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V
ARTES
1817
SCIENTIA
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کی تم A
F
John Lorimer, M.D.
1.D. MDCCXCIV, EL. 62,
Fellow of the Royal College of Physiciansat Edinburgh
2. Deoaldron del.
"Bajore
ch
A CONCISE
ESSAY ON MAGNETISM;
WITH
AN AÇCOUNT OF THE DECLINATION AND INCLINATION
OF THE
MAGNETIC NEEDLE;
AND
AN ATTEMPT TO ASCERTAIN THE CAUSE OF
THE VARIATION THEREOF.
BY
JOHN LORIMER,
M. D.
AND FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS OF EDINBURGH.
أو
Veniet tempus quo ifta quæ nunc latent, in lucem dies extrahet, et longioris
ævi diligentia.-Tempus veniet quo pofteri noftri tam aperta nos nefciifle
mirentur.-SENECA NAT. QUEST. L. VII. c. 25.
LONDON:
Printed for the AUTHOR, and fold by W. FADEN, Geographer to His
MAJESTY, the Corner of St. Martin's Lane.
M. DCC. XCV.
Wheldon
10-14-3/
TO
HIS ROYAL HIGHNESS
THE DUKE OF CLARENCE,
THIS ESSAY ON MAGNETISM
IS,
BY PERMISSION,
HUMBLY INSCRIBED,
BY
HIS ROYAL HIGHNESS's
MOST OBEDIENT AND
MOST HUMBLE SERVANT,
J. LORIM E R.
}
SHORT HISTORICAL ACCOUNT
OF THE
DISCOVERY OF MAGNETISM,
AND OF THE
MAGNETIC NEED L E.
Origin of I.
the name.
IT
T would be to little purpoſe at this time to
inquire particularly whether the Magnet
had its name from the fhepherd Magnus, who, as Ni-
cander and Pliny affirm, difcovered it upon Mount Ida
by the iron of his crook, and the nails in his ſhoes; or
whether it was fo titled from Magneſia, that part of Ly-
dia, where, according to Lucretius, it was firſt found.
The Grecians, indeed, who were acquainted with the
various names it then went by, and likewife with its at-
tractive property, have ſometimes called it Siderites, from
σιδηρος,
idngos, iron; but more frequently the Heraclean ftone,
from the city Heraclea in Magnefia; and Snellius may be
right in ſaying, that Euripides was the firſt who gave
it the name of Magnes, though Sophocles called it Lapis
Lydius.
2. With
vi
Attractive
property.
HISTORICAL ACCOUNT OF THE
2. With reſpect to the properties of the magnet,
father Kircher endeavours to prove, that its at-
traction was known to the Hebrews; and from Plutarch
it ſeems to appear, that the Egyptians were not, ignorant
of it. Pythagoras, Ptolemy, Hippocrates, Empedocles,
Democritus, Leucippus, Epicurus, and many more of
the ancients, knew and admired this wonderful property
of the magnet. It was on account of this quality that
Thales and Anaxagoras gave it a foul; and Plato, who
called it the ftone of Hercules, faid, that the cauſe of its
attraction was divine. Alfo Ariftotle, Theophraftus, Di-
ofcorides, Galen, and others amongst the ancients, have
made particular mention of it.
Invention of
the mariner's
compaſs.
3. The diſcovery of the verticity or directive
property of the magnet or loadſtone, and the
communication of that verticity to iron, or, in other
words, the invention of the mariner's compaſs, though
only a confequence of the former property, appears to
be but of a modern date. It is indeed pretended, that
the eaſtern nations were well acquainted with this pro-
perty of the magnet, long before the Europeans had learn-
ed any thing about it. Some fay that Solomon knew
the uſe of the compaſs, and that thereby he was enabled
to fend his mariners to Peru, which was then called Par-
vaim
DISCOVERY OF MAGNETISM.
vii
vaim and Ophir*. Other authors affirm, that the Chineſe
about that time, or even earlier than the days of Solo-
mon, were acquainted with this moſt uſeful quality of
the magnet. This, however, has been much doubted t.
Flavius Blond affirms, that in or about the year 1302,
one John Goia, a noble citizen of Amalphi, a town of
Principato, in the kingdom of Naples, firft difcovered
the mariner's compafs; and for this he quotes the fol-
lowing verfe from Antony of Palermo, recorded by the
Neapolitan hiftorians; viz.
Primo dedit nautis ufum magnetis Amalphi.
The arms of the territory of Principato has, it feems,
ever fince been a mariner's compafs. It has alfo with
equal confidence been afferted, that Marco Paulo, the
Venetian, learned the uſe of the mariner's compafs from
the Chineſe; and that he first made it known in Italy.
about the year 1260. But this cannot be true
for M.
Paulo did not fet out on his journey to China before the
year 1269, nor did he return before the year 1295. It
feems however from the documents, which will be ad-
*
See Pineda de rebus Solomonis. L. iv. c. 15.
† See Du Hald's Hiftory of China, and the learned Renaudot's Dif
fertation on the Chineſe Sciences, &c.
See Purchaſe's Pilgrim, vol. III.
duced
i
viii
HISTORICAL ACCOUNT OF THE
duced in the fequel, that the directive property of the
magnet, and the communication of that property to
iron, was known in Europe before this time; though to
all probability it was not uſed in navigation till ſome time
after; which may very reaſonably be attributed to the
clumfy way of fufpending the magnetic needle, which
was at firft uſed.
In the works of Claude Fauchet, entitled, Recueil de
l'origine de la Langue et Poefie François, fol. 555, there is
a quotation from an old French poem, called la Bible
Guiot, in which the mariner's compafs is evidently men-
tioned. This fame paffage is likewife quoted by Muf-
chenbrock, in his Differtatio de Magnete. The fingula-
rity of this paffage having excited my curioſity, I made
fome enquiry after the above-mentioned old
mentioned old poem,
in confequence of which, I found that there was a
curious and interefting quarto manufcript of the 13th
century, on vellum, in what was then juftly called the
Royal Library at Paris. The firft article in this manu-
fcript book, which was never publiſhed, is la Bible Guiot;
* The affertion of Dr. Wallis feems to be well founded; viz. that the
magnetic needle, or compaſs, was brought to perfection by gradual ſteps
and partial improvements, and that of theſe the Engliſh may claim con-
fiderable fare,
3
the
DISCOVERY OF MAGNETISM.
ix
the author of which, viz. Guiot de Provins, as mention-
ed in the poem itſelf, was at the court of the Emperor
Frederic Barbaroffa, held at Mentz in the year 1181,
when the emperor's two fons were knighted *.
Here follows this remarkable paffage, extracted from
la Bible Guiot, in its original antiquated language, which
has perhaps fuffered much from the careleffneſs of tran-
ſcribers, as may appear from the ſubjoined notes. The
tranflation, which follows this paffage, was made by a na-
tive of Provence, and as literal as poffible.
* See Chron. Abbot. Urfperg. p. 311.
b
EXTRACT
X
HISTORICAL ACCOUNT OF THE
EXTRACT FROM LA BIBLE GUIOT.
