Complete analogy of the earth to a magnet. By com-
paring these results with those which have been already de-
cribed in the case where the needle was carried successively over
a magnetic bar, the complete identity of the phenomena will be
apparent, and it will be evident that the earth and the needle
comport themselves in relation to each other exactly as do a
small and a great magnet, over which it might be carried, the
point where the needle is horizontal being over the magnetic
equator, and those two points where it is vertical being the
1658. The magnetic equator. The needle being brought
to that point where it rests horizontal, the magnetic equator
will be at right angles to its direction. By transporting it suc-
cessively in the one or the other direction thus indicated, the
successive points upon the earth's surface where the needle rests
horizontal, and where the dip is nothing, will be ascertained.
The line upon the earth traced by this point is the magnetic
Its form and position not regular. This line is not, as
might be expected, a great circle of the earth. It follows
a course crossing the terrestrial equator from south to north,
on the west coast of Africa, near the island of St. Thomas,
at about 7 or 8 long. E., in a direction intersecting the
equator at an angle of about 12 or 13. It then passes
across Africa towards Ceylon, and intersects that island near
the point of the Indian promontory. It keeps a course
from this of from 8 to 9 of N. lat. through the Indian Archi-
pelago, and then gradually declining towards, the line again
intersects it at a point in the Pacific Ocean in long. 170 TV.,
the angle at which it intersects the line being more acute than
at the other point of intersection. It then follows a course a
few degrees south of the line, and striking the west coast of
South America near Lima, it crosses the South American con-
tinent, attaining the greatest south latitude near Bahia ; and
then again ascending towards the line, traverses the Atlantic and
strikes the coast of Africa, as already stated, "near the island of
The magnetic equator, unlike the ecliptic, is not any regular
curve, but follows the course we have just indicated in a
direction slightly sinuous.
Variation of the dip, going north or south It has been
TERRESTRIAL MAGNETISM. 171
explained, that proceeding from nortli or south, from the
magnetic equator, the needle dips on the one side or on the
other, the dip increasing with the distance from the magnetic
equator to which the needle is transported north or south.
Lines of equal dip. The lines of equal dip, therefore,
may be considered as bearing the same relation to the magnetic
equator, which parallels of latitude hear to the terrestrial equator,
being arranged nearly parallel to the former, though not in a
manner so regular as in the case of parallels of latitude.
1659. Magnetic meridians. If the horizontal needle be
transported north or south, following a course indicated by its
direction, it will be carried over a magnetic meridian. These
magnetic meridians, therefore, bear to the magnetic equator a
relation analogous to those which terrestrial meridians bear to
the terrestrial equator, but, like the lines of equal dip, they are
much more irregular.
1660. Method of ascertaining the declination of the needles.
Astronomy supplies various methods of determining in a
given place the declination of the needle. It may be generally
stated that this problem may be solved by observing any object
whose angular distance from the true north is otherwise known,
and comparing the direction of such object with the direction
of the needle. Let p, Jig. 470., be the place of observation ; let
p N be the direction of the true north, or, what is the same, the
direction of the terrestrial meridian ; and let
p N' be the direction of the magnetic needle,
or, what is the same, the magnetic meridian.
The angle N p N' will then be the declination
of the needle, being the angle formed by the
terrestrial and magnetic meridians (1655).
Let O be any object seen on the horizon in
the direction P o ; the angle o P N is called the
true azimuth of this object, and the angle OPN'
is called its magnetic azimuth.
This magnetic azimuth may always be ob-
served by means of an azimuth compass.
If, then, an object be selected whose true azimuth is otherwise
known, the declination of the needle may be determined by
taking the difference between the true and magnetic azimuths
of the object.
There are numerous celestial objects of which the azimuths
are either given in tables, or may be calculated by rules and
formulae supplied by astronomy ; such, for example, as the sun
and moon at the moments they rise or set, or when they are at
any proposed or observed altitudes. By the aid of such objects,
which are visible occasionally at all places, the declination of
the needle may be found.
Local declinations. At different places upon the earth's
surface the needle has different declinations. In Europe its
mean declination is about 17, increasing in going westward.
1661. Lines of no declination called agonic lines. There
are two lines on the earth's surface which have been called
AGONIC LINES, upon which there is no declination ; and where>
therefore, the needle is directed along the terrestrial meridian.
One of these passes over the American and the other over
the Asiatic continent, and the former has consequently been
called the AMERICAN and the latter the ASIATIC AGONIC. These
lines run north and south, but do not follow the course of
It has been ascertained that their position is not fixed, but is
liable to sensible changes in considerable intervals of time.
