SECT. VI. be about thirteen Degrees there; and from thence it decreases all the way to Places near the Promontory of das Agulhas, in which there is again no Declination. And from that Place towards India the Declination begins to the weftward. The Declination at Hamburgh is nine Degrees, at Amfterdam about five Degrees, tho' formerly it was more, OBSERVATIONS teftify, that the Declination is not always the fame, but changes in procefs of time. At London in 1580 it was found to be eleven Degrees fifteen Minutes; but in the Year 1622 it was only fix Degrees thirteen Minutes; and in 1634 it was four Degrees fix Minutes; Obfervations being made, both with old and new Needles. At Paris in 1640 it was three Degrees, and in 1610 it was eight Degrees, and the fame is obferved in other Places, BEFORE the Variation of the Compafs was obferved to alter, or be different at the fame place at different Times; it was fuppofed, by fome, that the general Pofition of the Needle, towards the North and South, and yet not exactly to these points in all Places, or rather in but few places, was occafioned by the Excavations of fome parts of this terraqueous Globe; or by magnetical Veins, collaterally refpecting the Needle, &c. But this Opinion was foon exploded, when the Variation of Variation was difcovered; for if the pofition of the Compafs (whether directly North and South, or with fome deviation from the Meridian, which is called the Variation) had been always the fame in the fame place, there would be room to believe that That pofition of the Compafs, was occafioned by those Excavations or Adamantine Rocks, &c. but as we find the Variation alters, there must be some other Reafons affigned for the different Pofition or direction of the magnetical Needle. But as a Principle to to go upon, I fhall rank fome authentic Obfervations, of the Variation of the Compass in their proper Order, with the time when they were taken, and from thence prove the probability of the Variation of the Compafs (however irregular it may feem) being reducible to a regular Motion. AND to this may be added, that at the writing here of Anno 1734, the Variation at London, is more than one Point wefterly. I have inferted this Catalogue of Obfervations of the Variation of the Compafs, to prove that the Variation alters, and not only fo, but in a fort of a regular Manner too, as having been above eleven Degrees eafterly in the Year 1580, and then gradually decreafing to fix, four, three, &c. and doubtless fome time between the Year 1640, and the Year 1666 there was no Variation; because in 1666 the Variation was wefterly, increafing to two, feven, and now above twelve Degrees; how far it may advance further that way, before it come at it's Period, Time only can determine; nor are we certain what was it's utmost limits easterly before the aforefaid Year 1580, or when it was at thofe limits; but certain it is that this regular Motion of the Variation, as obferved, corroborates, or rather confirms, that excellent Theory of the celebrated Dr Halley as to magnetical Poles, c. hereunto fubjoined (e). (e) The learned Dr Halley having made a Collection of the exactelt Obfervations he could procure, and carefully compared and confidered them together; thence deduces the following Conclufions, concerning the Variation of the magnetical Needle. See Philof. Tranfa&t. No. 148. 1. That in the Year 1683, the Variation was to the Welt over all Europe; but more in the eastern Parts than the weft ern. 2. That on the Coast of America, about Virginia, NewEngland, and New-found-land, the Variation is likewife wefterly; and increases all the way to thofe that go northerly along the Coaft fo as to amount to above twenty Degrees at Newfound-land, nearly thirty in Hudfon's Strieghts, and fifty feven in Baffin's Bay. But it diminishes to thofe that fail eaftward from this Coat. And from these two Obfervations it feems to follow, that there ought to be an easterly Variation, or at least not a westerly one, fomewhere between Europe and the North Part of America: and this we may conjecture happens about the most easterly of the Tercera Inlands. 3. That on the Coast of Bra Al there is an easterly Variation which increafes confiderably in failing to the fouthward; fo as to be twelve Degrees at Cape Fris, and twenty and a half o. PRO ver against the Plate River: and from thence failing southwesterly to the Strieghts of Magellan, it decreases to feventeen Degrees; and at the Well Entrance of thofe Strieghts it is but four teen. 4. That to the eastward of Brafil, properly fo called, this eaflerly Variation decreases, fo as to be very fmall at the Iflands of St Helen, and Ajcenfion; and to vanish about eighteen Degrees of Longitude Welt from the Cape of GoodHope, where the Needle points due North and South. 