Fourthly, That at the Eastward of Brasile, properly so call'd, this Easterly Variation decreases, so as to be very little at St. Helena and Ascension, and to be quite gone, and the Compass Point true about 18 Degrees of Longitude West from the Cape of Good-hope.
Fifthly, That to the Eastward of the aforesaid Places a Westward Variation begins, which Reigns in the whole Indian Sea, and arises to no less than Eighteen Degrees under the Equator it self, about the Meridian of the Northern part of Madagascar; and near the same Meridian, but in 39 Degrees South Latitude it is found full 27½ Degrees: From thence Easterly the West Variation decreases, so as to be little more than eight Degrees at Cape Comorin, and than three Degrees upon the Coast of Java; and to be quite extinct about the Molucca Islands, as also a little to the Westwards of Van Diemens Land found out by the Dutch in 1642.
Sixthly, That to the Eastward of the Molucca's and Van Diemens Land in South Latitude there arises another Easterly Variation, which seems not so great as the former, nor of so large Extent; for that at the Island Rotterdam it is sensibly less than upon the East Coast of New Guinea; and, at the rate it decreases, it may well be suppos'd, that about 20 Degrees farther East, or 225 Degrees East Longitude from London, in the Latitude of 20 Degrees South, a Westerly Variation begins.
Seventhly, That the Variations observ'd by the Honourable Sir John Norborough at Baldivia, and at the West Entrance of the Straights of Magellan do plainly shew, that That East Variation, noted in our third Remark, is decreasing apace; and that it cannot reasonably extend many Degrees into the South Sea from the Coast of Peru and Chili, leaving room for a small Westerly Variation, in that Tract of the unknown World that lies in the mid-way between Chili and New-Zealand, and between Hounds-Island and Peru.
Eighthly, That in Sailing North-West from St. Helena by Ascension, as far as the Equator, the Variation continues very small East, and as it were constantly the same: So that in this part of the World the Course, wherein there is no Variation, is evidently no Meridian, but rather North-West.
Ninthly, That the Entrance of Hudson's Straights, and the Mouth of the River of Plate, being nearly under the same Meridian, at the one place the Needle varies 29½ Degrees to the West; at the other 20½ Degrees to the East. This plainly demonstrates the impossibility of reconciling these Variations by the Theory of Bond; which is by two Magnetical Poles and an Axis, inclin'd to the Axis of the Earth; from whence it would follow, That under the same Meridian the Variation should be in all places the same way.
These things being premised may serve as a sure Foundation to raise the Superstructure of a Theory upon. But first it would not be amiss to shew hereby the mistake of Gilbert and Des Cartes: The first whereof supposes, that the Earth it self being in all its parts Magnetical, and the Water not; wheresoever the Land is, thither also should the Needle turn, as to the greater quantity of Magnetical Matter. But this in many Instances is not true; but most remarkably upon the Coast of Brazile, where the Needle is so far from being attracted by the Land, that it turns the quite contrary way, leaving the Meridian to lye N b E, which is just along the Coast. As to the Position of Des Cartes, that the Iron and Loadstones hid in the Bowels of the Earth and the Bottom of the Sea, may be the Causes that the Needle varies; if we consider for how great a part of the Earths Surface, ex. gr. in the whole Indian Sea, the Needle declines the same way, and that regularly, 'twill follow that the attracting Substance that occasions it, must be very far distant. Now by Experience we find the little force that Iron Guns have upon the Compass in Ships (their Vertue, though they be Demiculverin, or greater Cannon, being not perceptible at four or five Yards distance) and the Experiments now before the Royal Society do plainly shew, how little a Magnetism there is in most crude Iron Oars: What quantity thereof must be then suppos'd to make so powerful a Diversion at two or three Thousand Miles distance? Yet I cannot deny that in some places near the Shoar, or in Shoal-Water, the Needle may be irregularly directed from the aforesaid Causes, and that not a little, as Gassendus gives a notable instance of the Island Elba in the Mediterranean Sea: But these differences from the general Direction are always signs of the nearness of those Magnetical Substances, for the Production whereof that Island Elba has been famous from all Antiquity. Besides, against both Des Cartes and Gilbert, the change of the Variation, which has been within these Hundred Years last past more than 15 gr. at London, is an entire Demonstration; tho' Des Cartes does not stick to say, that the transportation of Iron from place to place, and the growth of new Iron within the Earth, where there was none before, may be the cause thereof. The same holds likewise against the Hypothesis of Magnetical Fibres, which Kircher maintains.
