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.