion of coition, within the orbe of virtue.
S hould a very large weight, which at a very small distance is drawn towards a loadstone, be divided into ever so many equal parts, and should the radius of the orbe of magnetick attraction be divided into the same number of parts, the like named parts of the weight will correspond to the intermediate parts of the radius.
The orbe of virtue extends more widely than the orbe of motion of any magnetick; for the magnetick is affected at its extremity, even if it is not moved with local motion, which effect is produced by the loadstone being brought nearer. A small versorium also is turned when a good distance off, even if at the same distance it would not flow towards the loadstone, though free and disengaged from impediment.
The swiftness of the motion of a magnetick body to a loadstone is dependent on either the power of the loadstone, on its mass, on its shape, on the medium, or on its distance within the magnetick orbe.
A magnetick moves more quickly towards a more powerful stone than towards a sluggish one in proportion to the strength, and by a comparison of the loadstones together. A lesser mass of iron also is carried more quickly towards a loadstone, just as also one that is a little longer in shape. The swiftness of magnetick motion towards a loadstone is changed by reason of the medium; for bodies are moved more quickly in air than in water, and in clear air than in air that is thick and cloudy.
By reason of the distance, the motion is quicker in the case of bodies near together than when they are far off. At the limits of the orbe of virtue of a terrella a magnetick is moved feebly and slowly. At very short distances close to the terrella the moving impetus is greatest.
A loadstone which in the outmost part of its orbe of virtue hardly moves a versorium when one foot removed from it, doth, if a long piece of iron is joined to it, attract and repel the versorium more strongly with its opposite poles when even three feet distant. The result is the same whether the loadstone is armed or unarmed. Let the iron be a suitable piece of the thickness of the little finger.
For the vigour of the loadstone excites verticity in the iron and proceeds in the iron and through the iron much further than it extends through the air.
The vigour proceeds even through several pieces of iron (joined to one another end to end), not so regularly, however, as through one continuous solid.
Dust of steel placed upon paper rises up when a loadstone is moved near above it in a sort of steely hairiness; but if the loadstone is placed below, such a hairiness is likewise raised.
Steel dust (when the pole of a loadstone is placed near) is cemented into one body; but when it desires coition with the loadstone, the mass is split and it rises in conglomerated parts.
But if there is a loadstone beneath the paper, the mass is split in the same way and many portions result, each of which consists of very many parts, and remains cemented together, as individual bodies. Whilst the lower parts of these pursue greedily the pole of the loadstone placed directly beneath, even they also are raised up as magnetick wholes, just as a small iron wire of the length of a grain or two grains of barley is raised up, both when the loadstone is moved near both beneath and above.
Why a Loadstone should be stronger in its poles in a different ratio; as well in the Northern regions as in the Southern.
T he extraordinary magnetick virtue of the earth is remarkably demonstrated by the subtility of the following magnetical experiment. Let there be given a terrella of no contemptible power, or a long loadstone with equal cones as polar extremities; but in any other shape which is not exactly round error is easy, and the experiment difficult. In the Northern regions, raise the true North pole of the terrella above the horizon straight toward the zenith; it is demonstrable that it raises up a larger iron spike on its North pole, than the South pole of the same terrella is able to raise, when turned in the same way toward the highest point of the sky. The same thing is shown by a small terrella placed in the same way above a larger.
Let a b be the earth or a somewhat large terrella, also a b a smaller terrella. There is set up above the Northern pole of the smaller terrella a spike larger than the pole b of the smaller terrella can raise, if it is turned round to the higher parts. And the pole a of the smaller terrella has its strength from the larger, declining from the Zenith to the plane of the horizon or to the level. But now, if, leaving the terrella disposed in the same way, you bring a piece of iron to the lower and Southern pole, it will attract and retain a greater weight than the Boreal pole could, if it were turned round to the lower parts. Which thing is demonstrated thus: let A be the earth or a terrella; E the Boreal pole or some place in some great latitude; B a rather large terrella above the earth or a smaller terrella on the top of a larger; D its Southern pole. It is manifest that D (the Southern pole) attracts a larger piece of iron, C, than F (the Boreal pole) will be able to, if it is turned round downward to the position D, toward the earth or the terrella in the Northern regions.