Icelle eftoile ne fe muet,
Une arts font qui mentir ne puet,
Par la virtu de la manete*
Une piere laide et brunete,
Ou il fers volenters fe joint.
Ont regardent lor i droit point
Puez c'une aguile lont touchie,
Et en un feftu lont fifhie
En longue ‡ la mette fens plus,
Et il feftui la tient defus;
Puis fe torne la point toute
Contre leftoile fans doute,
Quant il nuis eft tenebre et brune
Con ne voit eftoile ne lune,
Lor font a laguille alumer;
Puiz ne puent ils afforer,
Contre leftoile vers la pointe;
5
IO
15
Par ce font § il mariner cointe,
De la droite voie tenir;
C'eft uns ars qui ne puet mentir.
20
* Manete, magnete, magnes, the loadſtone. This word is impro-
perly written in different manuſcripts; as marinette, mariniere, mar-
niere, &c. In the next line alfo, M. Fauchet has noirette, where it
is brunete in my correfpondent's manuſcript.
† Lor, alors.
An ingenious friend of mine obferves, that longue, or langue, as fome
copies have it, may perhaps mean l'eau; and in the fame line mette,
may have been mettent.
& Ce font, fe fort..
LITERAL
DISCOVERY OF MAGNETISM.
xi
LITERAL TRANSLATION OF THE PRECEDING.
This fame (the pole) ftar does not move, (and)
They (the mariners) have an art which cannot
deceive,
By the virtue of the magnet,
An ugly browniſh ſtone
To which iron adheres of its own accord.
Then they look for the right point,
And when they have touched a needle (on it)
And fixed it on a bit of ſtraw
5
Lengthwiſe in the middle, without more,
TO
And the ſtraw keeps it above;
Then the point turns juſt
Againſt the ſtar undoubtedly,
When the night is dark and gloomy,
That you can fee neither ftar nor moon,
Then they bring a light to the needle;
15
Can they not then affure themfelves
Of the fituation of the ftar towards the point (of
the needle ?)
By this the mariner is enabled
To keep the proper courfe;
This an art which cannot deceive *.
20
* The expletive words, included in parenthefis in this tranflation,
were thought neceffary to make it the more intelligible, though they
are not in the original; only in my correfpondent's copy the fecond
Fine runs thus:
Une arts font (les Marins) qui mentir ne puet.
b 2
Gaffendus
xii
HISTORICAL ACCOUNT OF THE
Gaffendus adduces, as an argument of the French hav-
ing been the inventors of the compaſs, that the north
point thereof is always marked with a Flower de Luce*.
As for Goropius's pretence, that the compaſs must be
the invention of the Danes, Dutch, or Germans, becauſe
the thirty-two points on it are written, and pro-
nounced in the Dutch or Teutonic language, has no
better grounds than the Englith claim from the words
Compafs and Box t. Vincentius Belluacenfis, and Albertus
Magnus, who lived about the year 1245, as well as Li-
vinus Lemnius, make mention of the direction of the
poles of the magnet, as from a tract de lapidibus, which
had been attributed to Ariſtotle, but is fuppoſed to have
been the work of fome Arabian author, a little be-
fore, or about, their own time, which tract has been
fince loft.
Plautus in Mercatore, Act V. Scene II. has the follow-
ing remarkable paffage, viz. Huc fecundus ventus nunc
eft, cape modo vorforiam. Now fome authors will have
it, that by the word vorforiam or verforiam, is meant
the mariners compaſs; fome learned critics, however,
* Lib. x.. Diog. Laert. T. i. p. 139.
† It appears, however, that thoſe nations were early acquainted
with the ufe of the mariner's compaſs.
affirm,
DISCOVERY OF MAGNETISM.
xiii
affirm, that the word verforium meant a particular rope.
For my part, I am inclined to believe that it meant no-
thing more than the helm.
The first Authors
on the verticity of
the magnet.
4. Francis Cabeus, a Jefuit of Ferrara,
fays, that the firſt thing he knows pro-
feffedly wrote on the direction, or verticity, of the mag-
net, was an epistle of Petrus Peregrinus Gallus, about
the latter end of the 13th century, and that the peregri-
nations of this fame Peter, in magnetical philofophy,
were not far from the truth. A few years after, this
epiſtle was diſguiſed by one John Tafnier, who publiſhed
it in his own name, under the title of Opufculum per-
petua memoria digniſſimum de natura et effectibus magnetis.
Some authors of note affirm, that this Petrus Peregrinus
was no other than an affumed name of the Engliſh Friar
Bacon, who flouriſhed in the 13th century. By the fa-
vour of Mr. Senebier, I received the following account
of the above-mentioned letter:
"Epiftola Petri Peregrini de Marcourt, ad Sigerium
" de Foucancourt, Militem de Magnete..
"The work contains a deſcription of that ſtone, the
means of finding the poles, its property of attracting
"iron, and proves that the part of the magnet, which
"is turned to the north, attracts that which is turned to
"the
}
xiv
HISTORICAL ACCOUNT OF THE
"the fouth. It then teaches the manner of employing
"the magnet in aſtronomy, and of playing tricks, like
• thofe of Comus. It deferves to be remarked, that the
"author knew not that the magnet could be employed
"in navigation; for, though he frequently ſpeaks de
"Rella nautica, he never ſpeaks of the uſe that might be
"made of the magnetic needle in ſea voyages.
"Fide Bibliotheca Bibliothecarum, fol. 11. p. 1400.
"Catalogue of the Manufcripts in the Library of Geneva,
"by Senebier, p. 207."
Amongst the manufcripts of the univerſity of
Leyden, there is a volume containing many ſcientific
tracts, one of which is a letter of Peter Adfiger, which
is dated in the year 1269, and contains an account of al-
moſt all the properties of the magnet that are known at
this day. The attraction, repulfion, directive property of
the magnet, the communication of thofe properties to
iron, the conſtruction of the azimuth compaſs, the uſe
of the magnetic needle and the variation of it, are clearly
mentioned in this curious letter, which, in fact, is a con-
cife Effay on Magnetiſm *.
* Ample extracts of this letter, which is entitled Epiftola Petri Adh-
gerii, in Signationibus Nature Magnetis, have been publiſhed by Mr.
Cavallo, in the ſecond edition of his Treatife on Magnetiſin.
&
To
DISCOVERY OF MAGNETISM.
XV
To give an account of all the numerous authors who
have wrote on the properties of magnets, both natural and
artificial, during the laſt and preſent centuries, would far
exceed the brevity here intended; I fhall, therefore, juſt
add to thoſe already mentioned the names of Des Cartes,
Dr. Gilbert, Wm. Barlow, Peter Van Muſchenbroeck,
Savery, Dr. Knight, Mitchell, Canton, Paulus Frifius,
Aepinus, and Cavallo. I have, indeed, heard of a labo-
rious Italian Jefuit, by name Scarella, who fome years
ago publiſhed two quarto volumes on Magnetiſm, but I
could never procure the book. It feems, that Dr. Knight
had once propoſed a work of the like extent on this fub-
ject; but he ſoon after laid afide all thoughts of publiſh-
ing it.
