loadstones appears to be of very ancient date among the nations of the West,
there is strong historical evidence in proof of the striking fact that the
knowledge of the directive power of a magnetic needle and of its relation to
terrestrial magnetism was peculiar to the Chinese, a people living in the
extremest eastern portions of Asia. More than a thousand years before our
era, in the obscure age of Codrus, and about the time of the return of the
Heraclidae to the Peloponnesus, the Chinese had already magnetic carriages,
on which the movable arm of the figure of a man continually pointed to the
south, as a guide by which to find the way across the boundless grass plains
of Tartary; nay, even in the third century of our era, therefore at least
700 years before the use of the mariner's compass in European seas, Chinese
vessels navigated the Indian Ocean* under the direction of magnetic needles
pointing to the south.
[footnote] *Humboldt, 'Examen Critique de l'Hist. de la Geographie', t.
iii., p. 36.
I have shown, in another work, what advantages this means of topographical
direction, and the early knowledge and application of the magnetic needle
gave the Chinese geographers over the Greeks and Romans, to whom, for
instance, even the true direction of the Apennines and Pyrenees always
remained unknown.*
[footnote] *'Asie Centrale', t. i., Introduction, p. xxxviii-xlii. The
Western nations, the Greeks and the Romans, knew that magnetism could be
communicated to iron, 'and that that metal would retain it for a length of
time'. ("Sola haec materia ferri vires, a maguete lapide accipit,
'retinetque longo tempore." Plin., xxxiv., 14.) The great discovery of the
terrestrial directive force depended, therefore, alone on this, that no one
in the West had happened to observe an elongated fragment of magnetic iron
stone, or a magnetic iron rod, floating, by the aid of a piece of wood, in
water, or suspended in the air by a thread, in such a position as to admit
of free motion.
The magnetic power of our globe is manifested on the terrestrial surface in
three classes of phenomena, one of which exhibits itself in the varying
intensity of the force, and the two others in the varying direction of the
inclination, and in
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the horizontal deviation from the terrestrial meridian of the spot. Their
combined action may therefore be graphically represented by three systems of
lines, the 'isodynamic, isoclinic', and 'isogonic' (or those of equal force,
equal inclination, and equal declination). The distances apart, and the
relative positions of these moving, oscillating, and advancing curves, do
not always remain the same. The total deviation (variation or declination
of the magnetic needle) has not at all changed, or, at any rate, not in any
appreciable degree, during a whole century, at any particular point on the
Earth's surface,* as, for instance, the western part of the Antilles, or
Spitzbergen.
[footnote] *A very slow secular progression, or a local invariability of
the magnetic declination, prevents the confusion which might arise from
terrestrial influences in the boundaries of land, when, with an utter
disregard for the correction of declination, estates are, after long
intervals, measured by the mere application of the compass. "The whole mass
of the bottomless pit of endless litigation by the invariability of the
magnetic declination in Jamica and the surrounding Archipelago during the
whole of the last century, all surveys of property there having been
conducted solely by the compass." See Robertson in the 'Philosophical
Transactions' for 1806, Part ii., p. 348, 'On the Permanency of the Compass
in Jamaica since 1660'. In the mother country (England) the magnetic
declination has varied by fully 14 degrees during the period.
In like manner, we observe that the isogonic curves, when they pass in their
secular motion from the surface of the sea to a continent or an island of
considerable extent, continue for a long time in the same position, and
become inflected as they advance.
These gradual changes in the forms assumed by the lines in their translatory
motions, and which so unequally modify the amount of eastern and western
declination, in the course of time render it difficult to trace the
transitions and analogies of forms in the graphic representations belonging
to different centuries.
Each branch of a curve has its history, but this history does not reach
further back among the nations of the West than the memorable epoch of the
13th of September, 1492, when the re-discoverer of the New World found a
line of no variation 3 degrees west of the meridian of the island of Flores,
one of the Azores.*
[footnote] *I have elsewhere shown that, from the documents which have come
down to us regarding the voyages of Columbus, we can, with much certainty,
fix upon three places 'in the Atlantic line of no declination' for the 13th
of September, 1492, the 21st of May, 1496, and the 16th of August, 1498.
