MAG
(48*)
MAG
be tafces occafion to obferve, that the directive Power of the Loadftone feenis to be mechanical ; and to be derived from magnetic Effluvia, circulating continually tound it.
VI. The abfolute attractive Power of different armed Loadftones, is, ceteris paribus, according to the Quantity, r.ot of their Diameters or Solidities, but of the Surfaces of the Loadftones 5 or in a Duplicate Proportion of their Diameters.
VII. The Power of good Magnets unarmed, not fenfi- bly different in Strength, fimilar in Figure and Pofition, but unequal in Magnitude, is fome times a little greater, fome times a little lefs than in the Proportion of their fimilar Diameters.
VIII. The Loadftone attra&s Needles that have been touched, and others that have not been touched with equal Force, at Diftances unequal, viz. where the Di- ftances arc to one another as 5 to 2.
IX. Both Poles of a Loadftone equally attract Needles, till they be, tho' roughly, touched ; then it is, and then only, that that one Pole begins to attract one End, and repel the other : tho' the repelling Pole will ftill at- tract upon Contact, nay at very fmall Diftances notwith- itanding.
X. The attractive Power of Loadftones, in their fi- milar Pofition to, but different Diftances from Magnetic Needles, is in the Sefquiduplicate Proportion of the Di- ftances of their Surfaces from thofe Needles reciprocally 5 or as the mean Proportionals between the Squares and the Cubes of thofe Diftances reciprocally ; or as the Square Roots of the fifth Powers of thofe Diftances reciprocally. Thus the Magnetic Power of Attraction, at twice the Diftance from the Surface of the Loadftone, is between a fifth and fixth Part of that Power at the firft Diftance. At thrice the Diftance the Power is between the 15th and 1 dth Part, at four times the Diftance the Power, is 32 times as fmall, and at fix times the Diftance 88 times as fmall. Where it is to be noted, that the Diftances are not taken, as in the Law of Gravity, from the Cen- ter 5 but the Surface: all Experience affuring us, that the Magnetic Power refides chiefly, if not wholly, in the Surfaces of Loadftones and Iron ; without any parti- cular Relation to any Center at all. The Proportion here laid down was determined by Mr. Whifton, from a great Number of Experiments of Mr, Haiiksbee, Dr. Brook, Taylor, and himfelf. The Force they meafured by the Chords of thofe Arcs, by which the Magnet, at feveral Diftances, draws the Needle out of its natural Direction, to which Chords (as he has demonft rated) it is ever pro- portional. The Numbers in fome of their moft accurate Trials he gives us in the following Table, fetting down half the Chords, or the Sines of half thofe Arches of Declination, as the true Meafures of the Power of Mag- netifm.
Diftance i Inches.
Degrees of
Inclination.
Sines oft Arcs.
14 f" 13 f- %z'£r
10J- 9 k-
—175- — 349- —523- -«P 7 - -871- 1045- 1219-
Rat. Sefqtti- dupi.
-46S -zi<f
-170
-105
-87 - 70
XI. An Inclinatory, or Dipping-Needle, of fix Inches Radius, and of a Prifmatic or Cylindric Figure, when it ofcillates along the Magnetic Meridian, performs, here, every mean Vibration in about 6" or ?«o "' ; and every fmall Ofcillation in about 5 " i, or 350 '"; and the fame ]cind of Needle, fout Foot long, makes every mean Of- cillation in about 24 ", and every fmall one in about
iz "■
XII. The entire Power of Magnetifm in this Country, as it arrefls Needles a Foot long, is to that of Gravity nearly as 1 to 300 ; and as it affecf s Needles four Foot long, as 1 todoo.
XIII. The Quantity of Magnetic Power accelerating the fame Dipping-Needle, as it ofcillates in different vertical Planes, is ever as the Co-fines of the Angles made by thofe Planes, and the Magnetic Meridian taken on the Horizon.
Coroll Thus if we would eftimate the Quantity of For- ces in the horizontal and vertical Situations of Needles at London ; we fliall find that the latter, in Needles a Foot long, is, to the entire Force along the Magnetic Meridian, as jtf to too ; and in Needles four Foot long, as 9SC7 to icooo : whereas in the former, the entire Force in Nee- dles a Foot long, is as 18 to 100 ; and in thofe four Foot
long, as 2560 to 10000. Whence it follows, that the Power by which horizontal Needles are governed in thefe Parts of the World, is but one quarter of the Power by which the Dipping Needle is moved.