Magneticks acquire strength through magneticks, if they are properly placed according to their nature, in near neighbourhood and within the orbe of virtue. Wherefore when a terrella is placed on the earth or on a terrella, so that its Southern pole is turned round toward the Northern pole, its Northern pole, however, turned away from the Northern pole, the influence and strength of its poles are increased. And so the Northern pole of a terrella in such a position lifts up a larger spike than the Southern pole, if the Southern pole is turned away. Similarly the Southern pole in a proper and natural arrangement, acquiring strength from the earth or from a larger terrella, attracts and retains larger rods of iron. In the other part of the terrestrial globe toward the South, as also in the Austral portion of a terrella, the reasoning is converse; for the Southern pole of the terrella being turned away is more robust, as also the Northern pole when turned round. The more a region on the earth is distant from the æquinoctial (as also in a larger terrella), the larger is the accession of strength perceived; near the æquator, indeed, the difference is small, but on the æquator itself null; at the poles finally it is greatest.
On a Perpetual Motion Machine, mentioned by authors, by means of the attraction of a loadstone.
C ardan writes183 that out of iron and the Herculean stone can be made a perpetual motion machine; not that he himself had ever seen one, but only conceived the idea from an account by Antonius de Fantis184, of Treves. Such a machine he describes, Book 9, De Rerum Varietate. But they have been little practised in magnetick experiments who forge such things as that. For no magnetick attraction can be greater (by any skill or by any kind of instrument) than the retention. Things which are joined and those which are approaching near are retained with a greater force than those which are enticed and set in motion, and are moved; and that coition is, as we have shown above, a motion of both, not an attraction of one. Such a machine Peter Peregrinus feigned many centuries before or else depicted one which he had received from others, and one which was much better fitted for the purpose. Johannes Taysnier published it also, spoiled by wretched figures, and copied out the whole theory of it word for word. O that the gods would at length bring to a miserable end such fictitious, crazy, deformed labours, with which the minds of the studious are blinded!
How a more robust Loadstone may be recognized.
V ery powerful loadstones sometimes lift into the air a weight of iron equal to their own; a weak one barely attracts a slender wire. Those therefore are more robust which appeal to and retain larger bodies, if there is no defect in their form, or the pole of the stone is not suitably moved up. Moreover, when placed in a boat a keener influence turns its own poles round more quickly to the poles of the earth or the limits of variation on the horizon. One which performs its function more feebly indicates a defect and an effete nature. There must always be a similar preparation, a similar figure, and a like size; for in such as are very dissimilar and unlike, the experiment is doubtful. The method of testing the strength is the same also with a versorium in a place somewhat remote from a loadstone; for the one which is able to turn the versorium round at the greater distance, that one conquers and is held the more potent. Rightly also is the force of a loadstone weighed in a balance by B. Porta; a piece of loadstone is placed in one scale-pan, in the other just as much weight of something else, so that the scale-pans hang level. Soon a piece of iron lying on the table is adjusted so that it sticks to the loadstone placed in the scale, and they cling together most perfectly, according to their friendly points; into the other scale-pan sand is gradually thrown, and that until the scale in which the loadstone is placed is separated from the iron. Thus by weighing the weight of sand, the magnetick force becomes known. Similarly also it will be pleasing to try with another stone, in equilibrium, the weight of the sand being observed, and to find out the stronger by means of the weights of sand. Such is the experiment of Cardinal Cusan in his De Staticis185, from whom it would seem that B. Porta learnt the experiment. The better loadstones turn themselves round more quickly toward the poles or points of variation; then they also lead along and turn round more quickly, according to the greater quantity and mass of wood, a boat and other stuff. In a declination instrument, the more powerful force of a loadstone is looked for and required. Those therefore are more lively when they get through their work readily, and pass through and come back again with speed, and swiftly at length settle at their own point. Languid and effete ones move more sluggishly186, settle more tardily, adhære more uncertainly, and are easily disturbed from their possession.
Use of a Loadstone as it affects iron.
B y magnetick coition we test iron ore in a blacksmith's forge. It is burnt, broken in pieces, washed and dried, in which way it lays down its alien humours; in the bits collected from the washing is placed a loadstone, which attracts the iron dust to itself; this, being brushed off with feathers, is received in a crucible, and the loadstone is again placed in the bits collected from the washing, and the dust wiped off, as long as any remains which it will attract to itself. This is then heated in the crucible along with sal nitri187 until it is liquid, and from this a small mass of iron is cast. But if the loadstone draws the dust to itself quickly and readily, we conjecture that the iron ore is rich; if slowly, poor; if it seems altogether to reject it, there is very little iron in it or none at all. In like manner iron dust can be separated from another metal. Many tricks there are also, when iron is secretly applied to lighter bodies, and, being attracted by the motion of a loadstone which is kept out of sight, causes movements which are amazing to those who do not know the cause. Very many such indeed every ingenious mechanician will perform by sleight of hand, as if by incantations and jugglery188.
On Cases of Attraction in other Bodies.
