On the Magnet/III-4
CHAP. IIII.
Why Iron touched by a Loadstone acquires an opposite
verticity, and why iron touched by the true Northern side of a stone
turns to the North of the earth, by the true Southern side
to the South; and does not turn to the South when rubbed
by the Northern point of the stone, and when by
the Southern to the North, as all who have
written on the loadstone have
falsely supposed.
emonstration has already been given that the northern part of a loadstone does not attract the northern part of another stone, but the southern, and repels the northern part of another stone from its northern side when it is applied to it. That general magnet, the terrestrial globe, disposes iron touched by a loadstone in the same way, and likewise magnetick iron stirs this same iron by its implanted strength, and excites motion and controls it. For whether the comparison and experiment has been made between loadstone and loadstone, or loadstone and iron, or iron and iron, or the earth and loadstone, or the earth and iron conformed * by the earth or strengthened by the power of a loadstone, the strength and inclinations of each must mutually harmonize and accord in the same way. But the reason must be sought, why a piece of iron when touched by a loadstone acquires a disposition to motion toward the opposite pole of the earth, and not toward that pole of the earth to which that pole of that loadstone turned by which it was excited. It has been pointed out that iron and loadstone are of one primary nature; when the iron is joined to the loadstone, they become, as it were, one body, and not only is the end of the iron changed, but the remaining parts also are affected along with it. A, the north pole of a loadstone, is placed against the cusp of a piece of iron; the cusp of the iron has now become the southern part of the iron, 
because it is touching the northern part of the stone; the cross-end of the iron has become the northern. For if that contiguous magnetick substance be separated from the pole of the terrella, or from the parts near the pole, the one end (or the end which, whilst the connection was kept up, was touching the northern part of the stone) is the southern, whilst the other is the northern. So also if a versorium excited by a loadstone be divided into ever so many parts (however small), those parts when separated will, it is clear, arrange themselves in the same disposition as that in which they were disposed before, when they were undivided. Wherefore whilst the cusp remains over the northern pole A, it is not the southern end, but is, as it were, part of a whole; but when it is taken away from the stone, it is the southern end, because when rubbed it tended toward the northern parts of the stone, and the cross (the other end of the versorium) is the northern end. The loadstone and the iron make one body; B is the south pole of the whole; C (that is, the cross) is the northern end of the whole; divide the iron also at E, and E will be the southern end with respect to the cross; and E will likewise be the northern end in respect to B. A is the true northern pole of the stone and is attracted by the southern pole of the earth. The end of the iron which is touched by the true boreal part of the stone becomes the southern end, and turns to A, the north [pole] of the stone, if it be near; or if it be some distance from the stone it turns to the north [pole] of the earth. So always iron which is touched (if it is free and unrestrained) tends to the opposite part of the earth from that part to which the loadstone that touched it tends. Nor does it * make any difference how it is rubbed, whether straight up or slanting in some way. For in any case the verticity flows into the iron, provided it is touched by either end. 
Wherefore all the cusps at B acquire the same verticity, after they are separated, but opposite to that pole of the stone; wherefore also they are united to the loadstone at the pole B; and all the crosses in the present figure have the opposite verticity to the pole E, and are moved and laid hold of by E when they are in a convenient position. It is exactly the same in the case of the long stone F H divided at G; F and H always move, both in the whole and in the divided stone, to opposite poles of the earth, and O and P mutually attract one another, the one of them being the northern, the other the southern. For, supposing H to have been the southern in the whole stone and F the northern, P will be the northern with respect to H in the divided stone, and O the southern with respect to F. So also F and H mutually incline to a connection, if they are turned a very little toward one another, and run together at length and join. But supposing the division of the stone to have been meridional (that is, according to the line of a meridian, not of any parallel circle), then they turn Stone divided on a meridian. round, and A attracts B, and the end B is attracted to A and attracts A, until, 
being turned round, they are connected and cemented together; because magnetick attraction is not made along the parallels, but meridionally. For this reason pieces of iron placed on a terrella whose poles are A B, near the æquator along parallels, * do not combine or stick together firmly:
But if applied to one another along a meridian they are immediately * joined firmly together, not only on and near the stone, but even at some distance within the force of the controlling orbe. Thus they are joined and cemented together at E, but not at C in the other figure. For the opposite ends C and F meet and adhære together in the case of the iron just in the same way as A and B before in the case of the stone. But they are opposite ends, because the pieces of iron proceed from the opposite sides and poles of the terrella; and C in reference to the northern pole A is southern, and F is boreal in reference to the * southern pole B. In like manner also they are cemented together, if the rod C (being not too long) be moved further toward A, and F toward B, and they be joined together over the terrella, like A and B of the divided stone above. But now if the cusp A, * which has been touched by a loadstone, be the southern end, and you were to touch and rub with this the cusp of another iron needle B, which has not been touched, B will be northern, and will point to the south. But if you were to touch with the northern point B any other iron needle, still new, on its cusp, this again will be southern, and will turn to the north. The iron not only receives the necessary strength from the loadstone, if it be a good loadstone, but also imparts its acquired strength to another piece of iron, and the second to a third (always in strict accordance with magnetick laws). In all these demonstrations of ours it should always be borne in mind that the poles of a stone, as well as those of iron, whether touched or untouched, are always in fact and by nature opposite to the pole toward which they point and are so designated by us, as we have laid down above. For in them all it is always the northern * which tends to the south, either of the earth or of the stone, and the southern which tends to the north of the stone. Northern parts are attracted by the southern of the earth; so in the boat they tend toward the south. A piece of iron touched by the northern parts of a loadstone becomes south at the one end and tends always (if it is near and within the orbe of the loadstone) to the north of the stone, and if it be free and left to itself at some distance from the stone, it tends to the northern part of the earth. 
The northern pole A of a loadstone turns to G, the south of the earth; a versorium touched at its cusp by the part A follows A, because it has become southern. But the versorium C, placed farther away from the loadstone, turns its cusp to F, the north of the earth, because * the cusp has become southern by contact with the boreal part of the stone. So the ends touched by the northern part of the stone are made southern, or are excited with a southern polarity, and tend toward the north of the earth; those touched by the southern pole are made northern, or are excited with a northern force, and turn to the south of the earth.
Versoria orientated by the terrella or the earth.