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._
Demonstration 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[203] 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 { 126} 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, [Illustration] 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, { 127} [Illustration] 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.