The Atlantic line of no declination at that period ran from northeast to
southwest. It then touched the South American continent a little east of
Cape Codera, while it is not observed to reach that continent on the
northern coast of the Brazils. (Humboldt, 'Examen Critique de l'Hist. de la
Geogr.', t. iii., p. 44-48.) From Gilbert's 'Physiologia Nova de Magnete',
we see plainly (and the fact is very remarkable) that in 1600 the
declination was still null in the region of the Azores, just as it had been
in the time of Columbus (lib. 4, cap. 1). I believe that in my 'Examen
Critique' (t. iii., p. 54) I have proved from documents that the celebrated
line of demarkation by which Pope Alexander VI. divided the Western
hemisphere between Portugal and Spain was not drawn through the most western
point of the Azores, because Columbus wished to convert a physical into a
political division. He attached great importance to the zone (raya) "in
which the compass shows no variation, where air and ocean, the later covered
with pastures of sea-weed, exhibit a peculiar constitution, where cooling
winds begin to blow, and where [as erroneous observations of the polar star
led him to imagine] the form (sphericity) of the Earth is no longer the
same."
The whole of Europe, excepting a small
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part of Russia, has now a western declination, while at the close of the
seventeenth century the needle first pointed due north, in London in 1657,
and in Paris in 1669, there being thus a difference of twelve years,
notwithstanding the small distance between these two places. In Eastern
Russia, to the east of the mouth of the Volga, of Saratow, Nischni-Nowgorod,
and Archangel, the easterly declination of Asia is advancing toward us. Two
admirable observers, Hansteen and Adolphus Erman, have made us acquainted
with the remarkable double curvature of the lines of declination in the vast
region of Northern Asia; these being concave toward the pole between
Obdorsk, on the Oby, and Turuchansk, and convex between the Lake of Baikal
and the Gulf of Ochotsk. In this portion of the earth, in northern Asia,
between the mountains of Werchojansk, Jakutsk, and the northern Korea, the
isogonic lines form a remarkable closed system. This oval configuration*
recurs regularly and over a great extent of the South Sea, almost as far as
the meridian of Pitcairn and the group of the Marquesas Islands, between 20
degrees north and 45 degrees
p 183
south lat.
[footnote] *To determine whether the two oval systems of isogonic lines, so
singularly included each within itself, will continue to advance for
centuries in the same inclosed form, or will unfold and expand themselves,
is a question of the highest interest in the problem of the physical causes
of terrestrial magnetism. In the Eastern Asiatic nodes the declination
increases from without inward, while in the node or oval system of the South
Sea the opposite holds good; in fact, at the present time, in the whole
South Sea to the east of the meridian of Kamt-schatka, there is no line
where the declination is null, or, indeed, in which it is less than 2
degrees (Erman, in Pogg., 'Annal.', bd. xxxi, 129). Yet Cornelius Schouten,
on Easter Sunday, 1616, appears to have found the declination null somewhere
to the southeast of Nukahiva, in 15 degrees south lat. and 132 degrees west
long., and consequently in the middle of the present closed isogonal system.
(Hansteen, 'Magnet. der Erde', 1819 § 28.) It must not be forgotten, in
the midst of all these considerations, that we can only follow the direction
of the magnetic lines in their progress as they are projected upon the
surface of the Earth.
One would almost be inclined to regard this singular configuration of
closed, almost concentric, lines of declination as the effect of a local
character of that portion of the globe; but if, in the course of centuries,
these apparently isolated systems should also advance, we must suppose, as
in the case of all great natural forces, that the phenomenon arises from
some general cause.
The horary variations of the declination, which, although dependent upon
true time, are apparently governed by the Sun, as long as it remains above
the horizon, diminish in angular value with the magnetic latitude of place.
Near the equator, for instance, in the island of Rawak, they scarcely amount
to three or four minutes, while they are from thirteen to fourteen minutes
in the middle of Europe. As in the whole northern hemisphere the north
point of the needle moves from east to west on an average from 8 1/2 in the
morning until 1 1/2 at mid-day, while in the southern hemisphere the same
north point moves from west to east,* attention has recently been drawn,
with much justice, to the fact that there must be a region of the Earth
between the terrestrial and the magnetic equator where no horary deviations
in the declination are to be observed.
