890 c O M 
certainly gather from thefe obfervations, that even al¬ 
though the theory of the variation Ihould be completed, 
we mult expert (by what we already know of magnetifrn 
in general) that the dilturbances of the needle, by local 
caufes intervening between it and the great influence by 
which it is chiefly directed, may be fo confiderabie as to 
aff.ct the pofition of the compafs needle in a very fenfible 
manner; for we know that the metallic fubltances in the 
bowels of the earth are in a Hate of continual change, and 
this to an extent altogether unknown. 
But, as the theory of Dr. Halley, in accounting for the 
variation of the compafs, is now generally received, we 
Ihall here (late his hypothefis : he luppofes, a3 mentioned 
above, four magnetic poles or points of attraction; near 
each pole of the equator two ; and that in thofe parts of 
the world which lie nearly adjacent to any'one of thefe 
magnetic poles, the needle is governed by it; the nearell 
pole being always predominant over the more remote. 
The pole which he luppofes nearell to us, he conjeftures 
to lie in or near the meridian of the Land’s-end of Eng¬ 
land, and not above feven degrees-from thebiortli pole; 
by this pole, the variations in all Europe and Tartary, 
and the North Sea, are chiefly governed ; though ftill 
with Tome regard to the other northern pole, whole fitu- 
ation he fupoles in the meridian pafling about the middle 
of California; and about fifteen degrees from the north 
pole of the world, to which the needle has chiefly refpect 
an all North America, and in the tw'O oceans on either 
fide of it, from the Azores weftward to Japan, and far¬ 
ther. The tw r o fouthern magnetic poles, lie imagines* 
are rather more diftant from the fouth pole of the world; 
the one about fixteen degrees from it, on a meridian 
twenty degrees to the weftward of the Magellanic Streights, 
or ninety-five degrees weft from London ; this pole com¬ 
mands the needle in all South America, in the Pacific 
O.cean, and the greateft part of the Ethiopic Ocean. The 
other magnetic pole leems to have the greateft power, and 
the large!! dominion of all, as it is the moll remote from the 
pole of the world, being little lefs than twenty degrees 
diftant from it, in the meridian which paffes through New 
Holland, and the illand Celebes, about one hundred and 
twenty degrees eail from London : this pole he thinks 
predominant in the fouth part of Africa, in Arabia, and 
the Red Sea, in Perfia, India, and its iilands, and all oyef 
the Indian fea, from the Cape of Good Mope eaftward, to 
the middle of the Great South Sea that divides Alia from 
America. 
Such, lie obferves, feems to be the prefer.t difpofition 
©f the magnetic virtue throughout the whole globe of the 
earth. He then fhews how this hypothefis accounts for 
all the variations that have been oblerved of late. It is 
inferred that the direction of the needle, in the temperate 
and frigid zones, depends chiefly upon the counterpoize 
of the forces of two magnetic poles of the lame nature; as 
alio why, under the fame meridian, the variation Ihould 
be in one place twenty-nine degrees and a half weft, and 
in another twenty degrees and a half eaft. In the torrid 
zone, and particularly about the equator, refpeft mull 
be had to all the four poles, and their politions mull be 
well confidered, otherwife it will not be eafy to determine 
what the variation Ihould be, the nearelt pole being always 
itrongell; yet fo, however, as to be fometimes counter¬ 
balanced by the united forces of two more remote ones. 
Thus, in failing from St. Helena, by the ille of Afcen- 
fion, to the equator, on the north-weft courfe, the varia¬ 
tion is very little eailerjy, and unalterable in that whole 
track; becaufe the South-American pole, (which is much 
the nearell in the aforefaid places,) requiring a great eaft- 
eriy variation, is counterpoifed by the contrary attrac¬ 
tion of the North-American and the Aliatic fouth poles ; 
each of which iingly is, in thefe parts, weaker than the 
American fouth pole; and upon the north-well courfe 
the diltance from this latter is very little varied ; and as 
w'e recede from the Afiatic fouth pole, the balance is ftill 
preferved by an accefs towards the North-American pole. 