V ery often the herd of philosophizers and plagiarists repeat from the records of others in natural philosophy opinions and errors about the attractions of various bodies; as that Diamond attracts iron, and snatches it away from a magnet; that there are various kinds or magnets, some which attract gold, others silver, brass, lead; even some which attract flesh, water, fishes. The flame of sulphur is said to seek iron and stones; so white naphtha is said to attract fire. I have said above that inanimate natural bodies do not attract, and are not attracted by, others on the earth, excepting magnetically or electrically. Wherefore it is not true that there are magnets which attract gold or other metals; because a magnetick substance draws nothing but magnetick substances. Though Fracastorio says that he has shown a magnet drawing silver; if this were true, it must have happened on account of iron skilfully mixed with that silver or concealed in it, or else because nature (as she does sometimes, but rarely) had mixed iron with the silver; iron indeed is rarely mixed with silver by nature; silver with iron very rarely or never. Iron is mixed with silver by forgers of false coin or from the avarice of princes in the coining of money, as was the case with the denarius of Antony189, provided that Pliny is recording a true incident. So Cardan (perhaps deceived by others) says that there is a certain kind of loadstone which draws silver; he adds a most foolish test of this: "If therefore" (he says) "a slender rod of silver be steeped in that in which a versatory needle has stood, it will turn toward silver (especially toward a large quantity) although it be buried; by this means anyone will be able easily to dig up concealed treasures." He adds that "it should be very good stone, such as he has not yet seen." Nor indeed will either he or anyone else ever see such a stone or such an experiment. Cardan brings forward an attraction of flesh, wrongly so named and very dissimilar from that of the loadstone; for his magnes creagus or flesh-magnet, from the experiment that it sticks to the lips, must be hooted out from the assembly of loadstones, or by all means from the family of things attractive. Lemnian earth, ruddle, and very many minerals do this, and yet they are fatuously said to attract. He will have it that there is another loadstone, as it were, a third species, into which, if a needle is driven and afterwards stuck into the body, it is not felt. But what has attraction to do with stupefaction, or stupor with a Philosopher's intellect, when he is discoursing about attraction? There are many stones, both found in nature and made by art, which have the power of stupefying. Sulphur flame is said by some to attract, because it consumes certain metals by its power of penetration. So white naphtha attracts flame, because it gives off and exhales an inflammable vapour, on which account it is kindled at some distance, just as the smoke of a recently extinguished candle takes fire again from another flame; for fire creeps to fire through an inflammable medium. Why the sucking fish Echineis or the Remora should stay ships has been variously treated by Philosophers, who are often accustomed to fit this fable (as many others) to their theories, before they find out whether the thing is so in nature. Therefore, in order that they may support and agree with the fatuities of the ancients, they put forward even the most fatuous ratiocinations and ridiculous problems, cliffs that attract, where the sucking fish tarry, and the necessity of some vacuum, I know not what, or how produced. Pliny and Julius Solinus make mention of a stone Chatochitis190. They say that it attracts flesh, and keeps hold of the hands, just as a loadstone does iron, and amber chaff. But that happens only from a stickiness and from glue contained in it, since it sticks more easily to the hands when they are warm. Sagda or Sagdo191, of the colour of a sard, is a precious stone mentioned by Pliny, Solinus, Albertus, and Evax192; they describe its nature and relate, on the authority of others, that it specially attracts wood to itself. Some even babble that woods cannot be wrenched away except they are cut off. Some also narrate that a stone is found which grows pertinaciously into ships, in the same way as certain testacea on long voyages. But a stone does not draw because it sticks; and if it drew, it would certainly draw shreds electrically, Encelius saw in the hands of a sailor such a stone of feeble virtue, which would hardly attract even the smallest twigs; and in truth, not of the colour of the sard. So Diamond, Carbuncle, Crystal, and others do attract. I pass over other fabulous stones; Pantarbe, about which Philostratus writes that it draws other stones to itself; Amphitane also, which attracts gold. Pliny in his origin of glass will have it that a loadstone is an attractor of glass, as well as of iron. For in his method of preparing glass, when he has indicated its nature, he subjoins this about loadstone. "Soon (such is the astute and resourceful craft) it was not content to have mixed natron; loadstone also began to be added, since it was thought to attract to itself the liquor of glass (as it does iron)." Georgius Agricola writes that to the material of glass (sand and natron) one part also of loadstone is added. "Because that force is believed, in our times just as in former times, to attract the liquor of glass to itself, as it attracts iron to itself, purges it when drawn, and makes clear glass from green or muddy; but the fire afterwards burns up the loadstone." It is true indeed that some sort of magnes (as the magnesia of the glass-makers imbued with no magnetick virtues) is sometimes put in