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dius is alfo an Arc of a great Circle of about 44 Degrees.
XXI. The refpective Motion of the internal Magnet, or the Velocity, v.g. of its North- Pole, appears to be 27 Deg. o Min. in 144 Degrees, i.e. one Degree in five Years ; fo that it makes an entire Revolution in 1920 Xears.
Hence as the Number of Degrees in the upper Earth's diurnal Revolution, is to the Number of Days in the Revolution of the internal Magnet, i, e. as,i is to 700000, fo is the refpective Motion of this Magnet from Eaft to Weft to the real Motion of the upper^Earth from Weft to Eaft; or to fpeakftrictly, fo is the Difference of their Motions from Weft to Eaft, to the entire Motion of the tipper Earth the fame way. This external fixed Earth has therefore communicated almoft all its Motion already to the internal Magnet, and can communicate no more than this Difference of their Motion, and that only in an infinite Term of Years 5 or, in other Words, this real in- ternal Motion can never be the 700000th Part fwifter than it is at prefent. This internal Motion therefore began with the Commencement of the diurnal Motion of the upper Earth 5 and has gone on ftill fafter and fafter by the Communication of that Motion thro' the intermediate Fluid. Since therefore Action and Reaction are equal, and tend to contrary Parts, this internal Loadftone, thus accelerated by the upper Part, strati have all along re- tarded that upper Earth, and made the diurnal Rotation ftill flower and flower. This Acceleration on one Side, and Retardation on the other, rauft have been very great at the firft beginning of the diurnal Motion, when the Difference of their Motion was equal to the entire Motion Itfelf, and muft have been diminifliing ever fince. To which Caufe is probably owing that Acceleration of the Moon's Motion with rcfpect to that of the Earth, fince the time of the old Aftronoraers, firft taken notice of by Dr. Hailey, and embraced by Sir I. 'Newton. And rhe fame Consideration feems to fuggeft a Method for deter- mining the Age of the World ; for were the Proportions of the Quantity of Matter in the upper Earth to the in- ternal Magnet, with t 'the Tenacity of the intermediate JFluid, fife, known, one might go back from the known Difference of their Velocity now, and find thofe Differen- ces and Quantities of Motion themfelves, k priori, in all pair. Ages ; or were the Velocity of the firft diurnal Ro- tation of the upper Earth known, we might geometrically determine, a priori, how long ago that Rotation began, or how antientour Earth is.
XXII. The Variation of Magnetic Needles from the Azimuth of the Meridians of the internal "Magnet 5 is de- rived from the Difference of the Strength of the feveral Parts of the internal Magnet's Surface 5 which as it is only to be known by Experience, that Variation cannot be de- termined beforehand, unlefs where there are good Ac- counts how much it had formerly been j it being proba- ble that it returns round, and will be the fame in any Year of the next Revolution of the internal Magnet, that it has been in the like Year of any former Revolution, or will itfelf have a Revolution in about 1920 Years.
XXIII. The two fixed Magnetic Poles, in our upper Earth, firft introduced by Dr. Hailey, as neceffary to iblve the Irregularity of the Variation of the Horizontal Needle from the Meridians of the moveable internal Mag- net, feem not to have any juft Foundation in Nature ; the like Irregularities being found in the common Terrell* or Spherical Loadftones ; and being beft accounted for from the Compofition of the Magjiets, which are found to have Parts of different Degrees of Purity, Strength, and Perfection 5 fo that where the Parts are weaker than or- dinary, the ftronger neighbouring Parts prevail, and draw the Needle that way : not but Dr. Gilbert's Notion of prominent and depreffed Parts on Magnets may have fome room, and be allowed to contribute fomewhat to fuch Variations. See Needle.