PASS. 
In this cafe no notice is taken of the European north 
pole, its meridian being a little removed from thofe of 
thefe places, and of itlelf requiring the fame variations 
which are here found. After the lame manner may the 
variations in other places about the equator be accounted 
for, upon Dr. Hailey’s hypothefis. 
To obferve the Variation of the Needle .—Draw a meridian 
line, as directed under Meridian; then a ftyle being erec¬ 
ted in the middle of it, place a needle upon it, and draw 
the right line which it hangs over. Thus will the quan¬ 
tity of the variation appear. Or thus: as the former me¬ 
thod of finding the variation cannot be applied at fea,' 
others have been deviled, the principal of which are as 
follow : lufpend a thread and plummet over the compafs, 
till the fhadow pafs through the center of the card; ob- 
ierve the rhumb, or point of the compafs which the Iha- 
dow touches when it is the (horteft. For the (hadow is 
then a meridian line; and confequently the variation is 
fhewii. Or thus: oblerve the point of the compafs upon, 
which the fun, or lome liar, riles and lets ; bileft the arch 
intercepted between the fifing and letting, and the line of 
bifeftion will be the meridian line; coniequently the va¬ 
riation is had as before. The lame may alfo be obtained 
from two equal altitudes of the fame liar, oblerved either 
by day or night. Or thus: oblerve the rhumb upon 
which the fun or liar riles and fets; and from the latitude 
ofthe place find the eaftern orweftern amplitude; for the 
difference between the amplitude, and the diltance ofthe 
rhumb oblerved, from the eaftern rhumb of the card, is. 
the variation fought. The ule of the variation is to cor- 
reft the courfes a ihip has fleered by the compafs, which 
mull always be done before they are worked, or calcu¬ 
lated. See the article Navigation. 
The Variation of the Variation, according to Dr. Halley’s 
theory, is fuppoied to be ow ing to the difference of velo¬ 
city in the motions of the internal and external parts of. 
the globe. From the obfervations that have been cited, 
it leems to follow, that all the magnetical poles have a 
motion weftward, but yet not exabtly round the axis of 
the earth, for then the variations would continue the 
fame in the fame parallel ef latitude, contrary to expe¬ 
rience. From the difagreement of fuch a fuppolition with 
experiments, therefore, the learned author of the theory 
invented the following hypothefis: Tiie external parts of 
the globe he conliders as the flieil, and the internal as a 
nucleus or inner globe ; and between the two he conceives 
a fluid medium. That inner earth having the fame com¬ 
mon center and axis of diurnal rotation, may revolve 
with our earth every twenty-four hours; only the outer 
fphere having its turbinating motion lomewhat fwifter or 
flower than the internal ball; and a very minute differ¬ 
ence in length ot time, by many repetitions, becoming 
fenfible ; the internal parts will gradually recede front the 
external, and they will appear to move, either eaftward 
or weftward, by the difference of their motions. Now, 
luppoiing fuch an interna! lphere having fuch a motion, 
the two great difficulties in the former hypothefis are 
ealily folved ; for if this exterior Iheli of earth be a mag¬ 
net, having its pole at a diltance from the poles of diur¬ 
nal rotation; and if the internal nucleus be likewiTe a 
magnet, having its poles in two other places, diftant alfo 
from the axis; and thefe latter, by a flow' gradual mo¬ 
tion, change their place in refpeft of the external; a rea- 
Ibnable account may then be given of the four magneti¬ 
cal poles before-mentioned, and alfo ofthe changes of the 
needle’s variation. 
Dr. Halley thinks that two of thefe poles are fixed, and 
the other two moveable; viz. that the fixed poles are the 
poles of the external cortex or Iheil of the earth ; and the 
other the poles of the magnetical nucleus, included and 
moveable within the former. From thefe obfervations 
he infers, that the motion is weftwards, and coniequently 
that the nucleus has not precifely attained the fame ve¬ 
locity with the exterior parts in their diurnal rotation ; 
but lo very nearly equals jt, that in three hundred and 
fixty- 