and mixed with the material of the glass; not, however, because it attracts glass. But when a loadstone is burnt, it does not lay hold of iron at all, nor is iron when red-hot allured by any loadstone; and loadstone also is burnt up by more powerful fires and loses its attractive potency. Nor is this a function of loadstone alone in the glass furnaces; but also of certain pyrites and of some easily combustible iron ores, which are the only ones used by our glass-makers, who make clear, bright glass. They are mixed with the sand, ashes, and natron (just as they are accustomed to make additions in the case of metallick ores whilst they are smelted), so that when the material slows down into glass, the green and muddy colour of the glass may be purged by the penetrating heat. For no other material becomes so hot, or bears the fire for such a convenient time, until the material of the glass is perfectly fluid, and is at the same time burnt up by that ardent fire. It happens, however, sometimes, that on account of the magnetick stone, the magnesia, or the ore, or the pyrites, the glass has a dusky colour, when they resist the fire too much and are not burnt up, or are put in in too great quantity. Wherefore manufacturers are seeking for a stone suitable for them, and are observing also more diligently the proportion of the mixture. Badly therefore did the unskilful philosophy of Pliny impose upon Georgius Agricola and the more recent writers, so that they thought the loadstone was wanted by glass-makers on account of its magnetick strength and attraction. But Scaliger in De Subtilitate ad Cardanum, in making diamond attract iron, when he is discussing magneticks, wanders far from the truth, unless it be that diamond attracts iron electrically, as it attracts wood, straws, and all other minute bodies when it is rubbed. Fallopius reckons that quicksilver draws metals by reason of an occult property, just as a loadstone iron, amber chaff. But when quicksilver enters metals, it is wrongly called attraction. For metals imbibe quicksilver, just as clay water; nor do they do this unless they are touching, for quicksilver does not allure gold or lead to itself from afar, but they remain motionless in their places.
On Bodies which mutually repel one another.
W riters who have discoursed on the forces of bodies which attract others have also spoken about the powers of bodies which repel, but especially those who have instituted classes for natural objects on the basis of sympathy and antipathy. Wherefore it would seem necessary for us to speak also about the mutual strife of bodies, so that published errors should not creep further, and be received by all to the ruin of true philosophy. They say that, just as like things attract for the sake of preservation, so unlike and contrary things for the same purpose mutually repel and put one another to flight. This is evident in the reaction of many things, but it is most manifest in the case of plants and animals, which attract kindred and familiar things, and in like manner reject foreign and unsuitable things. But in other bodies there is not the same reason, so that when they are separated, they should come together by mutually attracting one another. Animals take food (as everything which grows), and draw it into their interior; they absorb the nourishment by certain parts and instruments (through the action and operation of the anima). They enjoy by natural instinct only the things set in front of them and near them, not things placed afar off; and this without any alien force or motion. Wherefore animals neither attract any bodies nor drive them away. Water does not repel oil (as some think) because the oil floats on water; nor does water repel mud, because the mud, if mixed in water, settles down in time. This is a separation of unlike bodies or such as are not perfectly mixed as respects the material; the separated bodies nevertheless remain joined without any natural strife. Wherefore a muddy sediment settles quietly on the bottom of vessels, and oil remains on the top of the water and is not sent further away. A drop of water remains intact on a dry surface, and is not expelled from the dry substance. Wrongly therefore do those who discourse on these matters infer an antipathy (that is, the force of repelling by contrary passions); for there is no repelling force in them; and repulsion comes193 from action, not from passion. But their greek vocables please them too much. We, however, must inquire whether there is any body which drives anything else further off without material impetus, as a loadstone attracts. But a loadstone seems even to repel loadstone. For the pole of one loadstone repels the pole of another, which does not agree with it according to nature; by repelling, it turns it round in an orbit so that they may exactly agree according to their nature. But if a somewhat weak loadstone, floating freely on water, cannot readily be turned round on account of impediments, the whole loadstone is repelled and sent further away from the other. All electricks attract all things: they never repel or propel anything at all194. As to what is related about certain plants (as about the cucumber, which turns aside when oil is applied to it), there is a material change from the vicinity, not a hidden antipathy. But when they show a candle flame put against a cold solid substance (as iron) turn away to the side, and allege antipathy as the cause, they say nothing. The reason of this they will see clearer than the day, when we discourse on what heat is195. But Fracastorio's opinion that a loadstone can be found, which would drive iron away, on account of some opposing principle lurking in the iron, is foolish.
On Direction.
