_B._ Such iron were indeed a very vigorous loadstone. But the cause of it is the same that causeth fire or hot water, which have the same compounded motion, to work through a vessel of brass. For though the motion be altered by restraint within the heterogeneous body, yet being continued quite through, it restores itself. _A._ What is the cause why the iron rubbed over by a loadstone will receive the virtue which the loadstone hath of drawing iron to it? _B._ Since the motion that brings two bodies to meet must have contrary ways, and that the motions of the internal parts of the magnet and of the iron are contrary; the rubbing of them together does not give the iron the first endeavour to rise, but multiplies it. For the iron untouched will rise to a loadstone; but if touched, it becomes a loadstone to other iron. For when they touch a piece of iron, they pass the loadstone over it only one way, viz. from pole to pole; not back again, for that would undo what before had been done; also they press it in passing to the very end of the iron, and somewhat hard. So that by this pressing motion all the small circles about the points _a b c_, are turned the contrary way; and the halves of those small circles made on the arch D B will be taken away and the poles changed, so as that the north poles shall point south, and the south poles north, as in the figure. _A._ But how comes it to pass, that when a loadstone hath drawn a piece of iron, you may add to it another, as if they begat one another? Is there the like motion in the generation of animals? _B._ I have told you that iron of itself will rise to the loadstone; much more then will it adhere to it when it is armed with iron, and both it and the iron have a plain superficies. For then not only the points of contact will be many, which make the coherence stronger, but also the iron wherewith it is armed is now another loadstone, differing a little, which you perhaps think, as male and female. But whether this compounded motion and confrication causeth the generation of animals, how should I know, that never had so much leisure as to make any observation which might conduce to that? _A._ My next question is, seeing you say the loadstone, or a needle touched with it, naturally respecteth the poles of the earth, but that the variation of it proceedeth from some accidents in the superficies of the earth; what are those accidents?