5. That to the eastward of the forefaid Places, a weftward Variation begins; which ob. tains in the whole Indian Ocean; and amounts to eighteen Degrees under the very Equator; about the Meridian of the northern Part of Mada gafcar: and near the fame Meridian in thirty nine Degrees of South Latitude it amounts to twenty feven Degrees and a half. From thence easterly the West Variation decreases gra, dually, fo as fcarce to exceed eight Degrees at Cape Comorin, and not above three upon the Coast of Java; but about the Molucca Islands there is no Variation at all. The Cafe is likewise the same, a little to the weftward of Van Dieman's Land. 6. That to the eastward of the Molucca's and Van Dieman's Land, in South Latitude, there is PROPOSITION V. To find the Declination of the Needle from the North in any Place. FIND a Meridian Line by the Heavens, as we fhew'd by feveral ways in Chap. xx, and by that is found another easterly Variation, tho' less than the former both in Extent and Degree: for at the Island of Rotterdam it is fenfibly less than upon the Eaft Coaft of New Guinea. And to obferve the Proportion wherein it decreases, we may reasonably fufpect that it vanishes about twenty Degrees farther to the Eaft, or about two hundred and twenty five Degrees East Longitude from London, in twenty Degrees South Latitude; and that there a wefterly Variation begins. 7. That the Variations obferved at Baldivia, and the Weft Entrance of the Streights of Magellan, fhew that the Eaft Variation, explained in the third Observation, decreases apace, and cannot reasonably extend many Degrees into the South Sea, from the Coaft of Peru and Chili; leaving room for a small westerly Variation, in that Tract of the unknown World, betwixt Chili and New Zeland, and betwixt Hound's Ifland and Peru. 8. That in failing NorthWeft from St Helen's Inland by Afcenfion, as far as the Equators, the East Variation continues may very fmall, or as it were conftantly the fame. So that in this part of the World, the Tract of the Ocean where no Variation appears, is not extended in the Plane of any Meridian, but rather North-Weft. 9. That in the Entrance of Hudson's Streights, and the Mouth of the Plate River, tho' these are nearly under the fame Meridian, yet in the one the Needle varies twenty nine Degrees and a half to the Weft; and twenty and a half to the Eaft in the other. Whence plainly appears the Impoffibility of accounting for these Variations, by the Suppofition of two magnetical Poles, and an Axis inclined to the Axis of the Earth from whence it fhould follow, that under the fame Meridian the Variation fhould in all Places be the fame way. In order to explain these Phænomena, Dr Halley fagaciously fuppofes the Globe of the Earth to be a great Magnet, having four magnetical Poles; two near the North, and as many near the South Pole of the Earth; and that the Needle is governed by each of these Poles, whilft the Virtue of the nearest may the Declination of the Needle readily appear; but for Sea-mens Ufe the following Propofition will fhow it more readily. nearest Pole predominates over that of the more remote. But many Particulars being required to determine the Places of thefe Poles with exactnefs; the Doctor determines them thus by conjecture. He places the North magnetic Pole neareft to us, in or near the Meridian of the Land's End of England; and not above feven Degrees from the North Pole. By this Pole the Variations in all Europe, and Tartary, and the North Sea are principally govern'd; tho' it is fomewhat affected by that other northern magnetic Pole, fituated in a Meridian paffing thro' the middle of California, and about fifteen Degrees from the North Pole of the World. To this the Needie has chiefly refpect in all North America, and in the two Oceans, on either fide thereof, from the Azores weftward, up to Japan, and beyond. The two fouthern magnetical Poks are fomewhat farther diflant from the South Pole of the World; the one, being about fixteen Degrees from it, is in a Meridian twenty Degrees to the weftward of Magellan's Streights, or ninety five Degrees Weft from London. This commands the Needle in all South America, in the Pacific, and the greatest Part of the Ethiopic Ocean. PRO The fourth Pole feems to have the greatest Virtue, and largest Dominion, being furtheft diftant from the Pole of the World, viz. about twenty Degrees; in a Meridian paffing thro' New Holland, and the Celebes, about one hundred and twenty Degrees from London. This Pole predominates in the South of Africa, in Arabia, and the Red-Sea; in Perfia, India, and its Iflands, and all over the Indian Ocean from the Cape of Good-Hope eastwards, to the middle of the great South Sea, that divides Afia from America. It remains to be fhown how the Conclufions fet down are deduced from this Hypothefis. And that the whole may be the better understood; either a Globe or Map fhould be used, whereon the four magnetical Poles are to be placed in the Situations above described. And first, it is plain, that our European magnetical North Pole being in the Meridian of the Land's End of England, all Places more easterly than that, will have it on the Weft Side of their Meridian, and confequently the Needle respecting it, with it's northern Point, will have a wefterly Variation; which will ftill increase in failing eastwards up to fome Meridian of Ruffia, where it will be greatest; and from thence decreafe again. Thus at Breft the Variation is but one Dė gree |