the I N t R O D V C T I 0 M xiil * 
cc As to the fouth Poles, 1 take the AJian Pole, which 1 place about the Meridian of the Mand ‘Cele- 
« bes, to be fixed, and confequently the American Pole to move from the like Obfervations of the flow 
■« Decreafe of the' Variation on the Coaff of Java, and near the Meridian of the AJian Pole; tho’ I 
« mull confefs to have no Account of the Effects of the other beyond Magellan Streights. 
t£ If this be allowed me, it is plain, that the fixt Poles are the Poles of this external Shell or Cortex 
“ of the Earth, and the other two the Poles of the magnetical Nucleus, included and moveable within 
« the other. It likewife follows, that this Motion is Weftward, and by Confequence, that the aforefaid 
“ Nucleus has not precifely attained the fame Degree of Velocity, with the exterior Parts in their diur- 
«c na l Revolution, but fo nearly equals it, that in three hundred fixty-five Revolves, the Difference is 
«c fcarce fenfible/ This I conceive to arife from the Impulfe whereby this diurnal Motion was imprefs’d 
« on the Earth, being given to the external Parts, and from thence, in Time, communicated to the in- 
“ ternal, but not fo as perfe&ly to equal the Velocity of the firff Motion impreffed on them, and Mil 
“conveyed, by the fuperficiai Parts of the Globe. 
« As for the Quantity of this Motion, it is almoft impoffible to define it, both from the Nature of 
« this kind of Obfervation, which cannot be accurately performed, as alfo from the fmall Time thefe 
« Variations have been obferved, and their Change difcovered. It appears by all Circumftances, that 
« this Period is of many Centuries of Years, and as far as may be colle&ed from the Change of the 
c c Place where there was no Variation, by Reafon of the Equilibrium of the two fouthern magnetical 
“Poles, viz. from Cape d'Augulhas, to the Meridian of St. Helena (which is about 23 0 in ninety 
« Years) and of the Place where the wefterly Variation is in its Acme , or greatefl: Deflection, being 
<c about half as much, viz. from the Me of Diego Rioz , to the fouth-weft Parts of Madagascar, we 
“ may with fome Reafon conjecture, that the American Pole has moved Weft wards 46°, in that Time, 
“ and that the whole Period thereof is performed in feven hundred Years, or thereabouts ; fo that the 
“ nice Determination of this, and of feveral other Particulars in the magnetick Syftem, is referved for 
« remote Pofterity.^ All that we can hope to do, is, to leave behind us Obfervations that may be con- 
st fided in, and to propofe Hypothefes, which After-ages may examine, amend or rejeCt. Only here 
<c I muff take Leave to recommend to all Mafters of Ships, and other Lovers of natural Truths, that 
they ufe their utmoft Diligence to make, or procure to be made, Obfervations of thefe Variations in 
all Parts of the World, as well in the north, as fouth Latitude (after the laudable Cuftom of our Eajl- 
«« India Commanders) and that they be pleafed to communicate them to the Royal Society ,in order to leave 
<c as compleat a Hiftory, as may be, to thofe that are hereafter to compare all together, and to com pleat 
«« and perfeCl this abftrufe Theory.” 
There has not hitherto appeared any Thing more curious, or more correct, upon this Subject, than 
the foregoing admirable Difcourfe of the late learned Dr. Halley , whofe great Abilities,- and indefati- 
gable Diligence, have done the greatefl; Honour to this Nation, and fully fupported that Reputation 
which we had before acquired, of having done more in refpeCt to magnetical Difcoveries, than any 
other Nation in Europe. His Theory indeed, is at once fo fagacious, and fo ingenious, and withal, 
is delivered with fo great Modefty, fuch wife Precautions, and fo laudable a regard to Truth, that one 
may juftly affirm, it deferves much higher RefpeCt, and a more attentive Confideration, than is ufually 
due to Attempts of this Nature ; and, if after longer Experience, and a more particular Knowledge of 
the FaCts relating to the Variation in different Places, it ffiall appear capable of folving all the Diffi- 
culties, with which this SubjeCt has been hitherto burthened, Pollerity will, without doubt pay that 
due Veneration to the Memory of this Great Man, which his vaft Abilities and Angular Penetration 
have merited. 
But luppofing there fhould be fuch Deficiencies difcovered in his fecond Theory, as this candid and 
ingenious Man himfelf difcovered in his firft ; yet, before this can be done, we muff have fuch Ta- 
bles of Obfervations collected, as muff enable us to form a better Theory, or, which is ftill of greater 
Value, enable us to come at the Thing itfelf; or, in other Words, we muff be in a Condition to af- 
fign the true Period of Variation at any given Place : For, till this can be done, it will be Amply 
impoffible to fay, whether Dr. Halley ' s Theory will account for thefe Appearances or not: And if 
once, whether by Experiment, or Theory, a Method be found to eftablifh a certain Knowledge of 
the Period of Variation, the great End for which it is fought, will be attained, that is to fay, the 
eafieft Way of difcovering the Longitude will be found. 
As I look upon this to be the principal Advantage that would refult from our having a perfeCI and 
certain Knowledge of the Variation of the Needle, it is on this Account that I have infilled upon it 
fo largely, becaufe this is the Thing which, of all others, we moil want, towards compleating the 
Science of Navigation, which, though greatly improved by the Moderns, and therefore much fupe- 
rior to the Navigation of the Ancients, yet, as we before obferved, it is Hill deficient in many Points, 
and more particularly in this. 
At the fame Time that I mention this, I muff take Notice of another Thing, which, at firft 
Sight may feem to be a Paradox, but, at the fame Time, is an indifputable Truth, and withal a 
Truth of the higheft Importance. The Thing I mean is this, that the Defedts of modern Naviga- 
tion, though Disadvantageous to the Science, in refpedt to Practice, are, however, fo far from being 
difadvantageous when confidered in a fpeculative Light, that they are really and ffridtly fo many Ex- 
cellencies therein, when we compare the State of that Science now, to the State it was in amongff 
the Ancients ; for it was their Misfortune to have fo high an Opinion of their own Knowledge, as to 
charge upon Nature ; or, to fpeak more like a Chriftian, upon the Condudt of Divine Providence, 
thofe Defeats which were only in themfelves. Thus, for Inffance, they conceived, contrary to Truth, 
the greatefl Part of the Globe to be uninhabitable ; and from this Notion, pronounced thofe Difco- 
veries impoffible, which by the Induftry of fucceeding Ages have been made : Whereas with refpedt to 
Numb. 69. ^ e " the 