O n referring to the earlier books it will be found shown that a loadstone has its poles, and that a piece of iron has also poles, and rotation, and a certain verticity; finally, that the loadstone and the iron direct their poles toward the poles of the earth. Now, however, we must make clear the causes of these things and their admirable workings, pointed out indeed before, but not proven. All those who have written before us about these rotations have left us their opinions so briefly, so meagrely, and with such hesitating judgment that they seem hardly likely ever to persuade anyone, or even to be able to satisfy themselves; and all their petty reasons are rejected by the more prudent as useless, uncertain, and absurd, being supported by no proofs or arguments; whence also magnetick science, being all the more neglected and not understood, has been in exile. The true austral pole of a loadstone, not the boreal (as all before us used to think), if the loadstone is placed in its boat on the surface of water, turns to the North; in the case of a piece of iron also, whether it has been excited by a loadstone or not, the southern end moves toward the North. An oblong piece of iron of three or four digits' length196, when skilfully rubbed with a loadstone, quickly turns north and south. Wherefore mechanicians, taking a piece of iron prepared in this way, balance it on a pin in a box, and fit it up with the requisites of a sun-dial; or they prepare the versorium out of two curved pieces of iron with their ends touching one another, so that the motion may be more constant. In this way the mariners' versorium is arranged, which is an instrument beneficial, useful, and auspicious to sailors for indicating, like a good genius, safety and the right way. But it must be understood on the threshold of this argument (before we proceed further) that these pointings of the loadstone or of iron are not perpetually made toward the true poles of the world, do not always seek those fixed and definite points, or remain on the line of the true meridian; but usually diverge some distance to the East or to the West. Sometimes also at certain places on land or sea they do indicate exactly the true poles. This discrepancy is called the Variation of the iron or of the loadstone; and since this is brought about by other causes, and is merely a certain disturbance and perversion of the true direction, we are directing our attention in this place to the true direction of the compass and of the magnetick iron (which would be equally toward the true poles and on the true meridian everywhere on the earth, unless other obstacles and an untoward pervertency hindered it). Of its variation and the cause of the perversion we shall treat in the next book. Those who wrote about the world and about natural philosophy a century ago, especially those remarkable elementary philosophers, and all those who trace their knowledge and training to them down to our own times, those men, I say, who represented the earth as always at rest and, as it were, a useless weight, placed in the centre of the universe at an equal distance from the sky on every side, and its nature to be simple, imbued only with the qualities of dryness and cold, sought diligently for the causes of all things and of all effects in the heavens, the stars, the planets, in fire, air, waters and substances of mixed natures. Never indeed did they recognize that the terrestrial globe had, besides dryness and cold, some special, effective, and predominant properties, strengthening, directing, and moving the globe itself through its whole mass and its very deepest vitals; nor did they ever inquire whether there were any such. For this reason the crowd of philosophizers, in order to discover the reasons of the magnetical motions, called up causes lying remote and far away. And one man seems to me beyond all others worthy of censure, Martin Cortes, who, since there was no cause which could satisfy him in the whole of nature, dreamed that there was a point of magnetical attraction beyond the heavens, which attracted iron. Peter Peregrinus thinks that the direction arises from the poles of the sky. Cardan thought that the turning of iron was caused by a star in the tail of the Great Bear; Bessard, the Frenchman, opines that a magnetick turns toward the pole of the zodiack. Marsilius Ficinus will have it that the loadstone follows its own Arctick pole; but that iron follows the loadstone, straws amber; whilst this perhaps follows the Antarctick pole — a most foolish dream. Others have recourse to I know not what magnetick rocks and mountains. Thus it is always customary with mortals, that they despise things near home, whilst foreign and distant things are dear and prized. But we study the earth itself and observe in it the cause of so great an effect. The earth, as the common mother, has these causes inclosed in her innermost parts; in accordance with her rule, position, condition, verticity, poles, æquator, horizons, meridians, centre, circumference, diameter, and the nature of the whole interior of her substance, must all magnetical motions be discussed. The earth has been ordered by the highest Artificer and by nature in such a way that it should have parts dissimilar in position, bounds of the whole and complete body, ennobled by certain functions, by which it might itself remain in a definite direction. For just as a loadstone, when it is floated on water in a suitable vessel, or