As to theCaufesof Magnetifm, or the Manner in which thefe Phenomena of the Magnet are produced, we have yet no Hypothefis that will Satisfactorily account for them. Plutarch tells us, the Magnet attracts Iron, by emitting fome fpiritual Effluvia, whereby the contiguous Air being opened and driven on either Side, does again drive that contiguous to it 5 and thus the Action being communicated round, the Iron is thereby protruded : but this is contradicted by the equally vigorous Action of the Loadftone in Vacuo, and in the open Air. Others of the Antients afcribe the Action of the Magnet to a Soul that animates it ; and others to I know not what Sympathy between the Effluvia of the Iron and thofe of the: Magnet. The Opinion that principally pre- vails among the Moderns is that of Des Cartes, main- tained by Malsbranch, Rohault, Regis, &c. and even ad- mitted of and confirmed by Mr. Boyle, &c. In this 'tis fuppofed there is continually flowing, from the Poles of
the World, a fubtle, impalpable, and inviflble Mattel's channel'd or ftriated j which Matter circulating round the Earth, in the Plains of the Meridians, re-enters, at the Pole oppofite to that from which it iffued, and paffes a • gain thro' the Poles parallel to its Axis : That the Magnet has two Poles anfwerableto thofe of the Earth, and that out- of thefe there iffues a Matter like that juft men- tioned : That this Matter, entering in at one of theTFoles, gives the Impulfe whereby Iron tends to the Magnet, and produces what we call Attraction. Now befides the Mag- netical Matter re-entering the Poles of the Magnet, there is always a certain Quantity thereof circulating round the Magnet 5 compofing a kind of Vortex about it. The Space wherein this Matter moves, is the Sphere of Activity of the Magnet; within which its attractive Faculty is con- fined. As to its directive Faculty, or the Inclination of a Needle touched with it to the Polesof the World, and its Dip to a Point beneath the Horizon, it follows from the fame Principle j fince were the Magnet or Needle to have any other Situation, the Magnetic Matter would ftrikeon its Surface in vain ; and not being able to get Admiflion, would, by degrees, change its Situation, till fuch time as its Pores correfponded to the Courfe of the Magnetical Matter 3 which Situation having once acquired, it would ceafe to move 5 the Magnetical Matter then ceafing to difturb it. The Form of a Magnet therefore is fuppofed to confift in its being perforated by an infinite Number of parallel Pores ; fome whereof are difpofed to admit the ftriated Matter from the North Pole of the World, others that of the South 5 hence the North and South Poles of the Magnet.
As to the directive Power of the Magnet, Mr. Whifton t from the 1 ft, id, 3d, £f?c. Laws of Magnetifm, inclines to think it mechanical 5 and afcribes it to magnetic Effluvia circulating continually round the Loadftone j of which Circulation, he thinks, there are evident Indications in magnetic Experiments 5 as Mr. Boyle thinks there are of the Magnetifm or magnetic Effluvia of the Earth ; tho* thofe Effluvia were never yet rendered fenfible as Elec- tric Effluvia begin to be ', but the attractive Power Mr. Whifion thinks entirely 1m mechanical, as the Power of Gravity is 5 not being able to deyife any fuch Motion of a fubtle Fluid belonging to the Loadftone, as will ac- count for the attractive Power in the fefquidu plicate Pro- portion of the Diftances reciprocally 5 tho' if he could, yet would that be no more than to remove the immediate Power of the Supreme Being one Step further ; the laft Refort of all mechanical Principles whatever being into the immechanical Power and Efficiency of the Deity.
M. Hartfoeker maintains, that the Magnet is no more than a common Stone ; full of an infinite Number of hollow Prifms, which, by the diurnal Motion of the Earth, are ranged parallel to each other, and nearly pa- rallel to the Axis of the Earth. Thefe Prifms have their Cavities filled with an extremely fubtle Matter ; which, by the diurnal Motion of the Earth, is paffed from Prifm to Prifm : thus making a Circulation, and return- ing into the Prifms where it firft began : From thefe Principles he deduces all the Phenomena of the Magnet 5 and J/LJudry does the fame, from the Doctrine of Alkali and Acid.
There are Magnets found in moft of the Provinces of China, but the principal Ufe the Chinefe make of them is in Medicine. .Le CW/j&fe defcribes their manner of cutting them by a Machine, which, he fays, is vaftly more expe- ditious than any ufed among us.
The Magnet is alfo called Lapis Heracleus, from Hera* clea, a City of Magnefia, a Port of the antient Lydia t where it is fuppofed to have been firft found. Others derive the word Magnet from a Shepherd of that Name, who firft discovered it with the Iron of his Crook on Mount Ida. It is alfo called Lapis Naitticus, by reafon of its Ufe in Navigation ; and Siderites, from its attracting Iron, which the Greeks call «cA;p©-.
- MAGNETICAL AMPLITUDE, an Arch of the Ho-
rizon, contained between the Sun, at his Rifing or Set- ting, and the Eaft and Weft Point of the Compafs : found by obferving the Sun at his Rifing and Setting by an Amplitude Compafs. See Amplitude.
MAGNETICAL AZIMUTH. See Azimuth. MAGNETISM, a Term ufed by fome Chymifts, to fignify a certain Virtue, whereby one thing becomes af- fected at the fame time with another, either in the fame or a different manner. This is what they otherwife call Sympathy.
MAGNIFY, a Term chiefly ufed in reference to Mi- crofcopes, which are faid to magnify Objects, or to make them appear bigger than they really are ; but, in reali- ty, Microfcopesdo nor, nor cannot, magnify any Object, but only fhew it nearer and more of its Parts than before were taken notice of. See Microfcof>e.
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