_Names of | _Longitude |_Latitude._| _Anno | _Variation
Places._ | from_ Lon.| | Dom._| Observ'd._
| | | |
| _d_ _m_ | _d_ _m_ | | _d_ _m_
_London_ | 0 0 | 51 32 N | 1580 | 11 15 E
| | | 1622 | 6 0 E
| | | 1634 | 4 5 E
| | | 1672 | 2 30 W
_Paris_ | 2 25 E | 48 51 N | 1683 | 4 30 W
| | | 1640 | 3 00 E
| | | 1666 | 0 0
| | | 1681 | 2 30 W
_Uraniburg_ | 13 0 E | 55 54 N | 1672 | 2 35 W
----------------------------+------------+-----------+-------+------------
_Copenhagen_ | 12 53 E | 55 41 N | 1649 | 1 30 E
| | | 1672 | 3 35 W
_Dantzick_ | 19 0 E | 54 23 N | 1679 | 7 00 W
_Mompelier_ | 4 0 E | 43 37 N | 1674 | 1 10 W
_Brest_ | 4 25 W | 48 23 N | 1680 | 1 45 W
----------------------------+------------+-----------+-------+------------
_Rome_ | 13 0 E | 41 50 N | 1681 | 5 0 W
_Bayonne_ | 1 20 W | 43 30 N | 1680 | 1 20 W
_Hudson's_ Bay | 79 40 W | 51 00 N | 1668 | 19 15 W
In _Hud._ Straights | 57 00 W | 61 00 N | 1668 | 29 30 W
In _Baffin's_ Bay at } | | | |
Sir _Thomas Smith's_} | 80 00 W | 78 00 N | 1616 | 57 00 W
Sound } | | | |
----------------------------+------------+-----------+-------+-----------
At Sea | 50 0 W | 38 40 N | 1682 | 7 30 W
At Sea | 31 30 W | 43 50 N | 1682 | 5 30 W
At Sea | 42 0 W | 21 0 N | 1678 | 0 40 E
Cape St. _Aug._ of _Brazile_| 35 30 W | 8 0 S | 1670 | 5 30 E
Cape _Frio_ | 41 10 W | 22 40 S | 1670 | 12 10 E
----------------------------+------------+-----------+-------+------------
At Sea off of the Mou. } | | | |
of the River _Plate_ } | 53 00 W | 39 30 S | 1670 | 20 33 E
At the East Entrance } | | | |
of _Magellan_ Straits } | 68 00 W | 52 30 S | 1670 | 17 00 E
At the W. Entrance of } | | | |
the _Magellan_ Straits } | 75 00 W | 53 00 S | 1670 | 14 10 E
_Baldivia_ | 73 00 W | 40 00 S | 1670 | 8 10 E
----------------------------+------------+-----------+-------+------------
At Cape d'_Agulbas_ | 16 30 E | 34 50 S | 1622 | 2 00 W
| | | 1675 | 8 00 W
At Sea | 1 0 E | 34 30 S | 1675 | 0 00
At Sea | 20 0 W | 34 0 S | 1675 | 10 30 E
At Sea | 32 0 W | 24 0 S | 1675 | 10 30 E
----------------------------+------------+-----------+-------+------------
At St. _Helena_ | 6 30 W | 16 00 S | 1677 | 0 40 E
At _Ascension_ | 14 30 W | 7 50 S | 1678 | 1 00 E
At _Johanna_ | 44 00 E | 12 15 S | 1675 | 19 30 W
At _Monbasa_ | 40 00 E | 4 00 S | 1675 | 16 00 W
At _Zocatra_ | 56 00 E | 12 30 N | 1674 | 17 00 W
----------------------------+------------+-----------+-------+------------
At _Aden_, at the Mo. } | | | |
of the _Red Sea_ } | 47 30 E | 13 00 N | 1674 | 15 00 W
At _Diego Roiz_ | 61 0 E | 20 0 S | 1676 | 20 30 W
At Sea | 64 30 E | 0 0 | 1676 | 15 30 W
At Sea | 55 0 E | 27 0 S | 1676 | 24 00 W
----------------------------+------------+-----------+-------+------------
At _Bombay_ | 72 30 E | 19 0 N | 1676 | 12 00 W
At Cape _Comorin_ | 76 00 E | 8 15 N | 1680 | 8 48 W
At _Ballafore_ | 87 00 E | 21 30 N | 1680 | 8 20 W
At Fort St. _George_ | 80 00 E | 13 15 N | 1680 | 8 10 W
At the W. Point of _Java_ | 104 00 E | 6 40 S | 1676 | 3 10 W
----------------------------+------------+-----------+-------+------------
At Sea | 58 00 E | 39 0 S | 1677 | 27 30 W
At the Isle of St. _Paul_ | 72 0 E | 38 0 S | 1677 | 23 30 W
At _Van Dimen's_ Land | 142 0 E | 42 25 S | 1642 | 0 0
At _New Zealand_ | 170 0 E | 40 50 S | 1642 | 9 0 E
At _Three Kings Isle_ in } | | | |
_New Zealand_. } | 169 30 E | 34 35 S | 1642 | 8 40 E
----------------------------+------------+-----------+-------+------------
At the Isle _Rotterdam_ } | | | |
in the South Sea } | 184 00 E | 20 15 S | 1642 | 6 20 E
On the Coast of _N. Guin._ | 149 00 E | 4 30 S | 1643 | 8 45 E
At the W. P. of _N. Guin._ | 126 00 E | 0 26 S | 1643 | 5 30 E
Tho' I could wish we could obtain from the _Spaniards_ what Variations
they find in their Voyages from the _Manilhas_ towards _Acapulco_,
through the North part of the South Sea; as likewise what it is at
_Japan_ from the _Dutch_: Yet (considering the number of these
Observations I have collected, and that they are made in parts of the
World so remote from _Europe_, and from one another) I suppose that the
Theory that answers these will scarce fail in those Regions from whence
we have as yet no account. But first we must make some Remarks upon the
foregoing Table: And, First,
That in all _Europe_ the Variation at this time is West, and more in the
Eastern Parts thereof than the Western: As likewise, that it seems
throughout to be upon the increase that way.
