VARIATION. 



motion weftward : but if it be fo, it is evident that it is not 

 a rotation about the axis of the earth ; for then the varia- 

 tions would continue the fame in the fame parallel of lati- 

 tude (the longitude only changed), as much as the motion 

 of the magnetical poles : but the contrary is found by ex- 

 perience ; for there is no where, in the latitude of yii" 

 north, between England and America, a variation of ii'' 

 eaft, at this time ; as it was once here at London. Where- 

 fore it feems, that our European pole is become nearer the 

 Arftic pole than it was heretofore ; or elfe, that it has loft 

 part of its virtue. 



But whether thefe magnetic poles move altogether with 

 one motion, or with feveral ; whether equally, or unequally ; 

 whether circular, or libratory : if circular, about what centre ; 

 if libratorj-, after what manner ; are things yet unknown. 



This theory feems yet fomewhat obfcure and defeftive : 

 to fuppofe four poles in one magnetical globe, in order 

 to account for the variation, is a little unnatural ; but to 

 conceive thofe poles to move, and that by fuch laws as to 

 folve the variation of the variation, is ftill more extraordinary. 

 In effeft, the folution appears not much lefs implicit and 

 arbitrary than the problem. 



The learned author of the theory, therefore, found him- 

 felf under a neceflity to folve the phenomena of his folution ; 

 and with this view, he prefented the following hypothefis. 

 The external parts of the globe he confiders as the ihell, 

 and the internal as a nucleus, or inner globe ; and between 

 the two, he conceives a fluid medium. That inner earth, 

 having the fame common centre and axis of diurnal rotation, 

 may turn about with our earth each twenty-four hours. 

 Only the outer fphere having its turbinating motion fome 

 fmall matter either fwifter or flower than the internal ball ; 

 and a very minute difference in length of time, by many 

 repetitions, becoming fenfible ; the internal parts will, by 

 degrees, recede from the external ; and not keeping pace 

 with one another, they will appear gradually to move either 

 eaftward or wellward, by the difference of their motions. 



Now, fuppofe fuch an internal fphere, having fuch a 

 motion, the two great difficulties in the former hypothefes 

 are eafily folved ; for if this exterior ibell of earth be a 

 magnet, having its poles at a diftance from the poles of 

 diurnal rotation ; and if the internal nucleus be likewife a 

 magnet, having its poles in two other places, diftant alfo 

 from the axis ; and thefe latter, by a gradual and flow 

 motion, change their place in refpect of the external, we 

 may then give a reafonable account of the four magnetical 

 poles afore-mentioned, as likewife of the changes of the 

 needle's variation. 



The period of its motion being wonderfully great, and 

 there being hardly a hundred years fince thefe variations 

 have been duly obferved, it will be very hard to bring this 

 hypotheiis to a calculus ; efpecially fince, though the 

 variations do increafe and decreafe regularly in the fame 

 place, yet in different places ut no great diftance, there are 

 found fuch cafual changes thereof, as can no ways be ac- 

 counted for by a regular hypotheiis ; but feem to depend 

 upon the unequal and irregular diftribution of the mag- 

 netical matter within the fubftance of the external fliell, or 

 coat of the earth, which defleft the needle/rom the pofition 

 It would acquire from the effeft of the general magnetifm 

 of the whole. Of which the variations at London and 

 Paris give a notable iuftance ; for the needle has been con- 

 ftantly about i^^ more eafterly at Paris than at London : 

 though it be certain that, according to the general effeft, 

 the difference ought to be the contrary way ; notwithftand- 

 jng which, the variations, in both places, do change alike. 

 Heoce, and from fome other things of like nature, it 



feems plain, that the two poles of the external globe artr 

 fixed in tlie earth, and that if the needle were wholly go- 

 verned by them, the variations thereof would be always- 

 the fame, with fome irregularities, upon the account jull 

 now mentioned. But the internal fphere having fuch a 

 gradual tranflation of its poles does influence the needle, 

 and direft it varioufly, according to the refult of the at- 

 traftive or direftive power of each pole, and, confequently, 

 there muftbe a period of the revolution of this internal ball ; 

 after which the variations will return again as before. But 

 if it fliall in future ages be obferved otherwife, we muft then 

 conclude that there are more of thefe internal fpheres, and 

 more magnetical poles, than four ; which, at prefent, we 

 have not a fufficieiit number of obfervations to determine, 

 and particularly in that vaft Mar del Zur, or South fca, which 

 occupies fo great a part of the whole furface of the earth. 

 If, then, two of the poles be fixed, and two moveable, it 

 remains to afccrtain which they are that keep their place. 



The author thinks it may be fafely determined, that our 

 European north pole is the moveable one of the two 

 northern poles, and that which has chiefly influenced the 

 variations in thefe parts of the world ; for, in Hudfon's 

 Bay, which is under the direftion of the American pole, the 

 change is not obferved to be near fo faft as in thefe parts of 

 Europe, though the pole be much fai-ther removed from the 

 axis. As to the fouth poles, he takes the Afiatic pole to 

 be fixed, and, confequently, the American pole to move. 



This gi-anted, it is plain that the fixed poles are the 

 poles of tliis external fliell, or cortex, of the earth ; and 

 the other the poles of the magnetical nucleus, included and 

 moveable within the other. It likewife foUows, that this 

 motion is wellwards, and, by confequence, that the afore- 

 faid nucleus has not precifely attained the fame degree of 

 velocity with the exterior parts in their diurnal revolution ; 

 but fo very nearly equals it, that, in 365 revolutions, the 

 difference is fcarcely fenfible. That there is any difference 

 of this kind arifes hence, that the irapulfe by which the 

 diurnal motion was imprelfed on the earth, was given to the 

 external parts ; and from thence, in time, communicated to 

 the internal ; but not fo as yet perfeftly to equal the velocity 

 of the firft motion impreffed on, and llill conferved by, the 

 fuperficial parts of the globe. 



As to the precife period, we want obfervations to deter- 

 mine it, though tlie author thinks we may, with fome 

 reafon, conjefture, that the American pole has moved weft- 

 ward 46^^ i:i ninety years, and that the whole period thereof 

 is performed in about feven hundred years. 



Mr. Whifton, in his " New Laws of Magnetifm," raifes 

 feveral obieftions againft this theory. See Magnetism. 



M. Euler, the fon of the celebrated geometrician of that 

 name, has alfo controverted and cenfured the above theory : 

 he thinks that two magnetic poles placed on the furface of 

 the earth will fufficiently account for the variation ; and he 

 then endeavours to iliew, how we may determine the decli- 

 nation of the needle, at any time, and on every part of the 

 globe, from this hypothefis. But we muft refer for the 

 particulars of this reafoning. to the Hiftoire de 1' Academic 

 Royale des Sciences et Belles Lettres of Berlin, for 1757. 



Various other theories have been fuggefted by later 

 authors, but moft of thefe have been already alluded to 

 under our articles Magnetism, TiiPPisG-NeeJk, Sic. ; we 

 fliall, therefore, here conclude this article, by referring the 

 reader for a more minute account of the various theories, 

 particularly that of Coulomb, to Haiiy's " Elementary 

 Treatife on Natural Philofophy," tranflated by Gregory, 

 and to the firft chapter of the third volume of Biot's 

 " Traite de Phyfique." 



I Variation 