1662. Declination in different longitudes, at equator, and in
lat. 45. In proceeding in either direction, east or west from
these lines, the declination of the needle gradually increases,
and becomes a maximum at a certain intermediate point between
them. On the west of the Asiatic agonic the declination is
west, on the east it is east.
At present the declination in England is about 24 TV. ; in
Boston in the U. States it is 5^ W. Its mean value in Europe
is 17 W. At Bonn it is 20, at Edinburgh 26, Iceland 38,
Greenland 50, Konigsberg 13, and St. Petersburg 6.
The following table, however, will exhibit more distinctly
the variation of the declination in different parts of the globe.
The longitudes expressed in the first column are measured west-
ward from the meridian of Paris, and the declinations given in
the second column are those which are observed on the terres-
trial equator, those in the third column corresponding to the
mean latitude of 45.
Table of the Declinations of the magnetic Needle in different
Longitudes, and in Lat. Q and Lot. =45.
I.ongitu les West
of the Meridian
Lat. = 0.
Lat. = 45.
of Paris." 1 "
Lat. = 450.
1663. Isogonic lines. Lines traced upon the globe at a point
at which the magnetic needle has the same declinations, are
called ISOGONIC LINES. These, as well as the ISOCLINIC LINES, or
lines of equal dip, are irregular in their arrangement, and not
very exactly ascertained.
1664. Local dip. The local variations of the dip are also im-
perfectly known. In Europe it ranges from 60 to 70. In 1836
the dip observed at the undermentioned places was as follows :
Rio de Janeiro
- 54 C
- 61 C
- 68 C
- 71 C
- 14 C
- 13 C
1665. Position of magnetic poles. The determination of the
precise position of the magnetic poles, or the points where the
dip is 90, is attended with considerable difficulty, inasmuch as
for a considerable distance round that point the dip is nearly 90.
Hansteen considered that there were grounds for supposing
that there were two magnetic poles in each hemisphere. One
of these in the northern hemisphere he supposed to be west of
Hudson's Bay in 80 lat. N., and 96 long. W. ; and the other in
Northern Asia in 81 lat. N., and 116 long. E. The two
southern magnetic poles he supposed to be situate near the
southern pole. This supposition, however, appears to be at
present abandoned, and the observations of GAUSS lead to the
'conclusion that there is but one magnetic pole in each hemisphere.
In the northei-n voyages made between 1829 and 1833,
Sir James Ross found the dipping-needle to stand vertical in
the neighbourhood of Hudson's Bay at 70 5' 1 7" lat. N., and
114 55' 18" long. W. The dipping-needle, according to the
observations of Sir James Ross, was nowhere absolutely vertical,
departing from the vertical in all cases by a small angle, amount-
ing generally to one minute of a degree. This, however, might
be ascribed to the error of observation, or the imperfection of
instruments exposed to such a climate.
The existence of the magnetic pole, however, at or near the
point indicated, was proved by carrying roun,d it at a certain
distance a horizontal needle, which always pointed to the spot
in whatever direction it was carried. Gauss has fixed the
position of the magnetic pole in the southern hemisphere by
theory at 72 35' lat. S., and 152 30' long. E.
1666. Magnetic poles not antipodal. It will be perceived,
therefore, that the magnetic poles, unlike the terrestrial poles,
are not antipodal to each other ; or, in other words, they do not
form the extremities of the same diameter of the globe : they
are not even on the same meridian. If Gauss's statement be
assumed to be correct, the southern magnetic pole is on a
meridian 152 30' E. of the meridian of Greenwich, and there-
fore 207 30' W. of that meridian, whereas the northern
magnetic pole is on a meridian 114 55' 18" W. The angle,
therefore, under the two meridians passing through the two
poles will be about 921. It would follow, therefore, that these
points lie upon terrestrial meridians nearly at right angles to
each other, and that upon these they are at nearly equal distances
from the terrestrial poles ; the distance of the northern mag-
netic pole from the northern terrestrial pole being nearly 20,
and the distance of the southern magnetic pole from the southern
terrestrial pole being about 17^.
1667. Periodical variations of terrestrial magnetism. It
appears, from observations made at intervals of time more or
less distant for about two centuries back, that the magnetic
condition of the earth is subject to a periodical change; but
neither the quantity nor the law of this change is exactly
known. It was not until recently that magnetic observations
were conducted in such a manner as to supply the data
necessary for the development of the laws of magnetic varia-
tion, and they have not been yet continued a sufficient length
of time to render these laws manifest.
Independently of observation, theory affords no means of
ascertaining these laws, since it is not certainly known what
are the physical causes to which the magnetism of the earth
must be ascribed.
In the following table are given the declinations of the needle
observed at Paris between the years 1580 and 1835, and the
dip between the years 1671 and 1835.