Secondly, That on the Coast of _America_, about _Virginia_,
_New-England_ and _New-Foundland_, the Variation is likewise Westerly;
and that it increases all the way as you go Northerly along the Coast,
so as to be above 20 Degrees at _New-Found-Land_, nearly 30 gr. in
_Hudson's Straights_, and not less than 57 Degrees in _Baffin's Bay_;
also, that as you Sail Eastward from this Coast, the Variation
diminishes. From these two it is a Legitimate Corollary: That _Somewhere
between_ Europe, _and the North part of_ America, _there ought to be an
Easterly Variation, or at least no Westerly_. And so I conjecture it is
about the Eastermost of the _Tercera Islands_.
Thirdly, That on the Coast of _Brasile_ there is East Variation, which
increases very notably as you go to the Southward, so as to be 12
Degrees at _Cape Frio_, and over against the River of _Plate_ 20½
Degrees: And from thence Sailing South-Westerly to the Straits of
_Magellan_ it decreases 17 Degrees, and at the West Entrance but 14
Degrees.
Fourthly, That at the Eastward of _Brasile_, properly so call'd, this
Easterly Variation decreases, so as to be very little at St. _Helena_
and _Ascension_, and to be quite gone, and the Compass Point true about
18 Degrees of Longitude West from the Cape of _Good-hope_.
Fifthly, That to the Eastward of the aforesaid Places a Westward
Variation begins, which Reigns in the whole _Indian_ Sea, and arises to
no less than Eighteen Degrees under the Equator it self, about the
Meridian of the Northern part of _Madagascar_; and near the same
Meridian, but in 39 Degrees South Latitude it is found full 27½
Degrees: From thence Easterly the West Variation decreases, so as to be
little more than eight Degrees at _Cape Comorin_, and than three Degrees
upon the Coast of _Java_; and to be quite extinct about the _Molucca
Islands_, as also a little to the Westwards of _Van Diemens_ Land found
out by the _Dutch_ in 1642.
Sixthly, That to the Eastward of the _Molucca's_ and _Van Diemens_ Land
in South Latitude there arises another Easterly Variation, which seems
not so great as the former, nor of so large Extent; for that at the
Island _Rotterdam_ it is sensibly less than upon the East Coast of _New
Guinea_; and, at the rate it decreases, it may well be suppos'd, that
about 20 Degrees farther East, or 225 Degrees East Longitude from
_London_, in the Latitude of 20 Degrees South, a Westerly Variation
begins.
Seventhly, That the Variations observ'd by the Honourable Sir _John
Norborough_ at _Baldivia_, and at the West Entrance of the _Straights_
of _Magellan_ do plainly shew, that That East Variation, noted in our
third Remark, is decreasing apace; and that it cannot reasonably extend
many Degrees into the South Sea from the Coast of _Peru_ and _Chili_,
leaving room for a small Westerly Variation, in that Tract of the
unknown World that lies in the mid-way between _Chili_ and
_New-Zealand_, and between _Hounds-Island_ and _Peru_.
Eighthly, That in Sailing North-West from St. _Helena_ by _Ascension_,
as far as the Equator, the Variation continues very small East, and as
it were constantly the same: So that in this part of the World the
Course, wherein there is no Variation, is evidently no Meridian, but
rather North-West.
Ninthly, That the Entrance of _Hudson's Straights_, and the Mouth of the
River of _Plate_, being nearly under the same Meridian, at the one place
the Needle varies 29½ Degrees to the _West_; at the other 20½
Degrees to the _East_. This plainly demonstrates the impossibility of
reconciling these Variations by the Theory of _Bond_; _which is by two
Magnetical Poles and an Axis, inclin'd to the Axis of the Earth_; from
whence it would follow, That _under the same Meridian the Variation
should be in all places the same way_.
These things being premised may serve as a sure Foundation to raise the
Superstructure of a Theory upon. But first it would not be amiss to shew
hereby the mistake of _Gilbert_ and _Des Cartes_: The first whereof
supposes, that _the Earth it self being in all its parts Magnetical, and
the Water not; wheresoever the Land is, thither also should the Needle
turn, as to the greater quantity of Magnetical Matter_. But this in many
Instances is not true; but most remarkably upon the Coast of _Brazile_,
where the Needle is so far from being attracted by the Land, that it
turns the quite contrary way, leaving the Meridian to lye N b E, which
is just along the Coast. As to the Position of _Des Cartes_, _that the
Iron and Loadstones hid in the Bowels of the Earth and the Bottom of the
Sea, may be the Causes that the Needle varies_; if we consider for how
great a part of the Earths Surface, _ex. gr._ in the whole _Indian_ Sea,
the Needle declines the same way, and that regularly, 'twill follow that
the attracting Substance that occasions it, must be very far distant.
