THE Variation of the Compass (by which I mean the Deflection of the Magnetical Needle from the true Meridian) is of that great Concernment in the Art of Navigation, that the neglect thereof, does little less than render useless one of the noblest Inventions Mankind ever yet attained to. And for this cause all Ships of Consequence (especially those bound beyond the Equator) carry with them Instruments on purpose to observe this Variation: That so the Course steer'd by the Compass, may be reduc'd to the true Course in respect of the Meridian.
Now although the great utility that a perfect Knowledge of the Theory of the Magnetical Direction would afford to Mankind in general, and especially to those concern'd in Sea Affairs, seems as sufficient incitement to all Philosophical and Mathematical Heads, to take under serious Consideration the several Phænomena, and to endeavour to reconcile them by some general Rule: Yet so it is; that almost all the Authors, from whom a Discourse of this kind ought to have been expected, pass by in silence the Difficulties they here Encounter. And those that mention this Variation: By affirming it to proceed from Causes altogether uncertain (as are the casual lying of Iron Mines and Loadstones in the Earth) put a stop to all further Contemplation; and give discouragement to those that would otherwise undertake this Enquiry. 'Tis true, that not long since one Mr. Bond, an old Teacher of Navigation, put forth a small Treatise, wherein he pretends to calculate the Variation: But he limits his Hypothesis to the City of London, affirming himself (as he had a great deal of reason) that the same Calculus is not sufficient for other Places; whereby it appears that this Rule is far short of the so much desir'd general one.
Now although (through want of sufficient Observations, and some other Difficulties, which I shall anon shew) I cannot pretend perfectly to establish the Numbers and Rules of a Calculus, which shall precisely answer to the Variations of all parts of the World: Yet I suppose it will not be unacceptable to the Curious to propose something of a Light into this abstruse Mystery; which, if no other, may have this good Effect, to stir up the Philosophical Genii of the Age to apply themselves more attentively to this useful Speculation. But before I proceed, 'twill be necessary to lay down the Grounds upon which I raise my Conclusions; and at once to give a Synopsis of those Variations, which I have reason to look upon as sure, being mostly the Observations of Persons of good Skill and Integrity.
A
TABLE
OF
VARIATIONS.
Tho' I could wish we could obtain from the Spaniards what Variations they find in their Voyages from the Manilhas towards Acapulco, through the North part of the South Sea; as likewise what it is at Japan from the Dutch: Yet (considering the number of these Observations I have collected, and that they are made in parts of the World so remote from Europe, and from one another) I suppose that the Theory that answers these will scarce fail in those Regions from whence we have as yet no account. But first we must make some Remarks upon the foregoing Table: And, First,
That in all Europe the Variation at this time is West, and more in the Eastern Parts thereof than the Western: As likewise, that it seems throughout to be upon the increase that way.
Secondly, That on the Coast of America, about Virginia, New-England and New-Foundland, the Variation is likewise Westerly; and that it increases all the way as you go Northerly along the Coast, so as to be above 20 Degrees at New-Found-Land, nearly 30 gr. in Hudson's Straights, and not less than 57 Degrees in Baffin's Bay; also, that as you Sail Eastward from this Coast, the Variation diminishes. From these two it is a Legitimate Corollary: That Somewhere between Europe, and the North part of America, there ought to be an Easterly Variation, or at least no Westerly. And so I conjecture it is about the Eastermost of the Tercera Islands.
Thirdly, That on the Coast of Brasile there is East Variation, which increases very notably as you go to the Southward, so as to be 12 Degrees at Cape Frio, and over against the River of Plate 20½ Degrees: And from thence Sailing South-Westerly to the Straits of Magellan it decreases 17 Degrees, and at the West Entrance but 14 Degrees.