I fhall now proceed to give fome account of the prin-
cipal properties of the magnet, referving the declination
and variation of the needle for the latter part of this
Effay.
AN
A N
ESSAY ON MAGNETISM.
CHAPTER THE FIRS T.
Of MAGNETISM in General.
Done at New-York, in the Year 1782, for a very ingenious young MIDSHIPMAN.
A
SECTION I.
NATURAL magnet may not improperly be termed
an iron ore; or rather a kind of native iron or vir-
gin iron, as it is taken out of the earth. It reſembles
very much caft iron, and is endowed with the properties
of attracting iron or other magnets (1). An artificial
magnet is a piece of iron or ſteel, which has been endow-
ed or impregnated with theſe fame properties by art.
SECT. II. Every magnet, whether natural or artificial,
has two principal points or poles, one whereof is called
the North, and the other the South Pole. The fimilar
poles of all magnets repel, and the diffimilar attract one
another: that is, the north pole of one magnet repels the
north pole of another, but it attracts the fouth pole, and
vice versa (2).
B
SECT.
+
AN ESSAY
SECT. III. If a flender bar of prepared iron or ſteel be
properly placed between the poles of a good magnet, and
kept there for fome time, it will acquire a certain degree
of magnetifm, or, in other words, it will become an arti-
ficial magnet (3).
SECT. IV. If a good magnet, natural or artificial, be
made of a ſpherical form, in which cafe it is called a
Terrella, or little earth, and a fmall piece of iron wire or
needle is placed on its furface about midway between
the two poles thereof, this needle will lie in the direc-
tion of a meridian, or rather parallel to the axis of the
magnet; and, confequently, will direct one of its ends to
the north, and the other to the fouth pole.
Further, if this little needle is moved along the furface
of the magnet eastward or weftward all round, it will
keep the fame ends directed to the poles refpectively; but
if the needle is moved toward either of the poles north
or fouth, it will, befides keeping one of its ends directed
towards that pole, begin to elevate itſelf on the end
which is directed to the neareſt pole, the more and more
as it advances; fo that when it has arrived at this pole,
it will ſtand perpendicular upon it.
If again, the little needle is moved toward the other
pole of the magnet, the end which was elevated becomes
lower and lower with a kind of tremulous motion, till
the needle is nearly midway between the two poles, when
it
ON MAGNETISM.
3
it falls flat, and becomes parallel to the axis of the mag-
net as before; and if it is carried on towards this other
pole, it raiſeth itſelf gradually on the oppofite end in the
fame manner, till it becomes alfo perpendicular upon
that pole (4).
SECT. V. If a good magnet, natural or artificial, be
made to float in quickfilver or on water, or be ſuſpended
in fuch a manner as to have little friction, one of its
poles will direct itſelf towards the north, and the other
towards the fouth pole nearly; alfo if it is in the north-
ern hemisphere, (magnetically speaking) the north
pole of the magnet will defcend below the horizontal
line, while the fouth pole is elevated above it; but if it is
in the ſouthern magnetic hemiſphere, then the ſouth
pole of the magnet will defcend below, and the north
pole will be elevated above the horizontal line. In either
cafe, the meaſure of this inclination will be various, ac-
cording to the fituation of the place; but the whole ex-
actly fimilar to the experiments of the ſpherical magnet
or terrella, and little needle, already mentioned (5).
SECT. VI. Upon any part of the ſurface of this globe,
if a bar of prepared iron or ſteel be fufpended nearly in the
fame direction that the needle or magnet, having a free
and unconfined motion, arranges itſelf in; and if this
bar is kept in the fame pofition for fome time, it will
become an artificial magnet (6).
B 2
SECT.
4
AN ESSAY
SECT. VII. From what has been faid, we conclude,
that the globe of this earth is ſtrongly impregnated with
magnetiſm; for, like another magnet, it will commu-
nicate a confiderable degree of magnetic power to any
bar of prepared iron or steel, which may be upon its fur-
face, if it is kept in a certain poſition for fome time (fee
Sect. VI.). Alſo, that all fuch maſſes of native or virgin
iron, as are formed in the bowels of the earth, will be
impregnated with this fame general magnetic virtue, as
well as the bar already mentioned.
Further, that any fuch wires or flender bars of iron
impregnated with magnetifin, which have a free and un-
confined motion, will be arranged nearly in a northerly and
foutherly direction, as mentioned in Sect. V. by the mag-
netical power of the earth alone; and thence all the pro-
perties of the magnetic needle or mariners compaſs may
be explained; the variation thereof only excepted.
SECT. VIII. I fhall conclude this firſt chapter with ob-
ferving, that as it is not intended for a ſyſtem of magne-
tiſm, I have only mentioned here fuch leading principles
as were neceffary for explaining the declination and va-
riation of the magnetic needle, which is to be the ſubject
of the enfuing pages; yet I flatter myſelf, that whoever
is thoroughly mafter of the principles here laid down,
will very eaſily come to underſtand every other property
of the magnet, though he has never before made that
fubject his peculiar ftudy.
NOTES
ON MAGNETISM.
NOTES AND ILLUSTRATIONS
ON
5
I.
CHAPTER THE FIRST.
THERE are many rich iron ores which neither are,
nor in that ſtate can they be impregnated with mag-
netiſm; the pure calces of iron cannot be rendered mag-
netical.
QUERE Ift.
Would it not therefore appear, that iron in its metallic
form only is ſuſceptible of magnetifm? As iron and ſteel
are better purified than the natural magnet, and can
more eaſily be made into a proper form, ſo the artificial
are generally ſtronger than the natural magnets.
The longer a magnet is in proportion to its breadth,
and the broader it is in proportion to its thickneſs, cæteris
paribus, the ſtronger it generally is.
QUERE 2d.
Would it not therefore appear, that magnetifm acts in
proportion to the ſurface, and not as the quantity of mat-
ter like gravity? The magnetical machine of the late
Doctor Gowen, Knight, is a further proof of this article.
See Phil. Traní. Vol. LXVI. Part 2d. Anno 1776.
9
2. There
6
AN ESSAY
2. There are feveral ways of fhewing this property of
magnets; the moſt convenient I take to be by the com-
mon compaſs, the needle of which is an artificial magnet.
Take therefore another magnet and point to it, and it will
be found, that the north end of the magnet will repel
the north, but it will attract the fouth end of the
needle.
Reverſe the magnet, and the fouth end of the magnet
will repel the fouth end of the needle, but it will attract
the north end: and as often as you reverſe the magnet,
the needle will in like manner be reverfed. But to thofe
who are not much acquainted with this fubject, the moſt
convincing method of fhewing the attraction as well as
the repulfion of magnets, is by laying an artificial magnet
in a little boat of light wood, or the like (ſee Plate II.);
and ſetting it to float in a bowl of water, and pointing
another magnet to it end to end.
3. I do not propoſe this as the beſt or only method of
making artificial magnets; but I mentioned it here as a
fact, for the fole purpoſe of explaining fome matters
which I have further to offer in this Effay.