Now by Experience we find the little force that Iron Guns have upon the
Compass in Ships (their Vertue, though they be Demiculverin, or greater
Cannon, being not perceptible at four or five Yards distance) and the
Experiments now before the _Royal Society_ do plainly shew, how little a
Magnetism there is in most crude Iron Oars: What quantity thereof must
be then suppos'd to make so powerful a Diversion at two or three
Thousand Miles distance? Yet I cannot deny that in some places near the
Shoar, or in Shoal-Water, the Needle may be irregularly directed from
the aforesaid Causes, and that not a little, as _Gassendus_ gives a
notable instance of the Island _Elba_ in the Mediterranean Sea: But
these differences from the general Direction are always signs of the
nearness of those Magnetical Substances, for the Production whereof that
Island _Elba_ has been famous from all Antiquity. Besides, against both
_Des Cartes_ and _Gilbert_, the change of the Variation, which has been
within these Hundred Years last past more than 15 gr. at _London_, is an
entire Demonstration; tho' _Des Cartes_ does not stick to say, that the
transportation of Iron from place to place, and the growth of new Iron
within the Earth, where there was none before, may be the cause thereof.
The same holds likewise against the _Hypothesis_ of _Magnetical Fibres_,
which _Kircher_ maintains.
Now to propose something that may answer the several appearances, and
introduce nothing strange in Philosophy, after a great many close
Thoughts, I can come to no other Conclusion than that, _The whole Globe
of the Earth is one great Magnet, having four Magnetical Poles, or
Points of Attraction, near each Pole of the Equator. Two; and that, in
those parts of the World which lie near adjacent to any one of those
Magnetical Poles, the Needle is govern'd thereby, the nearest Pole being
always predominant over the more remote_. The parts of the Earth wherein
these Magnetical _Poles_ lie, cannot as yet be exactly determin'd for
want of sufficient _Data_ to proceed Geometrically; but, as near as
Conjecture can reach, I reckon that the _Pole_, which is at present
nearest to us, lies in or near the Meridian of the Lands-end of
_England_, and not above seven Degrees from the Pole Arctick; by this
_Pole_ the Variations in all _Europe_ and _Tartary_, and the _North Sea_
are principally govern'd, though with regard to the other Northern Pole,
whose situation is in a _Meridian_ passing about the middle of
_California_, and about 15 gr. from the North Pole of the World; to this
the Needle has chiefly respect in all the North _America_, and in the
two Oceans on either side thereof, from the _Azores_ Westward to
_Japan_, and farther. The two Southern Poles are rather farther distant
from the South Pole of the World: The one about sixteen Degrees
therefrom, is in a Meridian, some twenty Degrees to the Westward of
_Magellan Straights_, or ninety five Degrees West from _London_: This
commands the Needle in all the _South-America_, in the _Pacifick Sea_,
and the greatest part of the _Ethiopick_ Ocean. The Fourth and last
_Pole_ seems to have the greatest Power, and largest Dominions of all,
as it is the most remote from the _Pole_ of the World, being little less
than 20 Degrees distant therefrom in the Meridian, which passes through
_Hollandia Nova_, and the Island _Celebes_ about one hundred and twenty
Degrees East from _London_; this Pole is predominant in the South part
of _Africa_, in _Arabia_ and the _Red Sea_, in _Persia_, _India_, and
its Islands, and all over the _Indian Sea_, from the Cape of _Good-Hope_
Eastwards to the middle of the great South Sea, that divides _Asia_ from
_America_. This seems to be the present Disposition of the Magnetical
Vertue throughout the whole Globe of the Earth; it remains to shew how
this Hypothesis makes out all the Variations that have been observ'd of
late; and how it answers to our several Remarks drawn from the Table.
And first it is plain, that (our _European_ North _Pole_ being in the
Meridian of the Lands-end of _England_) all places more Easterly than
that will have it on the West side of their Meridian, and consequently
the Needle, respecting it with its Northern Point, will have a Westerly
Variation, which will still be greater as you go to the Eastwards, till
you come to some Meridian of _Russia_, where 'twill be greatest, and
from thence decrease again. Thus at _Brest_ the Variation is but 1¾
Degrees, at _London_ 4½ Degrees; but at _Dantzick_ seven Degrees
West. To the Westward of the Meridian of the Lands-end, the Needle ought
to have an Easterly Variation; were it not that (by approaching the
_American_ Northern _Pole_, which lies on the West side of the Meridian,
and seems to be of greater force than this other) the Needle is drawn
thereby Westwards, so as to counterballance the Direction given by the
_European Pole_, and to make a small West Variation in the Meridian of
the Lands-end it self. Yet I suppose that about the Meridian of the Isle
_Tercera_, our nearest _Pole_ may so far prevail as to give the Needle a
little turn to the East, though but for a very small space: The
Counterballance of those two _Poles_ permitting no considerable
Variation in all the Eastern Parts of the _Atlantick_ Ocean; nor upon
the West Coasts of _England_ and _Ireland_, _France_, _Spain_ and
_Barbary_. But to the Westwards of the _Azores_ the Power of the
_American_ Pole overcoming that of the _European_, the Needle has
chiefly respect thereto, and turns still more and more towards it as you
approach it. Whence it comes to pass, that on the Coast of _Virginia_,
_New-England_, _New-found-Land_, and in _Hudson's-Straights_ the
Variation is Westward; that it decreases as you go from thence towards
_Europe_, and that it is less in _Virginia_ and _New-England_, than in
_New-found-Land_, and _Hudson's-Straights_. This Westerly Variation
again decreases, as you pass over the _North America_; and about the
Meridian of the middle of _California_ the Needle again points due
North; and from thence Westward to _Yedzo_ and _Japan_, I make no doubt
but the Variation is Easterly, and half the Sea over no less than
fifteen Degrees, if there be any truth in this Hypothesis of mine.