Now to propose something that may answer the several appearances, and introduce nothing strange in Philosophy, after a great many close Thoughts, I can come to no other Conclusion than that, The whole Globe of the Earth is one great Magnet, having four Magnetical Poles, or Points of Attraction, near each Pole of the Equator. Two; and that, in those parts of the World which lie near adjacent to any one of those Magnetical Poles, the Needle is govern'd thereby, the nearest Pole being always predominant over the more remote. The parts of the Earth wherein these Magnetical Poles lie, cannot as yet be exactly determin'd for want of sufficient Data to proceed Geometrically; but, as near as Conjecture can reach, I reckon that the Pole, which is at present nearest to us, lies in or near the Meridian of the Lands-end of England, and not above seven Degrees from the Pole Arctick; by this Pole the Variations in all Europe and Tartary, and the North Sea are principally govern'd, though with regard to the other Northern Pole, whose situation is in a Meridian passing about the middle of California, and about 15 gr. from the North Pole of the World; to this the Needle has chiefly respect in all the North America, and in the two Oceans on either side thereof, from the Azores Westward to Japan, and farther. The two Southern Poles are rather farther distant from the South Pole of the World: The one about sixteen Degrees therefrom, is in a Meridian, some twenty Degrees to the Westward of Magellan Straights, or ninety five Degrees West from London: This commands the Needle in all the South-America, in the Pacifick Sea, and the greatest part of the Ethiopick Ocean. The Fourth and last Pole seems to have the greatest Power, and largest Dominions of all, as it is the most remote from the Pole of the World, being little less than 20 Degrees distant therefrom in the Meridian, which passes through Hollandia Nova, and the Island Celebes about one hundred and twenty Degrees East from London; this Pole is predominant in the South part of Africa, in Arabia and the Red Sea, in Persia, India, and its Islands, and all over the Indian Sea, from the Cape of Good-Hope Eastwards to the middle of the great South Sea, that divides Asia from America. This seems to be the present Disposition of the Magnetical Vertue throughout the whole Globe of the Earth; it remains to shew how this Hypothesis makes out all the Variations that have been observ'd of late; and how it answers to our several Remarks drawn from the Table. And first it is plain, that (our European North Pole being in the Meridian of the Lands-end of England) all places more Easterly than that will have it on the West side of their Meridian, and consequently the Needle, respecting it with its Northern Point, will have a Westerly Variation, which will still be greater as you go to the Eastwards, till you come to some Meridian of Russia, where 'twill be greatest, and from thence decrease again. Thus at Brest the Variation is but 1¾ Degrees, at London 4½ Degrees; but at Dantzick seven Degrees West. To the Westward of the Meridian of the Lands-end, the Needle ought to have an Easterly Variation; were it not that (by approaching the American Northern Pole, which lies on the West side of the Meridian, and seems to be of greater force than this other) the Needle is drawn thereby Westwards, so as to counterballance the Direction given by the European Pole, and to make a small West Variation in the Meridian of the Lands-end it self. Yet I suppose that about the Meridian of the Isle Tercera, our nearest Pole may so far prevail as to give the Needle a little turn to the East, though but for a very small space: The Counterballance of those two Poles permitting no considerable Variation in all the Eastern Parts of the Atlantick Ocean; nor upon the West Coasts of England and Ireland, France, Spain and Barbary. But to the Westwards of the Azores the Power of the American Pole overcoming that of the European, the Needle has chiefly respect thereto, and turns still more and more towards it as you approach it. Whence it comes to pass, that on the Coast of Virginia, New-England, New-found-Land, and in Hudson's-Straights the Variation is Westward; that it decreases as you go from thence towards Europe, and that it is less in Virginia and New-England, than in New-found-Land, and Hudson's-Straights. This Westerly Variation again decreases, as you pass over the North America; and about the Meridian of the middle of California the Needle again points due North; and from thence Westward to Yedzo and Japan, I make no doubt but the Variation is Easterly, and half the Sea over no less than fifteen Degrees, if there be any truth in this Hypothesis of mine. Therefore I propose this as a Trial, that the whole may be scann'd thereby; and I conceive it will not be hard to know of the Spaniards how it is, who so frequently sail through that Ocean, in their return from the Manilha Isles. This East Variation extends over Japan, Yedzo, East-Tartary, and part of China, till it meet with the Westerly, which is govern'd by the European North Pole, and which I said was greatest some where in Russia.
Towards the Southern Pole the effect is much the same, only that here the South Point of the Needle is attracted. Hence it will follow, that the Variation on the Coast of Brazile, at the River of Plate, and so on to the Straights of Magellan, should be Easterly (as in our third Remark); if we suppose a Magnetical Pole situate about twenty Degrees more Westerly than the Straights of Magellan. And this Easterly Variation doth extend Eastward over the greatest part of the Ethiopick Sea, till it be counterpoised by the Vertue of the other Southern Pole; as it is about mid-way between the Cape of Good-Hope, and the Isles of Tristan d' Acuntia. From thence Eastwards, the Asian South Pole (as I must take the liberty to call it) becoming prevalent, and the South point of the Needle being attracted thereby, there arises a West Variation, very great in quantity and extent, because of the great distance of this Magnetical Pole of the World. Hence it is, that in all the Indian Sea as far as Hollandia Nova, and farther, there is constantly West Variation; at that under the Equator it self it arises to no less than eighteen Degrees, where 'tis most. About the Meridian of the Island Celebes, being likewise that of this Pole, this Westerly Variation ceases, and an Easterly begins; which reaches, according to my Hypothesis, to the middle of the South-Sea, between Zelandia Nova, and Chili, leaving room for a small West Variation govern'd by the American South Pole, which I shew'd to be in the Pacifick Sea, in the sixth and seventh Remark.
What I have now said, does plainly shew the sufficiency of this Hypothesis for solving the Variations that are at this time observ'd in the temperate and frigid Zones, where the Direction of the Needle chiefly depends upon the Counterpoise of the forces of two Magnetical Poles of the same Nature; and I suppose I have shewn how it comes to pass, that under the same Meridian the Variation should be in one place 29½ West, and another 20½ East; as I have noted in my ninth Remark.