is hung by slender threads in the air, by its implanted verticity conforms its poles to the poles of the common mother in accordance with magnetick laws; so if the earth were to deviate from its natural direction and its true position in the universe, or if its poles were to be drawn aside (if this were possible) toward the sun-rising or the sun-setting or toward any other points whatsoever in the visible firmament, they would return again to the north and south by magnetical motion, and would settle at the same points at which they are now fixed. The reason why the terrestrial globe seems to remain more steadily with the one pole toward those parts and directed toward the Cynosure, and why its pole diverges by 23 degrees 29 minutes, with a certain variation not sufficiently investigated as yet by Astronomers, from the poles of the ecliptick, depends on its virtue magnetical. The causes of the precession of the æquinoxes and the progression of the fixed stars, and of the change, moreover, in the declinations of the sun and of the tropicks, must be sought from magnetick influences; so that neither that absurd motion of trepidation of Thebit Bencora197, which is at great variance with observations, nor the monstrous superstructures of other heavens, are any longer needed. A versatory iron turns to the position of the earth, and if disturbed ever so often returns always to the same points. For in the far regions of the north, in a latitude of 70 or 80 degrees (to which at the milder seasons of the year our sailors are accustomed to penetrate without injury from the cold); in the regions halfway between the poles; on the æquator in the torrid zone; and again in all the maritime places and lands of the south, in the highest latitude which has thus far been reached, always the iron magnetick finds its way, and points to the poles in the same manner (excepting for the difference of variation); on this side of the æquator (where we live), and on the other side to the south, less well known, but yet in some measure explored by sailors: and always the lily of the compass points toward the North. This we have had confirmed by the most eminent captains, and also by very many of the more intelligent sailors. These facts have been pointed out to me and confirmed by our most illustrious Sea-god, Francis Drake, and by another circumnavigator of the globe, Thomas Candish; our terrella also indicates the same thing. This is demonstrated in the case of the .
orbicular stone, whose poles are A and B; an iron wire CD, which is placed upon the stone, always points directly along the meridian toward the poles AB, whether the centre of the wire is on the central line or æquator of the stone, or on any other part situated between the æquator and the poles, as at H, G, F, E. So the cusp of a versorium on this side of the æquator points toward the north; on the other side the cross is always directed toward the south; but the cusp or lily198 does not, as some one has thought, turn toward the south beyond the æquator. Some inexperienced people indeed, who in distant parts beyond the æquator have seen the versorium sometimes become more sluggish and less prompt, thought that the distance from the arctick pole or from the magnetick rocks was the cause of this. But they are very much mistaken; for it is as powerful199, and adjusts itself as quickly to the meridian or to the point of variation in the southern as in the northern parts of the earth. Yet sometimes the motion appears slower, namely, when the supporting pin by lapse of time and long voyaging has become somewhat blunt, or the magnetick iron parts have lost, by age or rust, some of their acquired vigour. This may also be shown experimentally by the versatory iron of a small sun-dial placed on a very short pin set perpendicular to the surface of the stone, for the iron when touched by a loadstone points toward the poles of the stone and leaves the poles of the earth; for the general and remoter cause is overcome by the particular and powerful cause which is so near at hand. Magnetick bodies have of themselves an inclination toward the position of the earth and are influenced by a terrella. Two equal stones of equal strength adjust themselves to a terrella in accordance with magnetick laws. The iron conceives vigour from the loadstone and is influenced by the magnetical motions. Wherefore true direction is the motion of a magnetick body in regard to the verticity of the earth, the natures of both agreeing and working together toward a natural position and unity. For indeed we have found out at length, by many experiments and in many ways, that there is a disposing nature, moving them together by reason of their various positions by one form that is common to both, and that in all magnetick substances there is attraction and repulsion. For both the stone200 and the magnetick iron arrange themselves by inclination and declination, according to the common position of their nature and the earth. And the force of the earth by the virtue of the whole, by attracting toward the poles, and repelling, arranges all magneticks which are unfixed and loose. For in all cases all magneticks conform themselves to the globe of the earth in the same ways and by the same laws by which another loadstone or any magneticks do to a terrella.
The Directive or Versorial Virtue (which we call verticity): what it is, how it exists in the loadstone; and in what way it is acquired when innate.