Therefore I propose this as a Trial, that the whole may be scann'd
thereby; and I conceive it will not be hard to know of the _Spaniards_
how it is, who so frequently sail through that Ocean, in their return
from the _Manilha_ Isles. This East Variation extends over _Japan_,
_Yedzo_, _East-Tartary_, and part of _China_, till it meet with the
Westerly, which is govern'd by the _European_ North Pole, and which I
said was greatest some where in _Russia_.
Towards the Southern Pole the effect is much the same, only that here
the South Point of the Needle is attracted. Hence it will follow, that
the Variation on the Coast of _Brazile_, at the River of _Plate_, and so
on to the Straights of _Magellan_, should be Easterly (as in our third
Remark); if we suppose a Magnetical Pole situate about twenty Degrees
more Westerly than the Straights of _Magellan_. And this Easterly
Variation doth extend Eastward over the greatest part of the _Ethiopick_
Sea, till it be counterpoised by the Vertue of the other Southern
_Pole_; as it is about mid-way between the Cape of _Good-Hope_, and the
Isles of _Tristan d' Acuntia_. From thence Eastwards, the _Asian_ South
Pole (as I must take the liberty to call it) becoming prevalent, and the
South point of the Needle being attracted thereby, there arises a West
Variation, very great in quantity and extent, because of the great
distance of this Magnetical Pole of the World. Hence it is, that in all
the _Indian_ Sea as far as _Hollandia Nova_, and farther, there is
constantly West Variation; at that under the Equator it self it arises
to no less than eighteen Degrees, where 'tis most. About the Meridian of
the Island _Celebes_, being likewise that of this Pole, this Westerly
Variation ceases, and an Easterly begins; which reaches, according to my
Hypothesis, to the middle of the _South-Sea_, between _Zelandia Nova_,
and _Chili_, leaving room for a small _West Variation_ govern'd by the
_American_ South Pole, which I shew'd to be in the _Pacifick Sea_, in
the sixth and seventh Remark.
What I have now said, does plainly shew the sufficiency of this
Hypothesis for solving the Variations that are at this time observ'd in
the temperate and frigid _Zones_, where the Direction of the Needle
chiefly depends upon the Counterpoise of the forces of two Magnetical
Poles of the same Nature; and I suppose I have shewn how it comes to
pass, that under the same Meridian the Variation should be in one place
29½ West, and another 20½ East; as I have noted in my ninth Remark.
In the _Torrid Zone_, and particularly under the Equinoctial, respect
must be had to all four Poles, and their Positions well consider'd,
otherwise it will not be easie to determine what the Variations shall
be; the nearest _Pole_ being always the strongest; yet not so, as not to
be counterballanc'd sometimes by the united forces of two more remote; a
notable Instance whereof is in our eighth Remark, where I took notice,
that in sailing from St. _Helena_ by the Isle of _Ascension_, to the
Equator, on a N. W. Course, the Variation is very little Easterly, and
in that whole Tract unalterable; for which I give this Reason, That the
South _American_ Pole (which is considerably the nearest in the
aforesaid Places) requiring a great Easterly Variation, is counterpoised
by the contrary Attraction of the _North-American_ and the
_Asian-South-Pole_; each whereof singly are in these Parts, weaker than
the _American-South-Pole_; and upon the North West Course, the Distance
from this latter is very little varied; and as you recede from the
_Asian-South-Pole_, the Ballance is still preserv'd by the access
towards the _North-American-Pole_. I mention not in this Case the
_European-North-Pole_, its Meridian being little remov'd from those of
these places; and of it self requiring the same Variations we here find.
After the same manner we might proceed to conclude the Variations in
other places under and near the Equator; but I purposely leave it for an
Exercise to the Thoughts of the serious Reader, who is desir'd to help
his Imagination, by having before him a Map or Globe of the Earth: And
to mark thereon the Magnetical _Poles_ in the _Longitudes_ and
_Latitudes_ I assign them. (_Vide Plate 2._)
Thus, I hope, I have not lost my Pains and Study in this difficult
Subject; believing that I have put it past doubt, _That there are in the
Earth four such Magnetical Points or Poles, which occasion the great
variety and seeming irregularity which is observed in the Variations of
the Compass_. But to calculate exactly what it is, in any place
assign'd, is what I dare not yet pretend to, though I could wish it were
my happiness to be able to oblige the World with so useful a piece of
Knowledge; there are Difficulties that occur, that render the thing as
yet not feasible; for first there are a great many Observations
requisite, which ought to be made at the same time; not at Sea, but
ashore, with greater Care and Attention than the generality of Sailors
apply. And besides, it remains undetermin'd in what proportion the
attractive Power decreases, as you remove from the _Pole_ of a Magnet,
without which it were a vain attempt to go about to calculate. There is
yet a further Difficulty, which is the Change of the Variation, one of
the Discoveries of this last Century; which shews, that it will require
some hundreds of Years to establish a compleat Doctrine of the
Magnetical System. From the foregoing Table it should seem, that all the
Magnetical _Poles_ had a motion Westward: But if it be so, 'tis evident,
that it is not a Rotation about the Axis of the Earth; for then the
Variations would continue the same, in the same parallel of _Latitude_
(the _Longitude_ only chang'd) as much as is the motion of the
Magnetical _Poles_, but the contrary is found by Experience; for there
is no where in the Latitude of 15½ North between _England_ and
_America_, a Variation of eleven Degrees East at this time; as it was
once here at _London_; it seems therefore, that our _European_ Pole is
grown nearer the Pole _Arctick_ than it was heretofore, or else that it
has lost part of its Vertue. But whether these Magnetical Poles move
altogether with one motion, or with several; whether equally or
unequally; whether Circular or Libratory: If Circular, about what
Center; if Libratory, after what manner; are Secrets as yet utterly
unknown to Mankind, and are reserv'd for the Industry of future Ages.
