On the Magnet

...latitude, even in the most obscure night and the thickest fog, it can be most easily known through the instrument of declination. Therefore, to return our speech at last to you, most grave and learned Mr. Gilbert (whom I willingly acknowledge as my teacher in this magnetic philosophy), if these books of yours on the Magnet held nothing else besides this discovery of latitude from magnetic declination, now brought to light by you for the first time, our British, French, Belgian, and Danish ship captains, about to enter the British Sea or the Herculean Strait the Strait of Gibraltar under a dark sky, would deservedly judge them to be worth not a little gold. That discovery of yours regarding the entire magnetic terrestrial globe, although perhaps to most it will seem παραδοξότατον most paradoxical even to the point of amazement, is nevertheless so solidly fortified and confirmed on all sides by so many experiments and those so applicable and accommodated to the matter (Book 2, Chapter 34; Book 3, Chapters 4 and 12; and in almost the entire fifth book) that no room for doubt or contradiction is left. I come, therefore, to that cause of magnetic variation which has hitherto vexed the minds of all learned men; which no mortal has ever produced more probably than that which is now brought forth for the first time in these books of yours on the Magnet. The ὀρθοβορεοδειξίς correct north-pointing of the magnetic index in the middle of the ocean and in the middle of continents (or at least in the middle of their more valid and eminent parts), but near the shores its inclination to them by land and sea, agreeing with the experiments of the terrella little earth/model magnet (unequal in the likeness of the terrestrial globe, and in some parts eminent, or weak, or decayed, or imperfect in some other way), demonstrated in Book 4, Chapter 2, truly and very probably proves that that variation is nothing else but a certain deviation of the magnetic iron toward those stronger and more eminent parts of the Earth. From whence the reason is also easily evident for that irregularity which is mostly seen in magnetic variations, from the inequality and anomaly of those eminences and terrestrial forces. Nor indeed do I doubt that even all those who have either devised or admitted some attractive or respective points in the sky of the Earth, or mountains, cliffs, or magnetic poles, will immediately falter as soon as they have finished reading these books of yours on the Magnet, and will willingly come over to your opinion. As for those things which you discuss at the end concerning the circular motion of the Earth and the terrestrial poles, although they will perhaps seem very opinionated to some, I do not see, however, why they should not deserve some indulgence even among those who do not acknowledge the spherical motion of the Earth; since not even they can easily extricate themselves from the many difficulties that follow from the daily motion of the entire heaven. For first, it is done in vain through more what can be done through fewer, and in vain the whole heaven, and all the spheres...