On the Axis and Magnetick Poles.
L et the line be called the axis which is drawn in the earth (as in a terrella) through the centre to the poles. They are called πόλοι by the Greeks from πολεῖν, to turn, and by the Latins they are also called Cardines or Vertices; because the world rotates and is perpetually carried around them. We are about to show, indeed, that the earth and a terrella are turned about them by a magnetick influence. One of them in the earth, which looks towards the Cynosure, is called Boreal and Arctic; the other one, opposite to this, is called Austral and Antarctic. Nor do these also exist on the earth or on a terrella for the sake of the turning merely; but they are also limits of direction and position, both as respects destined districts of the world, and also for correct turnings among themselves.
Why at the Pole itself the Coition is stronger than in the other parts intermediate between the æquator and the pole; and on the proportion of forces of the coition in various parts of the earth and of the terrella.
O bservation has already been made that the highest power of alluring exists in the pole, and that it is weaker and more languid in the parts adjacent to the æquator. And as this is apparent in the declination, because that disponent and rotational virtue has an augmentation as one proceeds from the Æquator towards the poles: so also the coition of magneticks grows increasingly fresh by the same steps, and in the same proportion. For in the parts more remote from the poles the loadstone does not draw magneticks straight down towards its own viscera; but they tend obliquely and they allure obliquely. For as the smallest chords in a circle differ from the diameter, so much do the forces of attracting differ between themselves in different parts of the terrella. For since attraction is coition towards a body, but magneticks run together by their versatory tendency, it comes about that in the diameter drawn from pole to pole the body appeals directly, but in other places less directly. So the less the magnetick is turned toward the body, the less, and the more feebly, does it approach
and adhære. Oblique magnetics. Just as if A B were the poles and a bar of iron or a magnetick fragment C is allured at the part E; yet the end laid hold of does not tend towards the centre of the loadstone, but verges obliquely towards the pole; and a chord drawn from that end obliquely as the attracted body tends is short; therefore it has less vigour and likewise less inclination. But as a greater chord proceeds from a body at F, so its action is stronger; at G still longer; longest at A, the pole (for the diameter is the longest way) to which all the parts from all sides bring assistance, in which is constituted, as it were, the citadel and tribunal of the whole province, not from any worth of its own, but because a force resides in it contributed from all the other parts, just as all the soldiers bring help to their own commander. Wherefore also a slightly longer stone attracts more than a spherical one, since the length from pole to pole is extended, even if the stones are both from the same mine and of the same weight and size. The way from pole to pole is longer in a longer stone, and the forces brought together from other parts are not so scattered as in a round magnet and terrella, and in a narrow one they agree more and are better united, and a united stronger force excels and is preeminent. A much weaker office, however, does a plane or oblong stone perform, when the length is extended according to the leading of the parallels, and the pole stops neither on the apex nor in the circle and orbe, but is spread over the flat. Wherefore also it invites a friend wretchedly, and feebly retains him, so that it is esteemed as one of an abject and contemptible class, according to its less apt and less suitable figure.
The Magnetick Virtue which is conceived in Iron is more apparent in an iron rod than in a piece of iron that is round, square, or of other figure.
D uly was it said before that the longer magnet attracts the greater weight of iron167; so also in a longish piece of iron which has been touched the magnetick force conceived is stronger when the poles exist at the ends. For the magnetick forces which are driven from the whole in every part into the poles are not scattered but united in the narrow ends. In square and other angular figures the influence is dissipated, and does not proceed in straight lines or in convenient arcs. Suppose also an iron globe have the shape of the earth, yet for the same reasons it drags magnetick substances less; wherefore a small iron sphere, when excited, draws another piece of iron more sluggishly than an excited rod of equal weight.
Showing that Movements take place by the Magnetical Vigour though solid bodies lie between; and on the interposition of iron plates.
F loat a piece of iron wire on the surface of water by transfixing it through a suitable cork; or set a versatory piece of iron on a pin or in a seaman's compass (a magnet being brought near or moved about underneath), it is put into a state of motion; neither the water, nor the vessel, nor the compass-box offering resistance in any way. Thick boards do not obstruct168, nor earthen vessels nor marble vases, nor the metals themselves; nothing is so solid as to carry away or impede the forces excepting an iron plate. Everything which is interposed (even though it is very dense) does not carry away its influence or obstruct its path, or indeed in any way hinder, diminish, or retard it. But all the force is not suppressed by an iron plate, but it is in some measure diverted aside. For when the vigour passes into the middle of an iron plate within the orbe of the magnetick virtue or placed just opposite the pole of the stone, that virtue is scattered in very large measure towards its extremities; so that the edges of a small round plate of suitable size allure iron wires on every side. This is also apparent in the case of a long iron wand, which, when it has been touched by a magnet in the middle, has a like verticity at either end.