_An Account of the Cause of the Change of the Variation of the
Magnetical Needle, with an Hypothesis of the Structure of the
Internal Parts of the Earth; as it was proposed to the _Royal
Society_ in one of their late Meetings. By Mr. _Edmund Halley_._
Having in the precedent Discourse delivered a Theory of the Variation of
the Magnetical Compass, wherein I did collect as many Observations as at
that time I could procure, and having carefully compar'd them together,
I came at length to this general conclusion, _That the Globe of the
Earth might be supposed to be one great Magnet, having four Magnetical
Poles or Points of Attraction, near each Pole of the Equator two; and
that in those parts of the World which lie near adjacent to any one of
those Magnetical Poles, the Needle is chiefly govern'd thereby; the
nearest Pole being always predominant ever the more remote_. And I there
have endeavour'd to state and limit the present Position of those Poles
in the Surface of our Globe, which the Reader pleasing to consult, will
save us the pains of repeating. But after all, tho' that Discourse was
favourably receiv'd both at home and abroad, as seeming to render a
tolerable account of the observ'd Variations, yet I found two
Difficulties not easie to surmount; the one was, that no Magnet I had
ever seen or heard of, had more than two opposite _Poles_, whereas the
Earth had visibly four, and perhaps more. And secondly, it was plain
that these _Poles_ were not, at least all of them, fixt in the Earth,
but shifted from place to place, as appear'd by the great Changes in the
Needles Direction within this last Century of Years, not only at
_London_, (where this great Discovery was first made) but almost all
over the Globe of Earth; whereas it is not known or observ'd that the
_Poles_ of a Load-stone ever shifted their place in the Stone, nor
(considering the compact hardness of that Substance) can it easily be
suppos'd; though the Matter of Fact be too notorious and universal, not
to be accounted for.
These Difficulties had wholly made me despond, and I had long since
given over an Inquiry I had so little hopes of, when in accidental
Discourse, and least expecting it, I stumbl'd on the following
Hypothesis; in delivering whereof, if I shall seem to advance any thing
that looks like Extravagant or Romantick, the Reader is desir'd to
suspend his Censure, till he have consider'd the force and number of the
many Arguments which concur to make good so new and so bold a
Supposition.
Though it be sufficiently known and allow'd, that the Needles Variation
changes, it will be necessary however to give a few Instances, whereby
it may appear that this Change is gradual and universal, and the effect
of a great and permanent motion: For which take the following Examples.
At _London_, in the Year 1580, the Variation was observ'd by Mr.
_Burrows_ to be 11° 15' East. In _Anno_ 1622, the same was found by Mr.
_Gunter_ to be but 6° 0' East. In the Year 1634, Mr. _Gellibrand_ found
it 4° 5' East. In 1657, Mr. _Bond_ observ'd that there was no Variation
at _London_. _Anno_ 1672, my self observ'd it 2° 30' to the West; and in
the Year 1692, I again found it 6° 00' West. So that in 112 Years the
Direction of the Needle has chang'd no less than seventeen Degrees.
At _Paris_, _Orontius Finæus_ about the Year 1550, did account it about
eight or nine Degrees East Variation. _Anno_ 1640, it was found three
Degrees East. _Anno_ 1660, there was no Variation there, and _Anno_
1681, I found it to be 2° 30' to the West.
At _Cape d' Agulhas_, the most Southerly Promontary of _Africa_, about
the Year 1600, the Needle pointed due North and South without Variation,
whence the _Portugueze_ gave its name. _Anno_ 1622, there was two
Degrees West Variation. _Anno_ 1675, it was 8° 50' West; and in the Year
1691, it was curiously observ'd not less than eleven Degrees West.
At St. _Helena_, about the Year 1600, the Needle declin'd eight Degrees
to the East. _Anno_ 1623, it was but 6° 0' East. _Anno_ 1677, when I was
there, I observ'd it accurately on Shoar to be 0° 40' East; and in 1692
it was found about 1° to the Westward of the North.
At _Cape Comorine_ in _India_, in the Year 1620, there was 14° 20' West
Variation. In the Year 1680, there was 8° 48', but now lately in the
Year 1688, it was no more than 7° 30', so that here the Needle has
return'd to the East about seven Degrees in seventy Years.
