VARIATION. 



' netical poles, or points of attradtioii ; near cath pole of the 

 equator two ; and that in thofe parts of the world which lie 

 nearly adjacent to any one of thefe magnetical poles, the 

 needle is governed thereby ; the neareft pole being always 

 predaminant over the more remote." 



The pole which at prefcnt is neareft to us, he conjeftures 



to lie in or near the meridian of the Land's-End of England, 



and not above 7° from the Arftic pole : by this pole the 



! variations in all Europe and Tartary, and the North fea, are 



principally governed ; though ftill with fome regard to the 



! other northern pole, whofe lltuation is in the meridian paffing 



i about the middle of California, and about 15° from the 



north pole of the world, to which the needle has chiefly 



refpeft in all North America, and in the two oceans on 



.either fide thereof, from the Azores, weftwards, to Japan, 



I and farther. 



The two fouthern poles, he imagines, are rather farther 

 1 diftant from the fouth pole of the world ; the one about 16°, 

 ; in a meridian 20° to the weftward of Magellan ftraits, or 

 ' 95° weft from London : this commands the needle in all 

 I South America, in the Pacific fea, and the greateft part of 

 1 the Ethiopic ocean. The other feems to have the greateft 

 i power, and the largeft dominion of all, as it is the moft re- 

 • mote from the pole of the world, being diftant from it little 

 [ lefs than 20°, in the meridian which paftes through New 

 'Holland, and the ifland Celebes, about 120° eaft from 

 ; London ; this pole is predominant in the fouth part of 

 : Africa, in Arabia, and the Red fea, in Pcrfia, India, and 

 1 its idands, and all over the Indian fea, from the Cape of 

 J Good Hope eaftwards, to the middle of the Great South 

 I fea that divides Afia from America. 



I Such appears to have been the difpofition of the magnetic 

 ' attraftion in the time of Dr. Halley ; and from thefe data 

 ' this author draws the following conclufions ; viz. 



1. Then, it is plain that, as our European north pole is 

 in the meridian of the Land's-End of England, all places 

 more eafterly than that will have it on the weft fide of the 

 meridian ; and, confequently, the needle, refp^fting it with 

 its northern point, will have a wefterly variation, which will 

 ftill be greater as you go to the eaftward, till you come to 

 fome meridian of Ruflia, where it will be the greateft, and 

 from thence will decreafe again. Accordingly, in fadl, we 

 find, that at Brelt the variation is but 15^° ; at London, 4^° 

 (in 1683); and at Dantzic, 7° weft. Again, to the weil- 

 ward of the meridian of the Land's-End, the needle ought 

 to have an eafterly variation, were it not that, by approach- 

 ing the American northern pole, (which lies on the weft 

 fide of the meridian, and feems to be of greater force than 

 this other,) the needle is drawn thereby weft ward, fo as to 

 counterbalance tiie direftion given by the European pole, 

 and to make a fmall weft variation in the meridian of the 

 Land's-End itfelf. Yet, about the ifle of Tercera, it is 

 fuppofed'our neareft pole may fo far prevail as to give the 

 needle a little turn to the eaft, though but for a very little 

 fpace ; the counterbalance of thofe two poles admitting no 

 confiderable variation in all the eaftern parts of the Atlantic 

 ocean, nor upon the weft coafts of England and Ireland, 

 France, Spain, and Barbary. But to the weftward of the 

 Azores, the powers of the American pole overcoming that 

 of the European, the needle has chiefly refpeft to this, and 

 turns ftill more and moie towards it as we approach it. 

 Whence it happens, that on tlie coaft of Virginia, New 

 England, Newfoundland, and in Hudfon's ftraits, the 

 variation is weftward, that is, it decreafes as you go from 

 thence towards Europe ; and that it is lefs in Virginia and 

 New England than in Newfoundland and Hudfon's ftraits. 



2. Tliis wefterly variation, again, decreafes as you pafs 



over North America ; and about the meridian of the middle 

 of California, the needle again points due nbrth ; and from 

 thence weftward to Yedzo and Japan, it is fuppofed the 

 variation is eafterly, and half-fea over, not lefs than 15' ; 

 and that this eaft variation extends over Japan, Yedzo, Tar- 

 tary, and part of China, till it meets with the wefterly, 

 whicli is governed by the European north pole, and which 

 is the greateft fomewhere in Ruflia. 



3. Towards the fouth pole the efFeft is much the fame, 

 only that here the fouth point of the needle is attradled. 

 Whence it will follow, that the variation on the coaft of 

 Brafil, at the river of Plata, and fo on to the ftraits of 

 Magellan, flaould be eafterly, if we fuppofe a magnetical 

 pole, fituate about 20° more wefterly than the ftraits of 

 Magellan. And this eafterly variation extends eaftward 

 over the greateft part of the Ethiopic fea, till it be coiinter- 

 poifed by the virtue of the other fouthern pole, as it is about 

 mid-way between the Cape of Good Hope and the ifles of 

 Triftan d'Alcunha. 



4. From thence eaftwards, the Afiatic fouth pole be- 

 coming prevalent, and the fouth point of the needle being 

 attrafted thereby, there arifes a weft variation, very great 

 in quantity and extent, becaufe of the great diftance of this 

 magnetical pole from the pole of the world. Hence it is, 

 that in all the Indian fea, as far as Hollandia Nova, and 

 farther, there is conftantly a weft variation ; and that, under 

 the equator itfelf, it rifes to no lefs than 1 1°, where it is 

 moft. And that, about the meridian of the ifland of Cele- 

 bes, being Hkewife that of this pole, this wefterly variation 

 ceafes, and an eafterly one begins, which reaches to the 

 middle of the South fea, between Zelandia Nova and Chili, 

 leaving room for a fmall weft variation, governed by the 

 Amerian fouth pole. 



5. From the whole it appears, that the direftion of the 

 needle, in the temperate and frigid zones, depends chiefly 

 upon the counterpoife of the forces of two magnetical poles 

 of the fame nature ; as alfo why, under the fame meridian, 

 the variation ftiould be in one place 29^° weft, and in an- 

 other 2oi° eaft. 



6. In the torrid zone, and particularly under the equi- 

 noftial, refpeft muft be had to all four poles, and their 

 pofuions muft be well confidcred, otiierwife it will not be 

 eafy to determine what the variation ftiall be, the neareft 

 pole being always ftrongeft ; yet not fo as not to be coun- 

 terbalanced, fometimes, by the united forces of two more 

 remote. Thus, in failing from St. Helena, by the ifle of 

 Afcenfion, to the equator, on the nortli-weft courfe, the 

 variation is very little eafterly, and in that whole tradt is 

 unalterable ; becaufe the South American pok-, ( which is 

 confiderably the neareft in the aforefaid places, ) requiring 

 a great eafterly variation, is counterpoifed by the contrary 

 attraftion of the North American and the Afiatic fouth 

 poles ; each of which fingly, is, in thefe parts, weaker than 

 the American fouth pole : and upon the north-weft courfe, 

 the diftance from this latter is very little varied ; and as 

 you recede from the Afiatic fouth pole, the balance is ftill 

 preferved by an accefs towards the North American pole. 

 In this cafe, no notice is taken of the European north pole ; 

 its meridian being a little removed from thofe of thefe 

 places, and of itfelf requiring the fame variations which we 

 here find. 



After the fame manner may the variations in other places, 

 under and near the equator, be accounted for, upon Dr. 

 Hallcy's hypothefis. 



But in order to acsount for the variation of tie variation^ 

 it was obferved, that from many of the obferved phenomena, 

 it feemed to follow that all the magnetic poles have a 



motion 



