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ikl the Hypothefis mull be allowM very adequate, and fufficient for rhe Phenomena.
'theory of the Variation of the Variation.
Rom the Obfervations abovemenlion'd, it fams to fol- low, that all the Magnetical Poles have a Motion Weit- w-rd ■ but if it be fo, 'tis evident, that it is not a Rotation about' the Axis of the Earth ; for then the Variations would continue the fame, in the fame Parallel of Latitude (•the Longitude only changed) as much as the Motion ot the Magnetical Poles : But the contrary is found by Ex- perience t for there is no where, in the Latitude ot 51 and an half North, between England and America, a Variation of n Degrees Ball, at this time ; as it was once here at London. Wherefore, it feems that our European Pole is grown nearer the ArBick Pole than it was heretofore ; or elfe, that it has loft Part of its Virtue. .
But whether thefe Magnetick Poles move altogether with one Motion, or with feveral ; whether equally, or unequal- ly • whether circular, or libratory : If circular, about what Centre ; if libratory, after what manner ? are Things yet unknown. , . r A .
This Theory feems yet fomewhat oblcure, and detective : To fuppofe four Poles in one Magnetical Globe, in order to account for the Variation, is a little unnatural ; but to con- ceive thofe Poles to move, and that by fuch Laws as to lolve the Variation of the Variation, is flill more extraordinary. In effeft, the Solution appears as implicite and arbitrary as fhc Problem.
The Author, therefore, found himfelf under a Necelfity to folve the Phenomena of his Solution ; and with this View, prefentcd the following Hypothefis.
The external Parts of the Globe, he confiders as the Shell, 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 24 Hours. Only the outer Sphere having its turbinating Motion fume fmall Matter cither 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, will appear gradually to move, either Eaft- ward or Weflward, by the difference of their Motions.
Now, fuppofing fuch an internal Sphere, having fuch a Motion, the two great Difficulties in the former Hypothefis are eafily folv'd : For if this exterior Shell of Earth be a Magnet, having its Poles at a diflance from the Poles of diurnal Rotation, and if the internal Nucleus be hkewife a Maimer, 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 refpeft of rhe External : We may rhen give a reafonable Account of the four Magnetical Poles afore- mention'd, as likewife of the Changes of the Needle's Va- riations.
The Period of this Motion being wondertul great, and there being hardly an hundred Years fince thefe Variations have been duly obferv'd ; it will be very hard to bring this Hypothefis to a Calculus ; efpecially fince, tho the Varia- tions do increafe and decreafe regularly in the fame Place, yet in differing Places, at no great diflance, there are found fuch cafual Changes thereof, as can no ways be accounted for by a regular Hypothefis ; but leem to depend upon the unequal and irregular Difiribution of the Magnetical Matter within the Subflance of the external Shell or Coat of the Earth, which deflefl the Needle from the Pofition it would acquire from, the EfTea of rhe general Magnetifm of the whole. Of which, the Variations at London and Paris tnve a' notable Inftance ; for the Needle has been conftantly about t{ more Eaflerly at 'Paris than at London: Tho it be certain/that according to the general Effect, the Difference ounht to be the contrary way ; notwithffanding which, the Variations in both Places do change alike.
Hence, and from fome other things of like Nature, it feems plain, that the two Poles of the external Globe arc fix'd in the Earth ; and that if the Needle were wholly go- vcrn'd by rhem, the Variations thereof would be always the fame, with fome Irregularities upon the Account jufl now mention'd. But the internal Sphere having fuch a gra- dual Tranflation of its Poles, does influence the Needle, and direct it varioufly, according to the Rcfult of the attractive or directive Power of each Pole ; and confequently, there mult be a Period of the Revolution of this internal Ball ; after which, the Variations will return again, as before. Bur if it fhall in future Ages be obferv'd otherwife, wc mufl then conclude, that there arc more of thefe internal Spheres, and mere Magnerical Poles than four ; which, at prefent, we have not a fufficient number of Obfervations to deter- -hiine, and particularly in that vafl Mer del li'-ir which occu- pies fo great a Part of the whole Surface of the Earth.
If then two of the Poles be fix'd, and two moveable, it remains to afcettain, which they are that keep their Place.
The Doctor thinks it may be fafely detsrmin'd, that oat European North Pole is the moveable one of the two Nor- thern Poles, and that which has chiefly influene'd the Varia- tions in thefe Parts of the World : For in Hudfins Say, which is under rhe Direction of the American Pole, the Change is not obferv'd to be near fo faff, as in rhefe Parts of Europe, tho that Pole be much farther removed from the Axis. As to the South Pole?, he takes the AJic.n Pole to be fixed, and confequently rhe American Pole to move.