In all the other Examples the Needle has gradually mov'd towards the
West, and the places are too far asunder to be influenc'd by the removal
of any Magnetical Matter, which may by accident be transplac'd within
the Bowels, or on the Surface of the Earth. If more Examples are
desir'd, the Reader may be furnished with them in the _Portugueze
Routier_ of _Aleixo de Motta_ (written about the Year 1600) and in the
Voyage of _Beaulieu_, both publish'd in Mr. _Thevenot_'s first
Collection of curious Voyages, Printed at _Paris_, _Anno_ 1663; which he
is to compare with the Journals of our late _East India_ Voyagers, and I
am assur'd, that it will be thereby evident, that the Direction of the
Needle is in no place fix'd and constant, tho' in some it change faster
than in others: And where for a long time it has continu'd as it were
unalter'd, it is there to be understood, that the Needle has its
greatest Deflection, and is become Stationary in order to return, like
the Sun, in the Tropick. This, at present, is in the _Indian Sea_, about
the Island _Mauritius_, where is the highest West Variation, and in a
Tract tending from thence into the N. N. W. towards the _Red-Sea_ and
_Egypt_. And in all Places to the Westward of this Tract, all over
_Africa_ and the Seas adjoining, the West Variation will be found to
have encreas'd; and to the Eastwards thereof as in the Example of _Cape
Comorine_, to have decreased, _viz._ all over the _East-Indies_, and the
Islands near it.
After the like manner in that Space of East Variation, which, beginning
near St. _Helena_, is found all over the South _America_, and which at
present is highest about the Mouth of _Rio de la Plata_, it has been
observ'd, that in the Eastern Parts thereof, the Variation of the Needle
gradually decreases; but whether on the contrary it increases in those
places which lie more Westerly than that Tract wherein the highest East
Variation is found; or how it may be in the vast _Pacifick Sea_, we have
not Experience enough to ascertain, only we may by Analogy infer, that
both the East and West Variations therein do gradually increase and
decrease after the same Rule.
These _Phænomena_ being well understood and duly consider'd, do
sufficiently evince, That the whole Magnetical System is by one, or
perhaps more motions translated, whether Eastwards or Westwards, I shall
anon discuss; that this moving thing is very great, as extending its
effects from Pole to Pole, and that the motion thereof is not _per
saltum_, but a gradual and regular motion.
Now considering the Structure of our _Terraqueous_ Globe, it cannot be
well suppos'd that a very great part thereof can move within it, without
notably changing its Center of Gravity and the Equilibre of its Parts,
which would produce very wonderful Effects in changing the Axis of
diurnal Rotation, and occasion strange alteration in the Seas Surface,
by Inundations and Recesses thereof, such as History never yet
mention'd. Besides, the solid parts of the Earth are not to be granted
permeably by any other than fluid Substances, of which we know none that
are any ways Magnetical. So that the only way to render this motion
intelligible and possible, is to suppose it to turn about the Center of
the Globe, having its Center of Gravity fix'd and immoveable in the same
common Center of the Earth: And there is yet requir'd, that this moving
internal Substance be loose and detached from the external Parts of the
Earth whereon we live; for otherwise, were it affix'd thereto, the whole
must necessarily move together.
So then the External Parts of the Globe may well be reckon'd as the
Shell, and the Internal as a _Nucleus_ or inner Globe, included within
ours, with a fluid Medium between, which having the same Common Center
and Axis of diurnal Rotation, may turn about with our Earth each twenty
four Hours; only this outer Sphere having its turbinating motion some
small matter either swifter or slower than the internal Ball: And a very
minute Difference in length of time, by many Repetitions becoming
sensible, the Internal Parts will by degrees recede from the External,
and not keeping pace with one another, will appear gradually to move
either Eastwards or Westwards by the difference of their motions.
Now supposing such an Internal Sphere having such a motion, we shall
solve the two great Difficulties we encounter'd in my former Hypothesis:
For if this exteriour Shell of Earth be a Magnet, having its Poles at a
distance from the Poles of diurnal Rotation; and if the Internal
_Nucleus_ be likewise a Magnet, having its Poles in two other places,
distant also from the Axis; and these latter by a gradual and slow
motion change their place in respect of the External; we may then give a
reasonable account of the four Magnetical Poles I presume to have
demonstrated before; as likewise of the Changes of the Needles
Variations, which till now hath been unattempted.
The Period of this Motion being wonderful great, and there being hardly
an hundred Years since these Variations have been duly observ'd, it will
be very hard to bring this Hypothesis to a Calculus, especially since,
though the Variations do increase and decrease regularly in the same
place, yet in differing places, at no great distance, there are found
such casual Changes thereof as can no ways be accounted for by a regular
Hypothesis; as depending upon the unequal and irregular distribution of
the Magnetical Matter within the Substance of the External Shell or Coat
of the Earth, which deflect the Needle from the Position it would
acquire from the effect of the general Magnetism of the whole. Of this
the Variations at _London_ and _Paris_ give a notable Instance, for the
Needle has been constantly about 1°½ more Easterly at _Paris_ than at
_London_; though it be certain that according to the general effect, the
Difference ought to be the contrary way: Notwithstanding which, the
Variations in both places do change alike.
Hence, and from some other of like Nature, I conclude, That the two
Poles of the External Globe are fixt in the Earth, and that if the
Needle were wholly govern'd by them, the Variations thereof would be
always the same, with some little Irregularities upon the account I but
just now mention'd: But the Internal Sphere having such a gradual
translation of its Poles, does influence the Needle, and direct it
variously, according to the result of the attractive or directive Power
of each Pole; and consequently there must be a Period of the Revolution
of this Internal Ball, after which the Variations will return again as
before. But if it shall in future Ages be observ'd otherwise, we must
then conclude that there are more of these Internal Spheres, and more
Magnetical Poles than Four, which at present we have not a sufficient
number of Observations to determine, and particularly in that vast _Mar
del Zur_, which occupies so great a part of the whole Surface of the
Earth.