4. To fhew this experiment to advantage, as it is a
very material one; befides the Terrella, it will be necef-
fary to have a ſmall glaſs tube, with a round bulb, like
thoſe which are uſed for thermometers; into which we
!
drop
ON
MAGNETIS M.
drop a bit of a fine fewing needle, not above two-tenths
of an inch long, which being applied to the Terrella, as
in Plate III. may be eafily guided round it, as is directed
in Sect. IV.
5. But to exhibit thefe two properties at once, the
best method is to have an inftrument conftructed in the
manner of a dipping needle, fuch as I contrived anno
1764. This machine, which I have called an Univerfal
Magnetic Needle, or Obfervation Compafs, is minutely
deſcribed in the Philofophical Tranfactions, vol. LXV.
anno 1775; but a delineation of that inftrument, which
was omitted in the Phil. Tranf., will be found in Plate I.
of this Effay.
6. If this is a bar of hardened steel, it will take
confiderable time before it acquires any magnetical pro-
perties; but when it has once become magnetical, it
will not lofe it eafily; nor can the poles thereof be re-
verfed any other way, than by fuch means as would have
given it magnetiſm originally, if it had none.
This, therefore, we call fixed magnetifm. But if the
bar is of foft iron, it takes no length of time to become
magnetical; for, in the northern hemifphere, the mo-
ment you put fuch a bar in the proper poſition, (fee
Sect. VI.) the lower end of it becomes a north pole, and
the upper end a fouth pole. (If it is in the ſouthern he-
8
mifphere
8
AN ESSA
mifphere the lower end is the fouth, and the upper end
the north pole.) Reverſe the ends of the bar, and the
poles are accordingly reverfed, as often as you repeat the
experiment.
The magnetiſm of this bar is therefore faid to be
moveable, and almoſt every tongs or poker, with a com-
mon compafs, is fufficient to fhew the experiment, (fee
Plate IV.) but by being often heated and cooled, and by
generally ſtanding in a perpendicular poſition, if their
magnetiſm ſhould appear to be fixed, let the lower end
be ſtruck againſt the floor, or the ftones of the chimney,
and the poles will immediately be changed; a fiart
ftroke of a hammer will have the fame effect, and a fhock
of electricity will give magnetifm even to ſteel. Se-
veral conjectures might be ſtarted from theſe experi-
ments, which at prefent however we fhall omit.
CHAPTER
Ο Ν MAGNETIS M.
9
CHAPTER THE SECON D.
Of the Declination of the Magnetic Needle, commonly called
the Variation of the Mariner's Compaſs.
SECTION I.
FROM attentively confidering the principles laid down
and explained in the foregoing part of this Effay, we
conclude, that as the globe of this earth is a great mag-
net, fo like every other magnet it must have two points
or poles, which will act upon any magnetic or compaſs
needle, in the fame manner that another magnet or
terrella would do; that is, it will cauſe one end of it to
be directed to one of its magnetic poles, and the other
end of it to the other. See Part I. Sect. 2, 4, 5, 6,
and 7*.
It is likewiſe evident, that, according to the pofition of
thefe magnetic poles, the compafs needle will be diffe-
rently affected, and from thence we may deduce the fol-
lowing cafes.
*But in this place it is proper to obſerve, that what we call the
north magnetic pole of the earth, is in fact a fouth pole; or, in other
words, it is a magnetic pole, contrary to that pole of the needle which
is attracted by, or directed towards, the north pole of the earth; becauſe
the magnetic attraction takes place only between poles of different deno-
minations and for the fame reafon, what we call the fouth magnetic
pole of the earth, is in fact a north pole.
:
C
CASE
IO
AN ESSAY
CASE I.
If the magnetic poles of this earth had coincided with
the true poles thereof, there could have been no declina-
tion or variation of the mariner's compafs in any part of
the world, (i. c.) if the earth is uniformly magnetical;
for in that cafe the needle, in pointing to the magnetic
poles, muſt always have pointed to the true poles alfo.
This needle would therefore be neceffarily directed along
the courfe of the meridian, or, in other words, it would
have no declination either to the eaſt or weft thereof.
CASE II. See PL. V.
If the magnetic poles were fituated in the fame meri-
dian, and in oppofite parallels upon that meridian which
paffes through the magnetic and true poles, from one of
the magnetic poles to the other, and upon the oppofite
meridian all along, there could be no declination, for the
reafon mentioned in the former cafe: likewife, upon the
equator there would be no declination; for though if one
of the magnetic poles was only to act upon the needle,
in paffing along the equator to the diſtance of 90° in lon-
gitude eaſt or weft, the declination would increaſe; fo
that at 90 diſtance from the line of no declination, it
would be equal to the angle contained between the mag-
I
netic
{
ON MAGNETISM.
II
netic and the true poles; yet as the other magnetic pole
in this cafe is always within the fame diſtance of the
needle, it will act upon the oppofite end of it with equal
force, and confequently will keep it parallel to itſelf all
round the equator: but in going from the equator
north or fouth, the declination will be increaſed, ſo as
to be 180° on the little arcs or ſpaces of the meridian con-
tained between the true and magnetic poles; which is the
greateft poffible declination in all cafes whatſoever.
I have further to obferve on this cafe, that the lines of
no declination, including theſe arcs of 180°, form two
great circles of the globe, along the meridian and the equa-
tor, croffing one another at right angles, and dividing the
furface of the globe into four quarters, two in each he-
miſphere; the one hemifphere having weft declination
in the north, and eaſt declination in the fouth half
thereof. In the oppofite hemiſphere it is juſt the reverſe,
fo that each of the arcs or femicircles of no declination
have eaſt declination on one fide of them, and weft de-
clination on the other. The ſmall arcs of 180° declina-
tion, which are between the true and magnetic poles, I
reckon in all cafes as part of the lines of no declination;
for there indeed the needle conforms itſelf to the me-
ridian as well as in the other parts of the circle, though
its ends are reverſed.
In ſhort, as all the lines of declination, or Halleyan
lines as they are very properly called, do coincide and
terminate in the magnetic and true poles, ſo theſe
C 2
✔
arcs
J2
AN ESSAY
arcs of 180° are a kind of ftopgaps, making with each
of thefe lines, as in the prefent cale, a curvelincar figure,
returning into itſelf, which figures from 180° between
the poles to o declination upon the equator, do cach of
them include a fpace larger than the other, till at last
they fill up the whole quarter of the furface of the globe,
and conform themselves as nearly as poffible to the fhape
and figure thereof. (See the Plate.) As a variety of this
cafe I have juſt to mention, that the magnetic poles may
be fituated in the fame meridian, but in parallels which
are not oppoſite. In that cafe, the only alteration which
could happen is, that in the hemifphere in which the
magnetic and the true poles are nearest to one another,
the figures formed by the Halleyan lines become fmaller,
and the correſponding figures in the oppofite hemi-
ſphere, larger.
The line of no declination, which in this cafe repre-
fents the equator, would alfo be proportionably nearer
to the poles which are neareſt one another.
CASE III. See PL. V.
