That a loadstone attracts loadstone, iron and other magnetical bodies, has been shown above in the previous book, and also with what strength the magnetick coition is ordered; but now we must inquire how that vigour is disposed in a magnetick substance. And indeed an analogy must be inferred from a large loadstone. Any magnetick substance joins itself with a loadstone strongly, if the loadstone itself is strong; but more weakly, when it is somewhat imperfect or has been weakened by some flaw. A loadstone does not draw iron equally well with every part; or a magnetick substance does not approach every part of a loadstone alike; because a loadstone has its points, that is its true poles, in which an exceptional virtue excels. Parts nearer the pole are {72} stronger, those far away more weak, and yet in all the power is in a certain way equal. The poles of a terrella are A, B; the æquinoctial is C, D. At A and B the alluring force seems greatest. [Illustration] At C and D there is no force alluring magnetick ends to the body, for the forces tend toward both poles. But direction is powerful on the æquator. At C, D, the distances are equal from both poles; therefore iron which is at C, D, when it is allured in contrary ways, does not adhære with constancy; but it remains and is joined to the stone, if only it incline to the one or other side. At E there is a greater power of alluring than at F, because E is nearer the pole. This is not so because there is really greater virtue residing at the pole, but since all the parts are united in the whole, they direct their forces towards the pole. From the forces flowing from the plane of the æquinoctial towards the pole, the power increases. A fixed verticity exists at the pole, so long as the loadstone remains whole; if it is divided or broken, the verticity obtains other * positions in the parts into which it is divided. For the verticity always changes in consequence of any change in the mass, and for this cause, if the terrella be divided from A to B, so that there are two stones, the poles will not be A, B, in the divided parts, but F, G, and H, I. [Illustration] {73}