If then two of the Poles be fixt and two moveable, it remains to
ascertain which they are that keep their place; and though I could wish
we had the Experience of another Century of Years to found our
Conclusions upon, yet I think we may safely determine, That our
_European North Pole_ (which in the precedent Discourse I suppos'd near
the Meridian of the Lands-end of _England_, and about seven Degrees
therefrom) is that That is moveable of the two Northern Poles, and that
That has chiefly influenc'd the Variations in these parts of the World:
For in _Hudson_'s Bay, which is under the Direction of the _American_
Pole, the Change is not observ'd to be near so fast as in these parts of
_Europe_, though that Pole be much farther remov'd from the Axis.
As to the South Poles, I take the _Asian_ Pole, which I place about the
Meridian of the Island _Celebes_ to be the fixt, and consequently the
_American_ Pole to move; from the like Observation of the slow Decrease
of the Variation on the Coast of _Java_, and near the Meridian of the
_Asian_ Pole; though I must confess to have no account of the effects of
the other beyond _Magellan_'s Streights.
If this be allow'd me, 'tis plain that the fixt Poles are the Poles of
this External Shell or Cortex of the _Earth_, and the other two the
Poles of a Magnetical _Nucleus_ included and moveable within the other.
It likewise follows, that this Motion is Westwards, and by consequence
that the aforesaid _Nucleus_ has not precisely attained the same degree
of Velocity with the exteriour Parts in their diurnal Revolution; but so
very nearly equals it, that in 365 Revolves the difference is scarce
sensible. This I conceive to arise from the Impulse whereby this diurnal
Motion was imprest on the Earth, being given to the External Parts, and
from thence in time communicated to the Internal; but not so as
perfectly to equal the Velocity of the first Motion impress'd on, and
still conserv'd by the superficial Parts of the Globe.
As to the quantity of this Motion it is almost impossible to define it,
both from the Nature of this kind of Observation, which cannot be very
accurately perform'd, as also from the small time these Variations have
been observ'd, and their Change discover'd. It appears by all
Circumstances, that its Period is of many Centuries of Years, and as far
as may be collected from the Change of the Place, where there was no
Variation, by reason of the Equilibre of the two Southern Magnetical
Poles, _viz._ from _Cape d' Agulhas_ to the Meridian of St. _Helena_
(which is about 23 _degr._ in about ninety Years) and of the place where
the Westerly Variation is in its ἀκμὴ or greatest Deflection, being
about half so much, _viz._ from the Isle of _Diego Roiz_ to the South
West Parts of _Madagascar_. We may with some Reason conjecture, that the
_American_ Pole has mov'd Westwards forty six Degrees in that time, and
that the whole Period thereof is perform'd in seven hundred Years, or
thereabouts; so that the nice Determination of this, and of several
other Particulars in the Magnetick System is reserv'd for remote
Posterity; all that we can hope to do, is to leave behind us
Observations that may be confided in, and to propose an Hypothesis which
after Ages may examine, amend or refute. Only here I must take leave to
recommend to all Masters of Ships, and all others, Lovers of Natural
Truths, that they use their utmost Diligence to make, or procure to be
made, Observations of these Variations in all parts of the World, as
well in the North as South Latitude (after the laudable Custom of our
_East India_ Commanders) and that they please to communicate them to the
_Royal Society_, in order to leave as compleat a History as may be to
those that are hereafter to compare all together, and to compleat and
perfect this abstruse Theory.
And by the way it will not be amiss to amend a receiv'd Error in the
Practice of observing the Variation, which is, to take it by the
Amplitude of the Rising and Setting Sun, when his Center appears in the
visible Horizon; whereas he ought to be observ'd when his under Limb is
still above the Horizon about ⅔ of his Diameter, or twenty Minutes,
upon the score of the Refraction, and the height of the Eye of the
Observer above the Surface of the Sea: Or else they are to work the
Amplitudes as they do the Azimuth, reckoning the Suns Distance from the
Zenith 90° 36': This, though it be of little consequence near the
Æquinoctial, will make a great Error in high Latitudes, where the Sun
rises and sets obliquely.
But to return to our Hypothesis, In order to explain the Change of the
Variations, we have adventur'd to make the Earth hollow, and to place
another Globe within it; and I doubt not but this will find Opposers
enough. I know 'twill be Objected, That there is no Instance in Nature
of the like thing; that if there was such a middle Globe it would not
keep its place in the Center, but be apt to deviate therefrom, and might
possibly chock against the Concave Shell, to the ruin, or at least
endammaging thereof; That the Water of the Sea would perpetually leak
through, unless we suppose the Cavity full of Water; That were it
possible, yet it does not appear of what use such an inward Sphere can
be of, being shut up in Eternal Darkness, and therefore unfit for the
Production of Animals or Plants; with many more Objections, according to
the Fate of all such new Propositions.
To these, and all other that I can foresee, I briefly Answer, That the
Ring environing the Globe of _Saturn_ is a notable Instance of this
kind, as having the same common Center, and moving along with the
Planet, without sensibly approaching him on one side more than the
other. And if this Ring were turn'd on one of its Diameters, it would
then describe such a Concave Sphere as I suppose our External one to be.