Table of Declinations observed at Paris.
11 30' E
22 25' W
1 30 W
Table of the Dip observed at Paris.
1668. Intensity of terrestrial magnetism. The intensity of
terrestrial magnetism, like that of a common magnet, may be
estimated by the rate of vibration which it produces in a
magnetic needle submitted to its attraction. This method of
determining the intensity of magnetic force is in all respects
analogous to those by which the intensity of the earth's attrac-
tion is determined by a common pendulum (549). The same
needle being exposed to a varying attraction will vary its rate
of vibration, the force which attracts it being proportional to
the square of the number of vibrations which it makes in a
given time. Thus, if at one place it makes ten vibrations per
minute, and in another only eight, the magnetic force which
produces the first will be to that which produces the second
rate of vibration, as 100 to 64.
1669. Increases from equator to poles. In this manner it
has been found that the intensity of terrestrial magnetism is
least at the magnetic equator, and that it increases gradually in
approaching the poles.
1670. Isodynamic lines. Those parts of the earth where
the magnetic intensities are equal are called isodynamic lines,
and resemble in their general arrangement, without however
coinciding with them, the isoclinic curves or magnetic parallels
of equal dip.
1671. Their near coincidence with isothermal lines. It
has been found that there is so near a coincidence between
the isodynamic and the isothermal lines, that a strong pre-
sumption is raised that terrestrial magnetism either arises from
terrestrial heat, or that these phenomena have at least a common
1672. Equatorial and polar intensities. It appears to
follow from the general result of observations made on the
intensity of terrestrial magnetism, that its intensity at the
poles is to its intensity at the equator nearly in the ratio
of 3 to 2.
1673. Effect of the terrestrial magnetism on soft iron> If
anything were wanted to complete the demonstration that the
globe of the earth is a true magnet, it would be supplied by
the effects produced by it upon substances susceptible of mag-
netism, but which are not yet magnetized. It has been already
shown that when a bar of soft iron is presented to the pole of
a magnet, its natural magnetism is decomposed, the austral fluid
being attracted to one extremity, and the boreal fluid repelled
to the other, so that the bar of soft iron becomes magnetized,
and continues so as long as it is exposed to the influence of the
magnet. Now, if a bar of soft iron be presented to the earth in
the same manner, precisely the same effects will ensue. Thus, if
it be held in the direction of the dipping-needle, so that one of its
ends shall be presented in the direction of the magnetic attraction
of the earth, it will become magnetic, as may be proved by any
TERRESTRIAL MAGNETISM. 177
of the tests of magnetism already explained. Thus, if a sensi-
tive needle be presented to that end of the bar which in the
northern hemisphere is directed downwards, austral magnetism
will be manifested, the boreal pole of the needle being attracted,
and the austral pole repelled. If the needle be presented to
the upper end of the bar, contrary effects will be manifested ;
and if it be presented to the middle of the bar, the neutral line
or equator will be indicated. If the bar be now inverted, the
upper end being presented downwards, and vice versa, still
parallel to the dipping-needle, its poles will also be inverted,
the lower, which previously was boreal, being austral, and vice
If the bar be held in any other direction, inclined obliquely
to the dipping-needle, the same effects will be manifested, but
in a less degree, just as would be the case if similarly presented
to an artificial magnet ; and, in fine, if it be held at right angles
to the direction of the dipping-needle, no magnetism whatever
will be developed in it.
1674. Its effects on steel bars. If the same experiments be
made with bars of hard iron or steel, no sensible magnetism
will at first be developed ; but if they be held for a consider-
able time in the same position, they will at length become
magnetic, as would happen under like conditions with an arti-
ficial magnet. Iron and steel tools which are hung up in work-
shops in a vertical position are found to become magnetic, an
effect explained by this cause.
1675. Diurnal variation of the needle. Besides the changes
in the magnetic state of the earth, the periods of which are
measured by long intervals of time, there are more minute and
rapid changes depending apparently upon the vicissitudes of the
seasons and the diurnal changes.
The magnitude of the diurnal variation depends upon the
situation of the place, the day, and the season, but is obviously
connected with the function of solar heat. At Paris it is ob-
served that during the night the needle is nearly stationary ;
at sunrise it begins to move, its north pole turning westwards,
as if it were repelled by the influence of the sun. About noon,
or more generally between noon and three o'clock, its western
variation attains a maximum, and then it begins to move east-
ward, which movement continues until some time between nine
and eleven o'clock at night, when the needle resumes the
position it had when it commenced its western motion in the
The amplitude of this diurnal range of the needle is, accord-
ing to Cassini's observations, greatest during summer and least
during winter. Its mean amount for the months of April,
May, June, July, August, and September is stated at from
13 to 15 minutes; and for the months of October. November,
December, January, and March, at from 8 to 10 minutes.