[footnote] *Arago, in the 'Annuaire', 1836, p. 284, and 1840, p. 330-338.
This fourth curve, which might be called the 'curve of no motion', or,
rather, 'the line of no variation of horary declination', has not yet been
discovered.
The term 'magnetic poles' has been applied to those points of the Earth's
surface where the horizontal power disappears, and more importance has been
attached to these points than properly appertains to them;* and in like
manner, the curve, where the inclination of the needle is null, has been
termed the 'magnetic equator'.
[footnote] *Gauss, 'Allg. Theorie des Erdmagnet.', 31.
The position of this line and its secular change of configuration have been
made an object of careful investigation in modern times. According to the
admirable work of Duperrey,* who crossed the magnetic equator six times
between 1822 and 1825, the nodes of the two equators, that is to say, the
two points at which the line without inclination intersects the terrestrial
equator, and consequently passes from one henisphere into the other, are so
unequally placed, that in 1825 the node near the island of St. Thomas, on
the western
p 184
coast of Africa, was 188 1/2 degrees distant from the node in the South Sea,
close to the little islands of Gilbert, nearly in the meridian of the Viti
group.
[footnote] *Duperrey, 'De la Configuration de l'Equateur Magnetique', in
the 'Annales de Chimie', t. xlv., p. 371 and 379. (See also, Morlet, in the
'Memoires presentes par divers Savans a l'Acad. Roy. des Sciences', t. iii.,
p. 132.
In the beginning of the present century, at an elevation of 11,936 feet
above the level of the sea, I made an astronomical determination of the
point (7 degrees 1' south lat., 48 degrees 40' west long. from Paris),
where, in the interior of the New Continent, the chain of the Andes is
intersected by the magnetic equator between Quito and Lima. To the west of
this point, the magnetic equator continues to traverse the South Sea in the
southern hemisphere, at the same time slowly drawing near the terrestrial
equator. It first passes into the northern hemisphere a little before it
approaches the Indian Archipelago, just touches the southern points of Asia,
and enters the African continent to the west of Socotora, almost in the
Straits of Bab-el-Mandeb, where it is most distant from the terrestrial
equator. After intersecting the unknown regions of the interior of Africa
in a southwest direction, the magnetic equator re-enters the south tropical
zone in the Gulf of Guinea, and retreats so far from the terrestrial equator
that it touches the Brazilian coast near Os Ilheos, north of Porto Seguro,
in 15 degrees south lat. From thence to the elevated plateaux of the
Cordilleras, between the silver mines of micuipampa and Caxamarca, the
ancient seat of the Incas, where I observed the inclination, the line
traverses the whole of South America, which in these latitudes is as much a
magnetic 'terra incognita' as the interior of Africa.
The recent observations of Sabine* have shown that the node near the island
of St. Thomas has moved 4 degrees from east to west between 1825 and 1837.
[footnote] *See the remarkable chart of isoclinic lines in the Atlantic
Ocean for the years 1825 and 1837, in Sabine's 'Contributions to Terrestrial
Magnetism', 1840, p. 134.
It would be extremely important to know whether the opposite pole, near the
Gilbert Islands, in the South Sea, has aproached the meridian of the
Carolinas in a westerly direction. These general remarks will be sufficient
to connect the different systems of isoclinic non-parallel lines with the
great phenomenon of equilibrium which is manifested in the magnetic equator.
It is no small advantage, in the exposition of the laws of terrestrial
magnetism, that the magnetic equator (whose oscillatory change of form and
whose nodal motion exercise an influence on the inclination of the needle in
the remotest districts of the world, in consequence of the altered magnetic
latitudes)* should traverse the
p 185
ocean throughout its whole course, excepting about one fifth, and
consequently be made so much more accessible, owing to the remarkable
relations in space between the sea and land, and to the means of which we
are now possessed for determining with much exactness both the declination
and the inclination at sea.
[footnote] *Humboldt, 'Ueber die seculäre Veränderung der Magnetischen
Inclination' (On the secular Change in the Magnetic Inclination), in Pogg.