If the magnetic poles were fituated in oppofite meri-
dians and oppofite parallels, upon thoſe meridians which
paſs through the magnetic and true poles, there could
be no declination, for the reafons mentioned in the for-
mer cafes. But upon the equator eastward and weft-
ward to the diſtance of 90° in longitude, the declina-
tion would actually increafe, fo as there to be equal to
the
2
ON MAGNET IS M.
13
the angle which meaſures the diftance between the true
and magnetic poles, and from thence would in the fame
manner decreaſe for the other 90° to the oppofite me-
ridian.
The Halleyan lines of 10°, 20°, &c. as far as the great-
eſt declination upon the equator, in this cafe become arcs
or curves, which conform themſelves as nearly as may be
to the courſe and direction of the lines of no declination,
and are called by Magnetifts the lines of the first order;
but the lines of the greateſt equatorial declination croſs
one another at the diſtance of 90° in longitude, from the
meridian or circle of no declination, fomething in form
like the letter X, or like to two Gothic arches joined at
the vertex. They are termed lines of the fecond order,
and may very properly be confidered as the boundary be-
tween the lines of the firſt and third order; as the lines
of no declination are always boundaries between the
lines of the eaſt and weft declination. In this caſe theſe
lines of no declination, including the arcs of 180°, form
only one great circle along the meridian, dividing the
furface of this globe into two hemifpheres; in the one
of which there is eaft declination, and in the other, weſt
declination.
From the greateſt equatorial declination to the arcs of
180°, the Halleyan lines of the third order, are curves re-
turning into themfelves, and in thape nearly reſembling
parabolas, erected upon the arcs of 180". As a variety
of this cafe alfo, I have only to add, that if the mag-
netic
14
A N
ESSAY
netic poles were fituated in oppofite meridians, but in
parallels which are not oppofite, then in that hemi-
fphere, in which the true and the magnetic poles ap-
proach neareſt to one another, the figures formed by
the Halleyan lines would be ſmaller, and in the oppofite
hcmifphere the correſponding figures would be larger in
proportion.
It was once the general opinion, that the poles of
every magnet muſt be diametrically oppofite to one
another, as the poles of natural magnets are generally
found to be fo; but Doctor Gowen Knight has demon-
ftrated by experiments, that the poles of magnets may be
diſpoſed in every poffible direction. See Phil. Tranf.
N° 474, 476, 484, &c. Anno 1744, 1746, 1747. (Sce the
Plate.)
CASE IV. See PL. VI.
I have only one cafe more to mention, but it is a very
extenſive one; viz.
When the magnetic poles are fituated neither in the
fame nor oppofite meridians: and this feems to have
been the real pofition of theſe poles, ever fince any ob-
fervations of the declination of the magnetic needle have
been made.
In this cafe, then, the lines of no declination cannot be
either in the direction of the meridian or along the
equator, as in the former cafes; but a kind of curves
which
J
ON
MAGNETISM.
15
which are varioufly inclined to both, and they divide the
furface of this globe into two parts; but thefe parts are
not he miſpheres, as in the laft cafe; for they may be of
very different extent.
If the magnetic poles are funted in meridians nearly
oppofite, the curvatures of thof: lincs are the lefs: that
is, they become more like to fe 3d. But as the magnetic
poles approach nearer to the fame meridian, the curva-
ture of the lines of no declination become greater till
they almoft touch one another, fomething in form like
the figure S, and at laft they complete the two great
circles, as in Cafe 2d.
The lines of the fecond order, which correfpond to
the greatest equatorial declination, if the magnetic poles.
are fituated in meridians nearly oppofite, have a declina-
tion nearly equal to the angle formed between the mag-
netic and true poles, as in Cafe 3d.
But as the magnetic poles approach toward the fame
meridian this declination decreafes, till at last it entirely
vanishes, together with the lines of the first order; and,
as in Caſe 2d, leaving only the lines of the third order.
The other Halleyan lines in this cafe are fo fimilar
to the former, that they require only to be referred
to it.
I have therefore just to add, that whether the mag-
netic poles are fituated in oppofite parallels or not, makes
as little difference in this cafe as in the former.
the Plate.)
(See
SECT.
16
AN ESSAY
SECT. II. Though I flatter myfelf, that the foregoing
pages will be found to contain all the cafes that are ne-
ceffary toward an explanation of the declination of the
magnetic needle, at any one time; yet to thofe who wifh
to be acquainted with this curious fubject more fully, I
would beg leave to recommend Mr. Euler's Reſearchcs,
in the Berlin Memoirs for the Year 1757, a performance
which will give ſome fatisfaction even to thoſe who can-
not enter into all his mathematical calculations; but for
the convenience of fuch gentlemen as may not ſoon have
it in their power to confult that volume, and as an ex-
ample of the fourth Cafe, I fhall tranſcribe from Mr.
Euler's Map, annexed to the paper above-mentioned, the
ſtate of Magnetiſm about the middle of this century.
According to this ingenious author, the North Magne-
tic Pole ſeems then to have been fituated near to the me-
ridian, which paffes by Cape St. Lucar, the South Point
of Calefornia, and between the latitudes of 70° and 80°
North.
(I do not think the obſervations are as yet fufficient
to authoriſe our being very particular *.)
The fituation of the South Magnetic Pole, at that time,
ſeems to have been above 60° more Weftwardly, and near
the latitude of 60° South †.
*Mr. Euler fays 73° or 76°. † According to the fame Author 55°.
SECT.
ON MAGNETISM.
17
SECT. III. From this pofition of the magnetic poles
then, the reſult is as follows. From the north magnetic
pole a line of no declination commences, and by a route
ſomewhat refembling the letter S, it traverſes the conti-
nent of North America and the Atlantic Ocean, &c. to
the fouth pole of the earth, and then, by the arc of
180° to the ſouth magnetic pole.
From the ſouth magnetic pole this line of no declination
proceeds, and in like manner traverſing the Pacific Ocean,
paffes by the iſlands of the Eaſt Indies, and through the
continent of Aſia to the north pole of this earth, and ſo
by the little arc of 180° to the north magnetic pole.
Theſe lines divide the furface of this globe into two
parts; and in going eastward from the firſt to the
ſecond line, over the eaſtern parts of North America,
Europe, Africa, and a great part of Afia, there is weft
declination; but from the fecond to the firſt, over the
eaftern parts of Afia, South America, the weſtern
parts of North America, and the Pacific Ocean, there is
eaft declination. The declination of the lines of the
fecond order, correfponding to the greateſt equatorial
declination, amounts in this cafe to 12°, and for the eaſt
declination they croſs one another in north latitude 24°,
and about 30° in longitude weſt of the meridian of Cale-
fornia: for weft declination the interſection is likewiſe
in 24° north latitude, upon that part of the coaſt of the
Red Sea, which is next to Arabia Felix.
D
If
38
AN ESSAY.