And since the Ring, in any Position given, would, in the same manner,
keep the Centre of _Saturn_ in its own, it follows, that such a Concave
Sphere may move with another included in it, having the same common
Centre. Nor can it well be suppos'd otherwise, considering the Nature of
Gravity; for should these Globes be adjusted once to the same common
Centre, the Gravity of the parts of the Concave would press equally
towards the Centre of the inner Ball, which equality must necessarily
continue till some External Force disturb it, which is not easie to
imagine in our Case. This perhaps I might more intelligibly express, by
saying that the inner Globe being posited in the Centre of the
Exteriour, must necessarily ascend which way soever it move; that is, it
must overcome the force of Gravity pressing towards the common Centre,
by an impulse it must receive from some outward Agent; but all outward
Efforts being sufficiently fenc'd against by the Shell that surrounds
it, it follows, that this _Nucleus_ being once fixt in the common
Centre, must always there remain.
As to the leaking of the Water through this Shell, when once a passage
shall be found for it to run through, I must confess it is an Objection
seemingly of weight; but when we consider how tightly great Beds of
Chalk or Clay, and much more Stone do hold Water, and even Caves arch'd
with Sand; no Man can doubt but the Wisdom of the Creator has provided
for the Macrocosm by many more ways than I can either imagine or
express, especially since we see the admirable and innumerable
Contrivances wherewith each worthless Individual is furnish'd both to
defend it self, and propagate its Species. What Curiosity in the
Structure, what Accuracy in the Mixture and Composition of the parts,
ought not we to expect in the Fabrick of this Globe, made to be the
lasting Habitation of so many various Species of Animals, in each of
which there want not many Instances that manifest the boundless Power
and Goodness of their Divine Author; and can we then think it a hard
Supposition, that the Internal Parts of this Bubble of Earth should be
replete with such Saline and Vitriolick Particles as may contribute to
Petrefaction, and dispose the transuding Water to shoot and coagulate
into Stone, so as continually to fortifie, and, if need were, to
consolidate any breach or flaw in the Concave Surface of the Shell.
And this perhaps may not without Reason be suppos'd to be the final
Cause of the admixture of the Magnetical Matter in the Mass of the
Terrestrial parts of our Globe, _viz._ To make good and maintain the
Concave Arch of this Shell: For by what the Excellent Mr. _Newton_ has
shewn in his _Principia Philosophiæ_, it will follow, that according to
the general Principle of Gravity, visible throughout the whole Universe,
all those Particles that by length of time, or otherwise, shall moulder
away, or become loose on the Concave Surface of the External Sphere,
would fall in, and with great force descend on the Internal, unless
those Particles were of another sort of Matter capable by their stronger
tendency to each other, to suspend the force of Gravity; but we know no
other Substances capable of supporting each other by their mutual
Attraction but the Magnetical, and these we see miraculously to perform
that Office, even where the Power of Gravity has its full effect, much
more within the Globe where it is weaker. Why then may we not suppose
these said Arches to be lin'd throughout with a Magnetical Matter, or
rather to be one great Concave Magnet, whose two Poles are the Poles we
have before observ'd to be fixt in the Surface of our Globe.
Another Argument, favouring this Hypothesis, is drawn from a Proposition
of the same Mr. _Newton_, where he determines the force wherewith the
_Moon_ moves the _Sea_ in producing the _Tides_: His Words are,
_Densitas Lunæ est ad densitatem Terra ut 680 ad 387 seu 9 ad 5
quamproximé. Est igitur corpus Lunæ densius ac magis terrestre quam
Terra nostra_, p. 466. Now if the Moon be more solid than the Earth, as
9 to 5, why may we not reasonably suppose the Moon, being a small Body,
and a secondary Planet, to be solid Earth, Water, Stone, and this Globe
to consist of the same Materials, only four Ninths thereof to be Cavity,
within and between the Internal Spheres; which I would render not
improbable.
To those that shall enquire of what use these included Globes can be, it
must be allow'd, that they can be of very little service to the
Inhabitants of this outward World, nor can the Sun be serviceable to
them, either with his Light or Heat. But since it is now taken for
granted, that the Earth is one of the Planets, and they all are with
Reason suppos'd Habitable, though we are not able to define by what sort
of Animals; and since we see all the parts of the Creation abound with
Animate Beings, as the Air with Birds and Flies, the Water with the
numerous varieties of Fish, and the very Earth with Reptiles of so many
sorts; all whose ways of Living would be to us incredible did not daily
Experience teach us. Why then should we think it strange that the
prodigious Mass of Matter, whereof this Globe does consist, should be
capable of some other improvement than barely to serve to support its
Surface? Why may not we rather suppose that the exceeding small quantity
of solid Matter, in respect of the fluid Æther, is so dispos'd by the
Almighty Wisdom, as to yield as great a Surface for the use of living
Creatures, as can consist with the conveniency and security of the
whole? We our selves, in Cities where we are pressed for Room, commonly
build many Stories one over the other, and thereby accommodate a much
greater multitude of Inhabitants.
But still it will be said, That without Light there can be no living,
and therefore all this _apparatus_ of our inward Globes must be useless:
To this I Answer, That there are many ways of producing Light which we
are wholly ignorant of; the Medium it self may be always luminous after
the manner of our _Ignes fatui_. The Concave Arches may in several
places shine with such a Substance as invests the Surface of the Sun;
nor can we, without a boldness unbecoming a Philosopher, adventure to
assert the impossibility of peculiar Luminaries below, of which we have
no sort of _Idea_. I am sure the Poets _Virgil_ and _Claudian_ have gone
before me in this Thought, inlightning their _Elysian Fields_ with Sun
and Stars proper to those infernal, or rather internal Regions. _Virg.
Æneid. 6._
_Largior hic compos Æther & lumine vestit
Purpureo; Solemque suum sua Sidera norunt._