'Annal.', bd. sv., s. 322.
We have described the distribution of magnetism on the surface of our planet
according to the two forms of 'declination' and 'inclination'; it now,
therefore, remains for us to speak of the 'intensity of the force' which is
graphically expressed by isodynamic curves (or lines of equal intensity).
The investigation and measurement of this force by the oscillations of a
vertical or horizontal needle have only excited a general and lively
interest in its telluric relations since the beginning of the nineteenth
century. The application of delicate optical and chronometrical instruments
has rendered the measurement of this horizontal power susceptible of a
degree of accuracy far surpassing that attained in any other magnetic
determinations. The isogonic lines are the more important in their
immediate application to navigation, while we find from the most recent
views that isodynamic lines, especially those which indicate the horizontal
force, are the most valuable elements in the theory of terrestrial
magnetism.*
[footnote] *Gauss, 'Resultate der Beob. des Magn. Vereins', 1838, 21;
Sabine, 'Report on the Variations of the Magnetic Intensity', p. 63.
One of the earliest facts yielded by observation is, that the intensity of
the total force increases from the equator toward the pole.*
[footnote] *The following is the history of the discovery of the law that
the intensity of the force increases (in general) with the magnetic
latitude. When I was anxious to attach myself, in 1798, to the expedition
of Captain Bandin, who intended to circumnavigate the globe, I was requested
by Borda, who took a warm interest in the success of my project, to examine
the oscillations of a vertical needle in the magnetic meridian in different
latitudes in each hemisphere, in order to determine whether the intensity of
the force was the same, or whether it varied in different places. During my
travels in the tropical regions of America, I paid much attention to this
subject. I observed that the same needle, which in the space of ten minutes
made 245 oscillations in Paris, 246 in the Havana, and 242 in Mexico,
performed only 216 oscillations during the same period at St. Carlos del Rio
Negro (1 degree 53' north lat. and 80 degrees 40' west long. from Paris), on
the magnetic equator, i.e., the line in which the inclination =0; in Peru (7
degrees 1' south lat. and 80 degrees 40' west long. from Paris) only
211;while at Lima (12 degrees 2' south lat.) the number rose to 219. I
found, in the years intervening between 1799 and 1803, that the whole force,
if we assume it at 1.0000 on the magnetic equator in the Peruvian Andes,
between Micuipampa and Caxamarca, may be expressed at Paris by 1.3482, in
Mexico by 1.3155, in San Carlos del Rio Negro by 1.0480, and in Lima by
1.0773. When I developed this law of the variable intensity of terrestrial
magnetic force, and supported it by the numerical value of observations
instituted in 104 different places, in a Memoir read before the Paris
Institute on the 26th Frimaire, An. XIII. (of which the mathematical portion
was contributed by M. Biot), the facts were regarded as altogether new. It
was only after the reading of the paper, as Biot expressly states
(Lametherie, 'Journal de Physique', t. lix., p. 446, note 2) and as I have
repeated in 'the Relation Historique', t. i., p. 262, note 1, that M. de
Rossel communicated to Biot his oscillation experiments made six years
earlier (between 1791 and 1794) in Van Diemen's Land, in Java, and in
Amboyna. These experiments gave evidence of the same law of decreasing
force in the Indian Archipelago. It must, I think be supposed, that this
excellent man, when he wrote his work, was not aware of the regularity of
the augmentation and diminution of the intensity as before the reading of my
paper he never mentioned this (certainly not unimportant) physical law to
any of our mutual friends, La Place, Delambre, Prony, or Biot. It was not
till 1808, four years after my return from America that the observations
made by M. de Rossel were published in the 'Voyage de l'Entrecasteaux', t.
ii., p. 287 , 291, 321, 480, and 644. Up to the present day it is still
usual, in all the tables of magnetic intensity which have been published in
Germany (Hausteen, 'Magnet. der Erde', 1819, s. 71; Gauss, 'Beob. des
Magnet. Vereins', 1838, s. 36-39; Erman, 'Physikal. Beob.', 1841, s.