If any perſon will take the pains to compare this
with the charts of Meffrs. Mountain and Dodſon for 1744
and 1756, he will find a furprifing fimilarity; and from
what has been faid, with the help of thefe charts upon
a common map of the globe, all the Halleyan lines may
be drawn by hand. I have been the more particular on
this part, as it is poffible that each of thefe pofitions of
the magnetic poles may have been, or hereafter may
happen to be, their real fituation; and as it is evident,
that the fourth Cafe has been their pofition ever fince
any obſervations have been made on that ſubject, ſo it is
probable that it will continue to be their fituation for
´many ages to come. I fhall therefore conclude this fe-
cond chapter, by obferving, that if in ſome inſtances our
beſt obſervations fhould not perfectly agree with calcula-
tions, we are not therefore entirely to reject the theory;
for, fuppofing the obfervations to be perfectly exact,
which is not always the cafe, yet as this earth is un-
doubtedly a very heterogeneous body, it may not be uni¬
formly magnetical, as fhould be the cafe, if it was to agree.
entirely with the calculations, &c.
CHAPTER
ON MAGNETIS M.
19
F
CHAPTER THE THIRD.
1
ROM what has been faid in the two preceding
chapters, we may conclude, ift. That the earth is a
great magnet, endowed with all the properties which have
been obſerved in common magnets; 2dly, That the mag-
netic needle owes its directive property to the vicinity of
this great magnet, in the fame manner as we have fhewn
to happen with a ſmall needle, when placed upon a ter-
rella, or globular magnet; and 3dly, That the direction
of the needle muſt either coincide with, or deviate from,
the meridian, according as the magnetic poles of the
earth happen to be fituated. After this, it will be ne-
ceffary to examine the real ſtate of the matter; namely,
to confider, according to the beſt obſervations, the lines of
declination throughout the world, a fummary view of
which will be contained in the prefent chapter.
3
SECTION I. That line, which I fhall call the Atlantic
line of no declination, feems to take its origin from the
north magnetic pole, and croffing the different meridians
in a ſouth-eaſterly direction, refembling in form the long
letter S, it traverfes the continent of North America,
enters the Atlantic Ocean to the northward of Charles-
town, and fo proceeds towards the fouth pole. Upon
D 2
the
20
AN ESSAY
the weſt ſide of this line there is eaſt declination, and
upon the eaſt fide thereof, weft declination; which laft
gradually increaſes as you go to the eaſtward, till you get
beyond the Cape of Good Hope, or about midway be-
tween the Atlantic and the Eaft India line of no declina-
tion, where it amounts to 31°, about the latitude of 48°
ſouth, and then it regularly decreaſes to the Eaſt India
line of no declination.
Again, as you go to the eastward of that line of no
declination, the eaſt declination increaſes rapidly till you
get to the eastward of New Zealand, where it is upwards
of 13° even in that latitude; but from thence, as you pro-
ceed eaſtward, for about 40° in longitude, this declination
appears to decreaſe; and again, it increaſes till you are to
the eaſtward of Cape Horn, where, in the latitude of 51°
fouth, it amounts to 21° 28', and then gradually decreaſes
to the Atlantic line of no declination aforefaid. Upon the
whole it would appear, that thefe obfervations agree
pretty nearly with the fourth general cafe deſcribed in
the preceding chapter, except in that decreaſing eaſt de-
clination to the eaſtward of New Zealand. But admitting
that the vaſt body of water in the great Pacific Ocean,
which cannot have any magnetic properties, fhould have
no effect in producing this irregularity, yet we are not to
expect even that the ſolid parts of this globe can be fo
uniformly magnetical throughout, as to anſwer entirely
with calculation in every part thereof.
SECT
ON
OŃ
MAGNETIS M.
21
SECT. II. The magnetic needle not only declines, or
varies from the true north, differently in different parts
of the earth at any one time, but likewife in the fame
place this declination is different at different times; I
would therefore call it, by way of diſtinction, the varia-
tion of the magnetic needle.
SECT. III. At London and Paris, where the moſt accu-
rate obſervations have been made, towards the latter end
of the fixteenth century (and we cannot pretend to much
earlier obſervations) there was between 11° and 12° of
eaft declination, which gradually decreaſed; fo that in
lefs than a hundred years afterwards, there was no de-
clination at all in thofe places. From 1657, at London,
and 1666, at Paris, a weſt declination began, and has ever
ſince increaſed gradually, though not uniformly, or in
the direct proportion of the times; for fuch is the nature
of the magnetic declination, that, like the apparent mo-
tion of the planets, fometimes it is fafter, fometimes
flower, at other times it is ftationary; analogous alſo to
the elongations of the inferior planets, at one time it is
to the eaſt, and at another time to the weft, alternately.
We may farther obferve, that the declination lines of
the fame name have always refpectively paffed London
fome years before the fame lines arrived at Paris; and
the like obfervations have been made in other parts of
the northern hemifphere, that is, in this hemiſphere
the
AN ESSAY
the Halleyan lines have regularly paffed thoſe places firft
which lay moſt wefterly, and fo in order, thoſe which
lay more to the eastward. For in the latter end of the
fixteenth century, there was an eaſtern declination over
moſt parts of Europe, while on the coaſt of North Ame-
rica a weſt declination prevailed; the line of no declina-
tion being then fituated about the Azores. This line of
no declination has ever fince moved gradually eastward,
the lines of eaſt declination receding before it, while the
lines of the weſt declination have gradually followed it.
SECT. IV. In the fouthern hemifphere, however, it is
quite otherwife; for about the latter end of the fixteenth
century, a line of no declination paffed near to the Cape
of Good Hope, upon the eaft fide of which there was
weft declination, and upon the weft fide thereof eaſt de-
clination; each of which declinations, in going eastward
or weftward, gradually increaſed to a certain degree, and
then in the fame manner decreaſed to nothing, fome-
where to the eaſtward of Java, one of the Eaſt India
iflands.
The declination in the Pacific Ocean has not as yet
(1775) been fo fully aſcertained, only in general we
find, that the declination is eafterly over moſt part of
that extenſive ocean. The line of no declination, which
was then fituated a little to the eastward of the Cape of
Good Hope, has ever fince been moving to the weſtward,
and
ON MAGNETISM.
23
and the lines of eaſt declination have gradually receded
before it, while thoſe of the weft declination have fol-
lowed it with a proportional pace; fo that at the Cape of
Good Hope there is now a confiderable weft declination
(about 24°. *) and the line of no declination has moved
many degrees to the weftward thereof.
SECT. V. From the preceding obſervations then it plain-
ly appears, that the Halleyan lines in the fouthern he-
mifphere do gradually move from eaſt to weſt, while the
motion of thofe lines in the northern hemiſphere is
from west to eaft; and here we ſhall reft the matter for
the preſent.
I ſhall juſt beg leave to obferve, that in treating of this
fubject, I have all along only endeavoured to explain the
manner in which magnetiſm acts upon this globe; yet
if the preceding conclufion be admitted, viz. that the
progreffive motion of the lines of declination in the
northern hemiſphere is conſtantly from weft to eaſt, and
in the ſouthern hemiſphere from eaſt to weſt, this diſ-
covery will be of as great ufe to us in framing, regu-
lating, or judging of our future charts or tables of the
declination or variation of the mariner's compaſs, and
will anſwer the purpoſes of navigation as well, as if we
were thoroughly acquainted with the primary caufes of
all the phenomena of magnetiſm.