D irective force, which is also called by us verticity, is a virtue which spreads by an innate vigour from the æquator in both directions toward the poles. That power, inclining in both directions towards the termini, causes the motion of direction, and produces a constant and permanent position in Nature, not only in the earth itself but also in all magneticks. Loadstone is found either in veins of its own or in iron mines, when the homogeneous substance of the earth, either having or assuming a primary form, is changed or concreted into a stony substance, which besides the primary qualities of its nature has various dissimilitudes and differences in different quarries and mines, as if from different matrices, and very many secondary qualities and varieties in its substance. A loadstone which is dug out in this breaking up of the earth's surface and of protuberances upon it, whether formed complete in itself (as sometimes in China) or in a larger vein, is fashioned by the earth and follows the nature of the whole. All the interior parts of the earth mutually conspire together in combination and produce direction toward north and south. But those magnetical bodies which come together in the uppermost parts of the earth are not true united parts of the whole, but appendages and parts joined on, imitating the nature of the whole; wherefore when floating free on water, they dispose themselves just in the same way as they are placed in the terrestrial system of nature. We had a large loadstone of twenty pounds weight, dug up and cut out of its vein, after we had first observed and marked its ends; then after it was dug out, we placed it in a boat on water, so that it could turn freely; then immediately the face which had looked toward the north in the quarry began to turn to the north on the waves and at length settled toward that point. For that face which looked toward the north in the quarry is the southern, and is attracted by the northern
parts of the earth, . in the same way as pieces of iron which acquire their verticity from the earth. About this point we intend to speak afterwards202 under change of verticity. But there is a different rotation of the internal parts of the earth, which are perfectly united to the earth and which are not separated from the true substance of the earth by the interposition of bodies as are loadstones in the upper portion of the earth, which is maimed, corrupt, and variable. Let A B be a piece of magnetick ore; between which and the uniform globe of the earth lie various soils or mixtures which separate the ore to a certain extent from the globe of the true earth. It is therefore influenced by the forces of the earth just in the same way as C D, a piece of iron, in the air. So the face B of some ore or of that piece of it is moved toward the Boreal pole G, just as the extremity C of the iron, not A or D. But the condition of the piece E F is different, which piece is produced in one connected mass with the whole, and is not separated from it by any earthy mixture. For if the part E F were taken out and floated freely in a boat by itself, it is not E that would be directed toward the Boreal pole, but F. So in those substances which acquire their verticity in the air, C is the southern part and is seen to be attracted by the Boreal pole G. In the case of others which are found in the upper unstable portion of the earth, B is the south, and in like manner inclines toward the Boreal pole. But if those pieces deep down which are produced along with the earth are dug up, they turn about on a different plan. For F turns toward the Boreal parts of the earth, because is the southern part; E toward the south, because it is the northern. So of a magnetick body, C D, placed close to the earth, the end C turns toward the Boreal pole; of one that is adnate to it B A, B inclines to the North; of one that is innate in it, E F, E turns toward the southern pole; which is confirmed by the . following
demonstration, and comes about of necessity according to all magnetick laws. Let there be a terrella with poles A B; from its mass cut out a small part E F; if this be suspended by a fine thread above the hole or over some other place, E does not seek the pole A but the pole B, and F turns to A; very differently from a rod of iron C D; because C, touching some northern part of the terrella, being magnetically carried away makes a turn round to A, not to B. And yet here it should be observed, that if the pole A of the terrella were moved toward the earth's south, the end E of the piece cut out by itself, if not brought too near to the stone, would also move of itself toward the south. But the end C of the piece of iron, placed beyond its orbe of virtue, will turn toward the north. The part E F of the terrella, whilst in the mass, produced the same direction as the whole; but when it is separated and suspended by a thread, E turns to B, and F to A. So parts having the same
verticity with the whole, when separated, are impelled in the contrary direction; for contrary parts solicit contrary parts. Nor yet is this a true contrariety, but the highest concordancy, and the true and genuine conformation of bodies magnetical in the system of nature, if they shall have been divided and separated: for the parts thus divided should be raised some distance from the whole, as will be made clear afterwards. Magnetick substances seek a unity as regards form; they do not so much respect their own mass. Wherefore the part F E is not attracted into its former bed; but when once it is unsettled and at a distance, it is solicited by the opposite pole. But if the small piece F E is placed back again in its bed or brought close to, without any substances intervening, it acquires its former combination, and, as a part of the whole once more united, accords with the whole and sticks readily in its former position; and E remains toward A, and F toward B, and they settle steadily in their mother's lap. The reasoning is the same when the stone is divided into equal parts through the poles. A spherical stone is divided into two equal parts along the axis A B; whether therefore the surface A B is in the one part facing upward (as in the former diagram) or lying on its face in both parts (as in the latter), the end A tends toward B. But it must also be understood that the point A is not carried with a definite aim always toward the point B, because in consequence of the division the verticity proceeds to other points, as to F G, as appears in the fourteenth chapter of this book. And L M are now the axes in each, and A B is no longer the axis; for magnetick bodies, as soon as they are divided, become single magnetick wholes; and they have vertices in accordance with their mass, new poles arising at each end in consequence of the division. Yet the axis and the poles always follow the leading of a meridian; because that force passes along the meridians of the stone from the æquator to the poles, by an everlasting rule, the inborn virtue of the substance agreeing thereto from the long and lasting position and the facing of a suitable substance toward the poles of the earth; by whose strength continued through many centuries it has been fashioned; toward fixed and determined parts of which it has remained since its origin firmly and constantly turned.