529-579), in England (Sabine, 'Report on Magnet. Intensity', 1838, p. 43-62;
'Contributions to Terrestrial Magnetism', 1843), and in France (Becquerel,
'Traite de Electr. et de Magnet.', t. vii., p. 354-367), to reduce the
oscillations observed in any part of the Earth to the standard of force
which I found on the magnetic equator in Northern Peru, so that, according
to the unit thus arbitrarily assumed, the intensity of the magnetic force at
Paris is put down as 1.348. The observations made by Lamanon in the
unfortunate expedition of La Perouse, during the stay at Teneriffe (1785),
and on the voyage to Macao (1787), are still older than those of Admiral
Rossel. They were sent to the Academy of Sciences, and it is known that
they were in the possession of Condorcet in the July of 1787 (Becquerel, t.
vii., p. 320); but, notwithstanding the most careful search, they are not
now to be found. From a copy of a very important letter of Lamanon, now in
the possession of Captain Duperrey, which was addressed to the then
perpetual secretary of the Academy of Sciences, but was omitted in the
narrative of the 'Voyage de La Perouse', it is stated "that the attractive
force of the magnet is less in the tropics than when we approach the poles,
and that the magnetic intensity deduced from the number of oscillations of
the needle of the inclination-compass varies and increases with the
latitude." If the Academicians, while they continued to expect the return
of the unfortunate La Perouse, had felt themselves justified, in the course
of 1787, in publishing a truth which had been independently discovered by no
less than three different travelers, the theory of terrestrial magnetism
would have been extended by the knowledge of a new class of observations,
dating eighteen years earlier than they now do. This simple statement of
facts may probably justify the observations contained in the third volume of
my 'Relation Historique' p. 615): "The observations on the variation of
terrestrial magnetism, to which I have devoted myself for thirty-two years,
by means of instruments which admit of comparison with one another, in
America, Europe, and Asia, embrace an area extending over 188 degrees of
longitude, from the frontier of Chinese Dzoungarie to the west of the South
Sea bathing the coasts of Mexico and Peru, and reaching from 60 degrees
north lat. to 12 degrees south lat. I regard the discovery of the law of
the decrement of magnetic force from the pole to the equator as the most
important result of my American voyage." Although not absolutely certain,
it is very probable that Condorcet read Lamanon's letter of July, 1787, at a
meeting of the Paris Academy of Sciences; and such a simple reading I regard
as a sufficient act of publication. ('Annuaire du Bureau des Longitudes',
1842, p. 463.) The first recognition of the law belongs, therefore, beyond
all question, to the comparison of La Perouse; but, long disregarded or
forgotten, the knowledge of the law that the intensity of the magnetic force
of the Earth varied with the latitude, did not, I conceive, acquire an
existence in science until the publication of my observations from 1798 to
1804. The object and the length of this note will not be indifferent to
those who are familiar with the connection with it, and who, from their own
experience, are aware that we are apt to attach some value to that which has
cost us the uninterrupted labor of five years, under the pressure of a
tropical climate, and of perilous mountain expeditions.
p 186
The knowledge which we possess of the quantity of this increase, and of all
the numerical relations of the law of intensity
p 187
affecting the whole Earth, is especially due, since 1819, to the unwearied
activity of Edward Sabine, who, after having observed the oscillations of
the same needles at the American north pole, in Greenland, at Spitzbergen,
and on the coasts of Guinea and Brazil, has continued to collect and arrange
all the facts capable of explaining the direction of the isodynamic system
in zones for a small part of South America. These lines are not parallel to
lines of equal inclination (isoclinic line), and the intensity of the force
is not at its minimum at the magnetic equator, as has been supposed, nor is
it even equal at all parts of it. If we compare Erman's observations in the
southern part of the Atlantic Ocean, where a faint zone (0.706) extends from
Angola over the island of St. Helena to the Brazilian coast, with the most
recent investigations of the celebrated navigator James Clark Ross, we shall
find that on the surface of our planet the force increases almost in the
relation of 1:3 toward the magnetic south pole, where Victoria Land extends
from Cape Crozier toward the volcano Erebus, which has been raised to an
elevation of 12,600 feet above the ice.*