* The author received this and other informations from Colonel
Gordon, commander of the Dutch forces at the Cape.
I
CHAPTER
24
AN ESSAY
CHAPTER THE FOURT H.
An Attempt to explain the Caufe of the Variation of the
Magnetic Needle.
WROTE IN THE YEAR 1793.
Si quid novifti rectius iftis,
Candidus imperti: fi non, his utere mecum.
Horat. Epift. VI. lib. 1.
I
SECTION I.
F we were to enter into the hiſtory of magnetiſm, the
diſcovery of the mariner's compaſs, or even that cu-
rious fubject the declination or variation of the magnetic
needle, we ſhould find that the compaſs was known pro-
bably before the year 1181, and the variation at leaſt as
early as the month of Auguft, 1269*, though that
ſubject was not brought to any kind of regularity till the
time of Dr. Halley. The reſearches and obfervations of
that ingenious gentleman have been of the utmoſt ſervice
in navigation, however whimſical his theory may have
appeared to fome people.-But this for the preſent we
paſs over.
* See the Edinburgh periodical publication, entitled, The Bee,
vol. xiii. N° 3. Columbus obferved the variation of the compafs in his
firſt voyage, anno 1492.-Muffchenbrek de Magnete, Exp. 97.
5
SECT.
ON MAGNETISM.
25
SECT. II. On the principles of the ingenious Mr. Euler
(fee the Berlin Acts, volume for the year 1757) the de-
clination of the magnetic needle may in great meaſure
be accounted for. That is, fuppofing the magnetic
poles of this earth to be, at any one time fince ac-
tual obſervations have been made on that fubject, only
two, but not ſituated diametrically oppoſite to each other.
(See Cavallo's Treatife on Magnetiſm, p. 117.) It is
alſo well known that the magnetic declination is not
only different in different parts of the earth at the fame
time, but that in a courſe of years it alſo becomes dif-
ferent in the fame place. Farther, that this declination
in the fame place is not only different in different years,
but even the afternoon declination is generally different
from the forenoon declination of the fame day.
SECT. III. This fubject appeared very unaccountable
till the year 1759, when the late Mr. John Canton ex-
plained it in a very ingenious manner; for having found
by experiment, that by heating a magnet it loſt part of
its attractive power, and by letting it cool, it recovered
that power again; he likewiſe diſcovered by repeated
obfervations, that while the eaſtern parts of the earth
were heated by the fun in the forenoon, and confe-
quently had their magnetic powers diminiſhed, the needle
generally inclined more to the weftward, and that after
the fun had paffed our meridian, and the weſtern parts
E
of
26
AN ESSAY
of the earth began to be heated, while thoſe on the eaſt
of the meridian were cooled, the declination of the
needle was lefs wefterly by feveral minutes, and that by
next morning it had returned to its former pofition
nearly. We fay nearly, for after a courſe of more than
twelve months attentive obſervation he found, that the
declination was upon the whole increaſed of about ten:
or twelve minutes of a degree. That this increaſe was
occafioned by the fun's heat, appeared the more pro-
bable, as it took place moftly in the fummer months;
and during the winter months there was but little altera-
tion, as appears from the following table * :
The mean difference of diurnal variation for each month in the
year 1759.
January
February
7. 8." July
1.3. 14."
8. 58.
Auguſt
12. 19.
March
kl. 17.
September
II. 43.
April
12. 26.
October
10. 36.
May
13. O.
June
13. 21.
November
December
8..
9.
6. 58.
SECT.
Mr.. Canton, it ſeems, made in 603 days about 4000. obfervations
on the ſubject, with an excellent variation.compaſs about nine inches in
diameter. In 574 of thoſe days the variation was regular, and confe-
quently only 29 irregular, according to his fyftem.. Mr. Canton's opinion
was, that when the variation increaſes from about eight or nine o'clock
in the morning till one or two in the afternoon, then becoming ſtationary
6
for
ON MAGNET IS M.
27
SECT. IV. It muſt be allowed, according to the obſer-
vations of ſeveral ingenious gentlemen, that the collective
magnetiſm of this earth ariſes from the magnetiſm of
all the ferruginous bodies therein contained, and that the
magnetic poles fhould therefore be confidered as the
centres of the powers of thoſe magnetic ſubſtances. Theſe
poles muſt therefore change their places according as the
magnetiſm of ſuch ſubſtances is affected; nd if with
Mr. Canton we allow that the general cauſe of the diur-
nal variation arifes from the fun's heat in the forenoon
and afternoon of the fame day, it will naturally occur,
that the fame caufe, being continued, may be ſufficient
to produce the general variation of the magnetic needle
for any number of years. For we muſt conſider that
ever fince any attentive obſervations have been made on
this ſubject, the natural direction of the magnetic needle
in Europe has been conſtantly moving, from weſt to eaſt,
and that in other parts of the world it has continued its
motion with equal conſtancy.
for ſome time, after that returning back again to its former ſtation in the
night, or by next morning, it is regular. But he calls it irregular when
the needle moves eastward in the hotter part of the morning, or weft-
ward in the latter part of the afternoon; alfo when it moves either way
in the night, or fuddenly, viz. when it moves both ways in a ſhort ſpace
of time. The caufes of thofe irregularities he attributes to fubterraneous
heat, to the Aurora Borealis, &c.
E 2
SECT.
28
AN ESSAY
SECT. V. As we must therefore admit that the heat in
the different feafons depends chiefly on the fun, and
upon the whole that the months of July and Auguft will
probably be found the hotteft, while January and Fe-
bruary are the coldeft months of the year; and that the
temperature of the other months falls into the reſpective
intermediate degrees; though from calculation we can
ſcarce pretend to afcertain the abfolute heat of any par-
ticular month or day; fo we muft confider the influence
of heat upon magnetiſm to operate in the like manner,
viz. that for a ſhort time it ſcarcely manifeſts itſelf; yet
in the courſe of a century, the conftancy and regularity
thereof becomes fufficiently apparent. It would there-
fore be idle to fuppofe, that fuch an influence could be
derived from an uncertain or fortuitous caufe. But if
it be allowed to depend upon the conſtancy of the fun's
motion*, and this appears to be a cauſe ſufficient to ex-
plain the phænomena, we ſhould (agreeably to Newton's
firſt law of philoſophizing) look no farther.
SECT. VI. As we therefore confider the magnetic pow-
ers of the earth to be concentrated in the magnetic poles,
and that there is a diurnal variation of the magnetic
needle, theſe poles muſt perform a fmall diurnal revolu-
tion proportional to fuch variation, and return again to
See Dr. John Hunter on Heat, Phil. Tranf. for 1788.
:
the
ON
MAGNETISM.
