tor, or Middle of the Stone towards the Pole, the Knife or Wire, will accordingly attrad the End of 
a poifed magnetical Needle: But if you take another Knife or Wire, and thruft it leifurely over the 
Pole towards the Equator, or Middle of the Equator, this Knife (hall expel, or drive away the fame 
End of the Needle, which the former Knife would attrad, which Experiment makes it very pro- 
bable, that the Operation of the Magnet depends on the FiuX of fo me fine Particles, which go out 
at one Pole, then round about, and in again at the other. 
XII. Becaufe it is one of the univerfal Laws of Nature, that Addon and Re-adion are always 
equal : Therefore, it is plain, the Iron muff attrad the Magnet as much as that doth the Irons and 
you m .y eafily experiment it to be fo in Fad:, if you place a Magnet, or a Piece of Iron on a 
Cork, fo as that it may fwim freely in the Water, for then you will fee that whichfoever ycu hold 
in your Hand will draw the other towards it. 
This Table of Fads, and Lift of Experiments, have been very exadly tranfcribed, that the Rea- 
der might fee as much as either Reafon or Experience can teach concerning the wonderful Nature of 
this Stone, and the various Circumftances which attend the Exertion of that Faculty by which it be- 
comes of fuch infinite Benefit and Advantage to Mankind. Without knowing thefe Circumftances, 
there would want many Grounds for enquiring into, and difcovering, as perhaps there are ftill want- 
ing, fufticient Grounds for fully explaining the Nature of Magnetifm, that is to fay, with fuch Clear- 
riels, as may fatisfy the Underftanding, and fuch Certainty, as may take away all Fear of Deception 
or Miftake in Pradice. If this Dodrine of Magnetifm was meerly a Point of Natural Philofophy, it 
Would be idle for me to fpend fo much Time about it; but as it is in Fad the great Secret which 
mail: fooner or later difclofe the Perfedion of that Science to which Books of this Nature belong, I 
mean the Knowledge of the entire Globe upon which we live, it becomes a moft neceffary Part of this 
Introdudion, becaufe the moft valuable Voyages contain feveral Particulars relating to this Matter, 
which are in Danger of appearing either trivial or tedious, if their Ufe and Importance be not fully 
fhewn. 
The feveral Hypothefes that have been devifed by fpeculative Men, in order to explain this Mat- 
ter, have hitherto proved not only erroneous and unfatisfadory, but by being for a Time judged other- 
wile, have hindered fuch Enquiries as might have turned to the Benefit of Mankind; for, as Lord Ba- 
con long ago obferved, Ignorance is not near fo great an Hindrance to Knowledge, as the Conceit of 
Knowledge: Thus for Inftance, Kircbers Notion of magnetical Fibres in the Earth, might make a 
Man fancy he could account for the Verticity of the Load-ftone. Again, Mr. Bond, a Countryman 
of our own, advanced fomething very plaufible about the Variation; for he fuppofed there were two 
magnetical Poles, which terminated an Axis inclined to that of the Earth, from whence it would fol- 
low, that under the fame Meridian, the Variation muft be the fame, and therefore as Experience 
fhews the Conclufion falfe, it follows, that his Account of the Matter could not be true. Dr. Gilbert ; 
our Countryman likewife, and whofe Labours on this Subjed can never be enough commended, guard- 
ed againft this Error, by the Opinion which he advanced, which is indeed very curious, and very in- 
genious, but at the Bottom likewife unfatisfadory. He thought that the Earth itfelf being in ail its 
Parts magnetical, and the Water not; the Needle, would consequently turn towards the Earth’ when- 
ever it lay, as to the greater Quantity of magnetical Matter; but to mention one out of a Multitude of 
Inftances that might be given, it is certain, that on the Coaftof Brazil , the Needle turns Quite the 
contrary Way, which is fufticient to deftroy this Notion. 
The famous M. Des Cartes invented another Scheme lefs liable to Contradidion, for he conjedured 
that Iron and Load-ftone, hid in the Bowels of the Earth, and the Bottom of the Sea, might be the 
Caufes of the Variation of the Needle ; but if this were true, how fhould the Variation of that Va- 
riation be accounted for? We know from Experience, that here at London , the Variation in one hun- 
dred Years has altered fifteen Degrees, and fomething more, for which none of thefe Notions account 
at all, and therefore the famous Dr. Halley took quite another Method, that is to fay, he took the 
only right Method, by firft colleding a large Table of Obfervations, and from thence endeavoured 
to raife a Theory. 
By this Means he went much farther than any of his Predeceftors, and his Scheme was this, that the 
Globe was a Magnet, having four magnetical Poles, two near each of the Poles of the Equator' and that 
the Needle was always governed by theneareft of thefe Poles. His Difcourfe upon this Subjed was ex- 
treamly well received, both at Home and Abroad, and perhaps, if it had fatisfied its Author, it mio-hf 
have fatisfied every body elfe ; but the ingenious Gentleman who devifed it, having a much greater 
regard to Truth than Fame, and to the general Advantage of Mankind, than his own particular Re- 
putation, foon faw that his Scheme, tho’ much better contrived, was at the Bottom as falacious as the 
reft; for he confidered, that no Magnet had any more than two Poles, whereas the Earth had vifi- 
bly four, and perhaps more ; and he likewife obferved, that according to his former Notion, thefe 
Poles fhifted their Place, which, according to tire Laws of Nature, and the particular Strudure’of the 
Magnet, was improbable and abfurd. 
He farther obferved, that this Variation was regular, and had a Certain Period, tho’ not hitherto 
perfectly difeovered; for, here at London , in the Year 1580, it was obferved to be n° iy> Eaft„~ I n 
1657, there was no Variation at all. In 1672, he obferved it himfelf 2 0 30 7 Weft; and in j6q 2 he 
obferved it to be exadly 6° Weft ; fo that in one hundred and twelve Years, it had changed iy° * To 
this it may not be improper to add, that in fifty-three Years, which have fince elapfed^it has varied 
1 i° more to the Weft. At Paris, in 1 550, the Needle varied 9 0 Baft. In 1666, there was" no Va- 
riation at all, and fince that, its Variation has increased to 14° Weft. The moft fouthern Promon- 
tory of Bfrica was, by the. Portugueze, about the Year 1600, called the Caf e dl Agulhas, o.r Cape 
Needles g 