How Iron acquires Verticity through a Loadstone, and how that verticity is lost and changed.
riction between an oblong piece of iron and a loadstone imparts to the former magnetick virtues, which are not corporeal nor inherent and persistent in any body, as we showed in the discussion on coition. It is plain that the iron, when it has been rubbed hard with one end and applied to the stone for a pretty long time, receives no stony nature, acquires no weight; for if, before the iron is touched by the stone, you weigh it in a small and very exact goldsmith's balance, you will see after the rubbing that it has exactly the same weight, neither diminished nor increased. But if you wipe the iron with cloths after it has been touched, or wash it in water, or scour it with sand or on a grindstone, still it in nowise lays aside its acquired strength. For the force is spread through the whole body and conceived in the inmost parts, and cannot in any way be washed or wiped away. Let an experiment then be made in fire, that untamed tyrant of nature. Take a piece of iron of the length of a palm and the thickness of a goosequill pen; let this iron be passed through a suitable round cork and placed on the surface of water, and observe the end which turns to the north; rub this particular end with the true southern end of a loadstone; the iron so rubbed turns toward the south. Remove the cork, and place the end which was excited in the fire until the iron is just red-hot; when it is cooled, it will retain the strength of the loadstone and the verticity, though it will not be so prompt, whether because the force of the fire had not yet continued long enough to overcome all its strength, or because the whole iron was not heated to redness, for the virtue is diffused through the whole. Remove the cork a second time, and putting the whole iron in the fire, blow the fire with the bellows, so that it may be all aglow, and let it remain a little longer time red-hot; when cooled (so, however, that, whilst it is cooling, it does not rest in one position), place it again on the water with the cork, and you will see that it has lost the verticity which it had acquired from the stone. From these experiments it is clear how difficult it is for the property of polarity implanted by the loadstone to be destroyed. But if a small loadstone had remained as long in the same fire, it would have lost its strength. Iron, because it does not so easily perish, and is not so easily burnt up as very many loadstones, retains its strength more stably, and when it is lost can recover it again from a loadstone; but a loadstone when burnt does not revive. But now that iron, which has been deprived of its magnetick form, moves in a different way from any other piece of iron, for it has lost its polar nature; and whereas before the touch of the loadstone it may have had a motion toward the north, and after contact toward the south; now it turns to no definite and particular point; but afterwards, very slowly and after a long time, it begins to turn in a doubtful fashion toward the poles of the earth (having acquired some power from the earth). I have said that the cause of direction was twofold, one implanted in the stone and iron, but the other in the earth, implanted by the disponent virtue; and for that reason (the distinction of poles and the verticity in the iron having now been destroyed) a slow and weak directive power is acquired anew from the verticity of the earth. We may see, therefore, with what difficulty and only by the application of hot fires and by long ignition of the iron heated to softness, the imparted magnetick virtue is eradicated. When this ignition has overcome the acquired polarity, and it has been now completely subdued and not awakened again, that iron is left unsettled and utterly incapable of direction. But we must further inquire how iron remains affected by verticity. It is manifest that it strongly affects and changes the nature of the iron, because the presence of a loadstone attracts the iron to itself with an altogether wonderful readiness. Nor is it only the part that is rubbed, but on account of the rubbing (on one end only) the whole iron is affected together, and gains by it a permanent though an unequal power. This is demonstrated as follows. Rub an iron wire on the end so that it is excited, and it will turn towards the north; afterward cut off some portion of it; you will see that it still turns toward the north (as before), but more feebly. For it must be understood that the loadstone excites a steady verticity in the whole iron (if the rod be not too long) more vigorous throughout the whole mass in a shorter bar, and as long as the iron remains touching the loadstone a little stronger. But when the iron is separated from contact with it, then it becomes much weaker, especially in the end that was not touched. Just as a long rod, one end of which is placed in the fire and heated, grows exceedingly hot at that end, less so in the parts adjoining and in the middle, whilst at the other end it can be held in the hand, and that end is only warm; so the magnetical vigour diminishes from the excited end to the other end; but it is present there instantly, and does not enter after an interval of time nor successively, as the heat in the iron; for as soon as a piece of iron has been touched by a loadstone it is excited throughout its whole length. For the sake of experiment, let there be a rod of iron 4 or 5 digits long, untouched by a loadstone; as soon as you touch one end only with a loadstone, the opposite end immediately, or in the twinkling of an eye, by the power that it has conceived, repels or attracts a versorium, if it be applied to it ever so quickly.