28
the fame point nearly.-Suppoſe then that the fun in his
diurnal revolution paffes along the northern tropic, of
along any parallel of latitude between it and the æquator,
when he comes to that meridian in which the magnetic
pole is fituated, he will be much nearer to it, than in any
other; and in the oppofite meridian he will of courſe be
the fartheft from it. As the influence of the fun's heat
will therefore act moft powerfully at the leaft, and lefs
forcibly at the greateſt diſtance, the magnetic pole will
confequently deſcribe a figure fomething of the elliptical
kind; and as it is well known that the greateſt heat of
the day is fome time after the fun has paffed the meri-
dian, the longeſt axis of this elliptical figure will lie
north-eaſterly in the northern, and fouth-eafterly in the
fouthern hemifphere. Again, as the influence of the
fun's heat will not from thofe quarters have fo much
power, the magnetic poles cannot be moved back to the
very fame point, from which they fat out; but to one
which will be a little more northerly and eafterly, or
more foutherly and eafterly, according to the hemifpheres
in which they are fituated. The figures therefore which
they deſcribe, may more properly be termed elliptoidal
fpirals *.
*The north magnetic pole may by this means be carried, with a flow
but conftant motion, more and more to the north eastward, till it ar-
rives at the region of the greatest cold, which, by the bye, is fuppofed to
be at fome degrees diftance from the natural pole of the earth. And
Bowie the other magnetic pole will be carried, &c.
I
SECT.
30
AN ESSAY
SECT. VII. In this manner the variation of the mag-
netic needle in the northern hemisphere may be ac-
counted for. But with refpect to the fouthern hemi-
ſphere we must recollect, that though the lines of
declination in the northern hemifphere have conſtantly
moved from west to eaft, yet in the fouthern hemi-
fphere, it is equally certain that they have moved from
eaſt to weſt, ever fince any obſervations have been made
on the ſubject*. Is it poffible then that the magnetic
pole in the ſouthern hemiſphere can move from eaſt to
weft, whilft that in the northern hemiſphere moves from
weſt to eaft?--I think not. But we muſt confider this
matter a little more attentively. In the first place it muſt
be obſerved, that in fpeaking of the declination or varia-
tion of the magnetic needle, we always refer to the north
end of the needle only. Thus, when the north end of
the needle points to the weſt of the meridian, we ſay it
has fo many degrees weft variation, though the fouth end
thereof points as many degrees to the eaſtward. Again,
when the north end of the needle points to the eastward
of the meridian, we ſay it has eaſt variation, though the
fouth end points to the weftward thereof. And the fame
language is uſed in the ſouthern as in the northern hemi-
ſphere; fo that if the fouth magnetic pole, which governs
the needle in that hemifphere, move to the eastward, oc-
See Cavallo's Treatife on Magnetiſm.
cafions,
ON MAGNET IS M.
31
cafions, as we fay, the needle to have weft variation; and,
on the contrary, if it move to the westward, it makes
what we term eaft variation. This therefore is the cauſe,
on account of which the lines of magnetic declination, or
Halleyan curves, as they are now commonly called, appear
to have a contrary motion in the ſouthern hemiſphere,
to what they have in the northern; though both the mag-
netic poles of the earth move in the fame direction, that
is from weft to eaſt*.
SECT. VIII. This might be made ſtill more evident by
a diagram, or by means of the mariner's compafs and a
common magnet: and here we might reft the matter for
the preſent, but I cannot help mentioning the idea of Dr.
Gowen Knight, a gentleman undoubtedly known to have
* In the northern hemifphere there was a line of no variation, which
had eaft variation on its eaftern fide, and weft variation on its western
fide. This line evidently moved from weft to eaſt, during the two laſt
centuries; the lines of eaft variation. moving before it, while the lines
of weſt variation followed it with a proportional pace. Thefe lines
firſt paſſed the Azores or Weſtern Iſlands, then the meridian of London,
and after a certain number of years ftill later, they paffed the meridian
of Paris. But in the fouthern hemifphere there was another line of no-
variation, which had eaſt variation on its western, and weft variation on
its eaſtern fide; the lines of eaſt variation moving before it, while
thofe of the weft variation followed it. This line of no variation firſt
paffed the Cape des Aiguilles, and then the Cape of Good Hope; the
lines of 5°, 10°, 15°, and 20°, weft variation following it, the fame as
was the cafe in the northern hemifphere, but in contrary direction.
been:
32
AN ESSAY
been well acquainted with practical as well as theoretical
magnetifm. His opinion was, that this earth had origi-
nally received its magnctifm, or rather that its magnetical
powers had been brought into action, by a fhock, which
entered at about the fouthern, and paffed out at the
northern tropic. His meaning appears to have been,
that this was the courfe of the magnetic fluid, and that
the magnetic poles were at firſt diametrically oppoſite to
each other.
SECT. IX. In this Effay our intention has all along been
to avoid fuppofitions merely theoretical; for we do not
pretend to explain the caufes of magnetifm on any theory
however plauſible. Yet we have no objection to Dr.
Knight's fuppofition, that the magnetic poles might at
firſt have been oppofite to each other; though, according
to Mr. Canton's doctrine, they would not have long con-
tinued fo; for from the intenſe heat of the fun in the
torrid zone, according to the principles already explained,
the north pole muſt have ſoon retired to the north-eaſt-
ward, and the fouth pole to the fouth-eastward. It is alſo
curious to obſerve, that on account of the fouthern hemi-
fphere being colder upon the whole than the northern
hemiſphere, the magnetic poles would have moved with
* It is well known that in the courfe of the year the fun is for
about eight days longer in the northern than in the fouthern hemi-
fphere; but without entering into all the caufes of this effect, it is
here ſufficient to obſerve that this fact has been indiſputably proved.
unequal
ON
MAGNETISM.
33
unequal pace; that is, the north magnetic pole would have
moved farther in any given time to the north-eaſt, than
the fouth magnetic pole could have moved to the fouth-
eaſt. And, according to the opinions of the moſt inge-
nious authors on this ſubject, it is generally allowed, that
at this time the north magnetic pole is confiderably nearer
to the north pole of the earth, than the fouth magnetic
pole is to the fouth pole of the earth.
Charlotte Street, Sept. 30, 1794.
P. S. Several ingenious fea officers are of opinion, that
in the weſtern parts of the Engliſh Channel the variation
of the magnetic needle has already begun to decreaſe;
having in no part of it ever amounted to 25°. There are,
however, other perfons who affert, that the variation is
ſtill increaſing in the Channel, and as far weftward as the
15th degree of longitude and 51° of latitude, at which
place they fay that it amounts to about 30°.
F
AN
34
AN ESSA Y, &c.
A N
EXTRACT
OF A
AS
L
ETTER
From the ingenious Dr. NOOTH, to the AUTHOR..
Dated Quebec, Nov. 1ft, 1793.
S I am well convinced, that you will with pleafüre-
receive any communication reſpecting the magnetic
influence, I fhall just tell you, that lately I accidentally
found that there was a diminution in the variation of
this place fince the year 1785. In that year Major
Williams and Major Holland, ran a meridian on the
plains of Abraham; and on the ſeveral ſtones that were
placed in the line of the meridian they marked the lon-
gitude, latitude, and variation. At that time the varia-
tion, by the mean of many compaffes, was 12° 35'; at pre-
fent I find it only 12° 5'; and this has been aſcertained by
at leaſt thirty needles, fo that there can be no doubt of:
the fact.
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