This granted, 'tis plain the fixed Poles are rhe Poles of this external Shell or Cortex of the Earth ; and the other the Poles of the Magnetical Nucleus, included and move- able within the other. It likewife follows, that this Motion is Weftwards ; and, by conlcquence, that the aforcfaid Nu- cleus has not precifcly attain'd the fame Degree of Velocity with the exterior Parts in their diurnal Revolutions ; but fo very nearly equals ir, rhar in 56"$ Revolves, the Difference is fcarce fenfible. That there is any difference, arifes hence, That the Impulfe whereby the diutnal Motion was imprefs'd on the Earth, was given to the external Parts 5 and from thence, in time, communicated to the internal : but net fo, as yet perfectly to equal the Velocity of the firlt Motion imprefs'don, and flill conferv'd by, the luperficial Parts of the Globe.
As to the precife Period, we want Obfervations to deter- mine it 5 tho the Author thinks we may with fome Reafon conjecture, that the American Pole has moved Wellward 46 Degrees in 90 Years ; and that the whole Period thereof is pcrfotm'd in about 700 Years.
Mr. IVUflon, in his Ne-iv Laws of Magnetifm, raifes fe- veral Objections againtt this Theory j -which fee under the Article Magnetism.
l"o olfcrve the Variation or Declination of the Needle.
Draw a Meridian-Line, as directed under the Article Me- ridian : Then, a Style being erected in the middle there- of, place a Needle thereon, and draw the right Line it hangs over. Thus will the Quantity of the Variation ap- pear. See Compass.
Or thus: As the former Method of finding the Declination cannot be applied at Sea, others have been thougtit of ; the Principal whereof follow. Sufpend a Thread and Plummet over the Compafs, till the Shadow pafs thro' the Centre of the Card : Obferve rhe Rhumb, or Point of the Compafs, which the Shadow touches when 'tis the fhorteft. For the Shadow is then a Meridian-Line : Confequently the Varia- tion is fhewn.
Or thus : Obferve the Rhumb wherein the Sun or fome Star rifes and fets : BitTecl the Arch intercepted between the riling and fetting ; the Line of Biffection will be the Meridian-Line : Confequently, the Declination is had as before. The fame is had from two equal Altitudes of the fame Star, obferved either by Day or Night.
Or thus : Obferve rhe Rhumb wherein rhe Sun, or a Srar rifes and fets ; and from rhe Laritude of the Place, find the Eattern or Weftern Amplitude : For the diffetence between the Amplitude, and the diltance of the Rhumb obferved, from the Eaftern Rhumb of the Card, is the Variation fought.
Or thus .' Obferve the Altitude of the Sun, or fome Star, S I, (Tab. Navigation, Fig. iff.) whofe Declination is known ; and note the Rhumb in the Compafs to which it then cor- refponds. Since then in the Triangle ZPS we have three Sides, viz. PZ, the Complement of the Elevation ol the Pole PR; S P, the Complement of the Declination DS; and ZS, the Complement of the Altitude SI ; the Angle PSZ is found by Spherical Trigonomerry, (fee Trian- gle :) the contiguous one to which, viz. AZS, mealures rhe Azimuth HI. The difference, rhen, between the Azi- muth, and the Diflance of the Rhumb obferved from the South, is the Variation fought.
Note, To have the Eaftern or Weftern Amplitude accu- rately, regard mufl be had ro the Refraction ; the Laws whereof are abundantly deliver'd under the Article Re- fraction.
For rhe more commodious obferving in whar Rhumb of the Compafs the Sun, or a Star is feen, ir will be proper to have two little Apertures, or glafs Windows oppofite to each other under the Limb fhereof; with a Telelcope Sight fitted to one of them, and to the other a fine Thread.
Variation of the Variation, or Declination, is the Change in the Declination of the Needle, obfetved at diffe- rent Times in the fame Place.
This Variation, firfl, 'tis faid, difcover'd by Gaffendus, is found to obferve one and the fame Law pretty regularly.
It is fuppofed owing to the difference of Velocity of the Motions of the internal and external Parts of the Globe. We want Obfervarions of two or rhree hundred Years, to de- termine the Quantity of this Variation in any certain Time : but from what Obfervations we have, Dr. Halley computes it to he at about 2 3 Degtees in 90 Years : So that in 700 Years it will come round ; and every Place have the fame Variation it had. See more at large under Variation, and Mag- netism. Varia-