There are, however, occasionally, days upon which its range
amounts to 25 minutes, and others when it does not surpass
5 or 6 minutes. Cassini repeated his magnetic observations in
the cellars constructed under the Paris observatory at a depth
of about a hundred feet below the surface, and therefore
removed from the immediate influence of the light and heat of
the day. The amplitude of the variations, and all the pecu-
liarities of the movement of the needle here, were found to be
precisely the same as at the surface.
In more northern latitudes, as, for example, in Denmark,
Iceland, and North America, the diurnal variations of the
needle are in general more considerable and less regular. It
appears, also, that in these places the needle is not stationary
during the night, as in Paris, and that it is towards evening
that it attains its maximum westward deviation. On the
contrary, on going from the north towards the magnetic equator
the diurnal variations diminish, and cease altogether on ar-
riving at this line. It appears, however, according to the ob-
servations of Captain Duperrey, that the position of the sun
north or south of the terrestrial equator has a perceptible
influence on the oscillation of the needle.
On the south of the magnetic equator the diurnal variations
are produced, as might be expected, in a contrary manner ; the
northern pole of the magnet turns to the east at the same hours
that, in the northern hemisphere, it turns to the west.
It has not yet been certainly ascertained whether in each
hemisphere these diurnal variations of the needle correspond in
the places where the eastern and western declinations also
The dip is also subject to certain diurnal variations, but
much smaller in their range than in the case of the horizontal
As a general result of these observations it may be inferred,
that if a magnetic needle were suspended in such a manner
as to be free to move in any direction Avhatever, it would,
during twenty-four hours, move round its centre of suspension
in such a manner as to describe a small cone, whose base would
be an ellipse or some other curve more or less elongated, and
whose axis is the mean direction of the dipping-needle.
1676. Disturbances in the magnetic intensity. The intensity
as well as the direction of the magnetic attraction of the earth
at a given place are subject to continual disturbances, independ-
ently of those more regular variations just mentioned.
These disturbances are in general connected with the elec-
trical state of the atmosphere, and are observed to accompany
the phenomena of the aurora borealis, earthquakes, volcanic
eruptions, sudden vicissitudes of temperature, storms, and other
1677. Influence of aurora borealis. During the appearance
of the aurora borealis in high latitudes, a considerable deflection
of the needle is generally manifested, amounting often to several
degrees. So closely and necessarily is magnetic disturbance
connected with this atmospheric phenomenon, that practised
observers can ascertain the existence of an aurora borealis by
the indications of the needle, when the phenomenon itself is not
1678. Effects of induction. The process by which artificial
magnets are produced are all founded upon the property of in-
duction (Ch. II.). When one of the poles of a magnet is presented
to any body which is susceptible of magnetism, it will have a
tendency to decompose the magnetic fluid in the body to which
it is presented, attracting one of its constituents and repelling
the other. If the coercive force by which the fluids are com-
bined be greater than the energy of the attraction of the mag-
net, no decomposition will take place, and the body to which
the magnet is presented will not be magnetized, but the coercive
force with which the fluids are united will be rendered more
feeble, and the body will be more susceptible of being magnetized
If, however, the energy of the magnetic force of the magnet
presented to it be greater than the coercive force with which
the fluids are united, a decomposition will take place, which will
be more or less in proportion as the force of the magnet exceeds
in a greater or less degree the coercive force which unites the
1679. Their application in the production of artificial mag-
nets. These principles being well understood, the methods of
producing artificial magnets will be easily rendered intelligible.
It has been already explained, that pure soft iron is almost,
if not altogether, divested of coercive force, so that a bar of this
substance is converted into a magnet instantaneously when the
pole of a magnet is presented to it; but the absence of coercive
force, which renders this conversion so prompt, is equally effica-
cious in depriving the bar of its magnetism the moment the
magnet which produces this magnetism is removed.
1680. Best material for artificial magnets. Soft iron, there-
fore, is inapplicable when the object is to produce permanent
magnetism. The material best suited for this purpose is steel,
especially that which has a fine grain, a uniform structure, and
is free from flaws. It is necessary that it should have a certain
degree of hardness, and that this should be uniform through its
entire mass. If the hardness be too great, it is difficult to im-
part to it the magnetic virtue; if not great enough, it loses its
magnetism for want of sufficient coercive force. To render
steel bars best fitted for artificial magnets, it has been found
advantageous to confer upon them in the first instance the
highest degree of temper, and thus to render them as hard and
brittle as glass, and then to anneal them until they are brought
to a straw or violet colour.
1681. Best form for bar magnets. The intensity of artificial