_B._ Suppose there be a hill upon the earth, for example, at _r_; then the stream of the air which was between _z_ and _x_ westward, coming to the hill, shall go up the hill’s side, and so down to the other side, according to the crooked line which I have marked about the hill by points; and this infallibly will turn the north point of the needle, being on the east side, more towards the east, and that on the other side more towards the west, than if there had been no hill. And where upon the earth are there not eminences and depressions, except in some wide sea, and a great way from land. _A._ But if that be true, the variation in the same place should be always the same, for the hills are not removed. _B._ The variation of the needle at the same place is still the same; but the variation of the variation is partly from the motion of the pole itself, which by the astronomers is called _motus trepidationis_; and partly from that, that the variation cannot be truly observed, for the horizontal needle and the inclinatory needle incline alike, but cannot incline in due quantity. For whether set upon a pin or an axis, their inclination is hindered, in the horizontal needle, by the pin itself: if upon an axis, if the axis be just, it cannot move; if slack, the weight will hinder it; but chiefly because the north pole of the earth draws away from it the north pole of the needle, for two like poles cannot come together. And this is the cause why the variation in one place is east, and another west. _A._ This is indeed the most probable reason why the variation varies that ever I heard given; and I should presently acknowledge that this parallel motion of the axis of the earth in the ecliptic, supposed by Copernicus, is the true annual motion of the earth, but that there is lately come forth a book called _Longitude Found_ , which makes the magnetical poles distant from the poles of the earth eight degrees and a half. _B._ I have the book. It is far from being demonstrated, as you shall find, if you have the patience to see it examined. For wheresoever his demonstration is true, the conclusion, if rightly inferred, will be this, that the poles of the loadstone and the poles of the earth are the same. And where, on the contrary, his demonstrations are fallacies, it is because sometimes he fancieth the lines he hath drawn, not where they are; sometimes because he mistakes his station; and sometimes because he goes on some false principle of natural philosophy; and sometimes also because he knoweth not sufficiently the doctrine of spherical triangles. _A._ I think that is the book there which lies at your elbow. Pray you read.