Hence alfo, fince the horizontal Needle is moved on- ly by a Part of the Power which moves the Dipping-Nee- dle ; and that it only points to a certain Place in the Ho- rizon, becaufe that Place is the neareft its original Ten • dency, of any, its Situation will allow it to tend to : When ever the Dipping-Needle Hands exaaiy perpendicular to the Horizon, the horizontal Needle will not refpefl one Point of the Compafs more than another, but will wheel about every way uncertainly.
XIV. The Times of Ofcillation and Vibration, both in dipping and horizontal Needles equally good, is as that of their Lengths directly ; and the actual Velocity of their Points along their Arcs always equal.
Hence Magnetic Needles are, ceteris paribus, flill bet- ter the longer they are 5 and that in the fame Proportion with their Lengths. See Needle.
XV. The Earth, on which we live, includes within it avail Sphetical Magnet, concentrical thereto, with its own Poles, Meridians, Equator, and Parallels ; and all much of the fame general Nature with thofe of fmall TerrelU, or Spherical Loadftones, in the poffeffion of the Curious among us.
XVI. The Power of a good Terrella, or Spherical Load- ftone, as it affects a Needle a Foot long, is equal to the Magnetic Power of that internal Loadftone about two and an half, or three Diameters off fuch Loadftone. From which Confideration the Quantity of Magnetic Attraction, at all Diftances from the internal Loadftone, for Needles a Foot long, may be determined ; and from the fame Confideration it appears, that the Diameter of this inter- nal Loadftone is about 11 ;o Miles. To which we may add, that, in regard Sir jfaac Newton has demonftrared, that the Power of Gravity diminifhes within the Earth, and is leffer there than at its Surface nearly, in the pro- portion of its greater Nearncfs to the Center ; the Magne- tic Power at 2900 Miles diftance from us, and nearly io(?o from the Earth's Center, which is || of the Power of Gravity here, will be fomewhat greater than the Power of Gravity there: Which Limit is worth our Attention, Gravity being ftronger than Magnetifm on the one Side' of it, and weaker on the other j we mean as it affects Needles of one foot Diameter. At that Limit, there- fore, at leaft near the Magnetic Poles, Iron, a Foot long, will be twice as heavy, and fall twice as faft as any other natural Body, viz. by the Union of thofe two equal Powers, Gravityand Magnetifm ; and of confequence, a- bove that Limit, fuch an Iron will be lefs than twice as heavy; below it, more than twice as heavy as any other natural Body.
XVII. The Earth's internal Loadftone is not fixed to our upper Parts, but is moveable with refpefl thereto, and act ually revolves on the Earth's Axis from Eaji to We/? in a certain long Period of Time ; as appears, be- yond Contradiction, from the conftant Variation of the horizontal Needle Weftward ; as well as the regular In- creafe of Inclination of the Dipping-Needle.
The only way to render this Motion, i. e. the Variation, pofflble and intelligible (to ufe Dr. Halley's Words) is to fuppofe it to turn about the Center of the Globe, having its Center of Gravity fixed and immoveable in the fame common Center of the Earth. This moveable internal Surface muft likewife be loofe, and detached from the external Parts of the Globe ; which may be reckoned the Shell, and the other the Nucleus, or inner Globe, included within it, with a fluid Medium between. Now from the Variations moving Weftwards, 'tis plain, that the forefaid Nucleus has not precifely attained the fame Degree of Velocity with the exterior Parts in their diurnal Revolution ; but fo nearly equals it, that in %Cj Revolves the Difference is fcarce fcnfible i and muft probably have arifen from hence, that the Impulfe, whereby the diurnal Motion was impreffed on the Earth, was given to the external Farts, and thence communi- cated to the internal.
XVIII. This internalMa^wef has one central Pole North- watds, in the nature of the Poles of our common Load- ftones j but its Southern Pole appears not to be central, but rather citcular ; and that at a great Diftance from the Southern Pole of the Earth.
XIX. The Northetn Magnetic Pole is now fituate a- bout theLatitude of 70T Deg. J, i.e. 13 Deg.4 from the North Pole of the Earth, and about 30 Deg." Eaftward from the Meridian of London.
XX. The Southern Magnetic circular Pole has its Center, or central Pole, nearly in the Parallel of 60 De- grees ; and in a Meridian palling along the Eaft Coaft of Burneo, about 1 1 7 Degrees Eaftward of London : its Ra- dius