B is a loadstone, C D a long rod magnetized in the middle A; E being the Boreal pole; C is an Austral end or pole; in like manner also the end D is another Austral pole. But observe here the exactness with which a versorium touched by a pole, when a round plate is interposed, turns towards the same pole in the same way as before the interposition, only weaker; the plate not standing in the way, because the vigour is diverted through the edges of the small plate, and passes out of its straight course, but yet the plate retains in the middle the same verticity, when it is in the neighbourhood of that pole, and close to it; wherefore the versorium tends towards the plate, having been touched by the same pole. If a loadstone is rather weak, a versorium hardly turns when a plate is put in between; for the vigour of the rather weak loadstone, being diffused through the extremities, passes less through the middle. But if the plate has been touched in this way by a pole in the middle and has been removed from the stone outside its orbe of virtue, then you will see the point of the same versorium tend in the contrary direction and desert the centre of the small plate, which formerly it desired; for outside the orbe of virtue it has an opposite verticity, in the vicinity the same; for in the vicinity it is, as it were, a part of the loadstone, and has the same pole.
A is an iron plate near the pole, B a versorium which tends with its point towards the centre of the small plate, which has been touched by the pole of the loadstone C. But if the same small plate be placed outside the orbe of magnetick virtue, the point will not turn towards its centre, but the cross E of the same versorium does. But an iron globe interposed (if it is not too large) attracts the point of the iron on the other side of the stone. For the verticity of that side is the same as that of the adjoining pole of the stone. And this turning of the cusp (that is, of the end touched by that pole) as well as of the cross-end, at a greater distance, takes place with an iron globe interposed, which would not happen at all if the space were empty, because the magnetick virtue is passed on and continued through magnetick bodies.
A is a terrella, B an iron globe; between the two bodies is F, a versorium whose point has been excited by the pole C. In the other figure A is a terrella, C its pole, B an iron globe; where the versorium tends towards C, the pole of the terrella, through the iron globe. So a versorium placed between a terrella and an iron globe vibrates more forcibly towards the pole of the terrella; because the loadstone sends an instantaneous verticity into the opposite globe. There is the same efficiency in the earth, produced from the same cause. For if a revolvable needle is shut up in a rather thick gold box (this metal indeed excels all others in density) or a glass or stone box, nevertheless that magnetick needle has its forces connected and united with the influences of the earth, and the iron will turn freely and readily (unhindered by its prison) to its desired points, North and South. It even does this when shut up in iron caverns, if they are sufficiently spacious. Whatever bodies are produced among us, or are artificially forged from things which are produced, consist of matter of the terrestrial globe; nor do those bodies hinder the prime forces of nature which are derived from their primary form, nor can they resist them except by contrary forms. But no forms of mixed bodies are inimical to the primary implanted earth-nature, although some often do not agree169 with one another. But in the case of all those substances which have a material cause for their inclining (as amber, jet, sulphur), their action is impeded by the interposition of a body (as paper, leaves, glass, or the like) when that way is impeded and obstructed, so that that which exhales170 cannot reach the corpuscle to be allured. Terrestrial and magnetick coition and motion, when corporeal impediments are interposed, is demonstrated also by the efficiencies of other chief bodies due to their primary form. The moon (more than all the stars) agrees with internal parts of the earth on account of its nearness and similarity in form. The moon produces the movements of the waters and the tides of the sea; twice it fills up the shores and empties them whilst it moves from a certain definite point in the sky back to the same point in a daily revolution. This motion of the waters is incited and the seas rise and fall no less when the moon is below the horizon and in the lowest part of the heavens, than if it had been raised at a height above the horizon. So the whole mass of the earth interposed171 does not resist the action of the moon, when it is below the earth; but the seas bordering on our shores, in certain positions of the sky when it is below the horizon, are kept in motion, and likewise stirred by its power (though they are not struck by its rays nor illuminated by its light), rise, come up with great force, and recede. But about the reason of the tides anon172; here let it suffice to have merely touched the threshold of the question. In like manner nothing on the earth can be hidden from the magnetick disposition of the earth or of the stone, and all magnetical bodies are reduced to order by the dominant form of the earth, and loadstone and iron show sympathy with a loadstone though solid bodies be interposed.
On the Iron Cap of a Loadstone, with which it is armed at the pole (for the sake of the virtue) and on the efficacy of the same.
C onceive a small round plate, concave in shape, of the breadth of a digit to be applied to the convex polar surface of a loadstone and skilfully attached; or a piece of iron shaped like an acorn, rising from the base into an obtuse cone, hollowed out a little and fitted to the surface of the stone, to be tied to the loadstone. Let the iron be the best steel, smoothed, shining, and even. A loadstone with such an appliance, which before only bore four ounces of iron, will now raise twelve. But the greatest force of a combining or rather united nature is seen when two loadstones, armed with iron caps, are so joined by their concurrent (commonly called contrary) ends, that they mutually attract and raise one another. In this way a weight of twenty ounces is raised, when either stone unarmed would only allure four ounces of iron. Iron unites to an armed loadstone more firmly than to a loadstone; and on that account raises greater weights, because the pieces of iron stick more pertinaciously to one that is armed. For by the near presence of the magnet they are cemented together, and since the armature173 conceives a magnetick vigour from its presence and the other conjoined piece of iron is at the same time endued with vigour from the presence of the loadstone, they are firmly bound together. Therefore by the mutual contact of strong pieces of iron, the cohesion is strong. Which thing is also made clear and is exhibited by means of rods sticking together, Bk. 3, chap 4174; and also when the question of the concretion of iron dust into a united body was discussed. For this reason a piece of iron set near a loadstone draws away any suitable piece of iron from the loadstone, if only it touch the iron; otherwise it does not snatch it away, though in closest proximity. For magnetick pieces of iron within the orbe of virtue, or near a loadstone, do not rush together with a greater endeavour175 than the iron and the magnet; but joined they are united more strongly and, as it were, cemented together, though the substance remain the same with the same forces acting.
An armed Loadstone does not endow an excited piece of Iron with greater vigour than an unarmed.
S uppose there are two pieces of iron, one of which has been excited by an armed loadstone, the other by one unarmed; and let there be applied to one of them another piece of iron of a weight just proportional to its strength, it is manifest that the remaining one in like manner raises the same and no more. Magnetick versoria also touched by an armed loadstone turn with the same velocity and constancy towards the poles of the earth as those magnetized by the same loadstone unarmed.
union with an armed Loadstone is stronger; hence greater weights are raised; but the coition is not stronger176, but generally weaker.
A n armed magnet raises a greater weight, as is manifest to all; but a piece of iron moves towards a stone at an equal, or rather greater, distance when it is bare, without an iron cap. This must be tried with two pieces of iron of the same weight and figure at an equal distance, or with one and the same versorium, the test being made first with an armed, then with an unarmed loadstone, at equal distances.
An armed Loadstone raises an armed Loadstone, . which also attracts a third; which likewise happens, though the virtue in the first be somewhat small.
M agnets armed cohære firmly when duly joined, and accord into one; and though the first be rather weak, yet the second one adhæres to it not only by the strength of the first, but of the second, which mutually give helping hands; also to the second a third often adheres and in the case of robust stones, a fourth to the third.
If Paper or any other Medium be interposed, an armed loadstone raises no more than an unarmed one.
O bservation has shown above that an armed loadstone does not attract at a greater distance than an unarmed one; yet raises iron in greater quantity, if it is joined to and made continuous with the iron. But if Paper be placed between, that intimate cohæsion of the metal is hindered, nor are the metals cemented together at the same time by the operation of the magnet.
That an armed Loadstone draws Iron no more than an unarmed one: And that an armed one is more strongly united to iron is shown by means of an armed loadstone and a polished cylinder of iron.
figure
I f a cylinder be lying on a level surface, of too great a weight for an unarmed loadstone to lift, and (a piece of paper being interposed) if the pole of an armed loadstone be joined to the middle of it; if the cylinder were drawn from there by the loadstone, it would follow rolling; but if no medium were interposed, the cylinder would be drawn along firmly united with the armed loadstone, and in no wise rolling. But if the same loadstone be unarmed, it will draw the cylinder rolling with the same speed as the armed loadstone with the paper between or when it was wrapped in paper.
Armed loadstones of diverse weights, of the same ore vigour and form, cling and hang to pieces of iron of a convenient size and proportionate figure with an equal proportion of strength. The same is apparent in the case of unarmed stones. A suitable piece of iron being applied to the lower part of a loadstone, which is hanging from a magnetick body, excites its vigour, so that the loadstone hangs on more firmly. For a pendent loadstone clings more firmly to a magnetick body joined to it above with a hanging piece of iron added to it, than when lead or any other non-magnetick body is hung on.
A loadstone, whether armed or unarmed, joined by its proper pole to the pole of another loadstone, armed or unarmed, makes the loadstone raise a greater weight by the opposite end177. A piece of iron also applied to the pole of a magnet produces the same result, namely, that the other pole will carry a greater weight of iron; just as a loadstone with a piece of iron superposed on it (as in this figure) holds up a piece of iron below, which it cannot hold, if the upper one be removed. Magneticks in conjunction make one magnetick. Wherefore as the mass increases, the magnetick vigour is also augmented.
An armed loadstone, as well as an unarmed one, runs more readily to a larger piece of iron and combines more firmly with a larger piece than with a lesser one.