The Redwood gazette. [volume] (Redwood Falls, Minn.) 1873-1940, 1875-08-19
Terrestrial Magnetism.


by its center of gravity, or placed upon ,
point, it will take determinate direction
toward point of the horizon which i
very nearly north and south. The fore,
which produces this direction is called
terrestrial magnetism. 1t is one of til,
modes Of manifestation of the natura
sources of electricity, since magnetism
itself is only particular form of electric
ity. The magnetic force of our globe i!
manifested at its surface by three classe!
Of phenomena, namely: the declination O
the magnetized needle, its inclination
and the intensity with which the force
acts. The declination is the angle that il
formed with the direction of the meridian
of the place by the direction of the magnet
lied needle placed upon a vertical pivot
The inclination is the angle that is fOrmcG
with the horizon in the magnetic meridian
by the direction of magnetized needle
sustained by its center of gravity, around
which it is able to turn freely in vertical
plane. These three Observation-Disclosures
inclination and intensity-nOt only vary
from one place to another, but in the same
place with time. They also manifest
irregular and accidental variations, desig
mated under the name of disturbances, the
existence of which is connected with the
presence of some natural phenomena
such in particular as that of the aurora
borealis. 1t is we'll established that the
forces which act upon the magnetized
needle emanate directly from the terres'
trial globe, and we are naturally led to re.
gard The earth as a great magnet, and as
having one pole situated to the north of us
attracting the. north pole of needle in
that direction.
FIVE suspend a magnetic needle by its
center of gravity, so that it may move
freely either in vertical or horizontal
plane, the extremity which turns toward
the north will incline below the horizon,
making at New York an angle with the
horizon of about 72 deg Hence we con-
clude that, if the earth be great magnet,
giving direction to the needle, its pole
must be situated, not on the north hor.
lion, but almost vertically beneath us.
If the earth is really magnet, the mag.
netism of soft iron ought to be decom.
posed by it, in the same manner as is done
by bar magnet, and such is the fact. II
a bar of soft iron is held in the direction
which magnetic needle assumes when
freely suspended, its lower end Immediate
ly becomes north pole and its upper
end south pole, as is shown by bringing
small magnetic needle near each end of
the bar. On inverting the bar it will be
found that its poles have Immediately
changed, the lower end being again a
north pole, and the upper one a south
pole. If the bar is held horizontally,
pointing east and west, no such effect
takes place.
A similar but slightly diminished eff
feet is produced on bar of iron suspend
ed in vertical position; and iron rods
which have remained long in a vertical
position frequently acquire permanent
magnetism.
When bar of iron is rendered mag
netism, stroke of hammer will some
will not be reversed when the bar is in
verted. But if several blows with the
hammer be struck when in the inverted
position its magnetism may be destroyed
or its poles be reversed.
The action exerted by the earth upon a
magnetic needle is simply to give direction
to the needle, for the weight of needle is
not increased by its magnetism. Hence it
is concluded that the attraction of the
earth for one pole Of the needle is exactly
equal to its repulsion for the other. If a
magnetic needle be placed upon cork
floating on water, it will soon adjust itself
to the magnetic meridian; but it has no
tendency to travel either toward the north
or south.
Although magnetic needle, when fully
suspended, generally points nearly north
and south, it is found in almost all parts
of the world that the north pole of the
needle deviates few degrees from the as
Economical meridian. This deviation is
called the magnetic declination. The de-
clination is Said to be east or west, accord
ing as the north pole of the needle de-
viates to the east or west of the true me-
ridian. The declination of the needle is
very different at different places on the
earth's surface. There are places where
the declination is 10 deg, 20 deg, 80 deg.
and even 80 deg. west; and there are places
where the declinatiOn is as much to the
east. AT most places on the earth's sur
face, the dipping needle will not rest in a
horizontal line, one pole pointing down"
ward and the other upward. This dip
varies at different places from 0 deg. to
80 deg, and observations to determine its
amount have been made in almost every
part of the world. In order to represent
all these observations conveniently upon a
chart, line is drawn connecting all those
places where the dip is the same. A line
connecting all those places where the nee
die rests horizontally is called the magnet
ic equator. This line exhibits numerous
sinuosities in its course around the globe,
but does not depart much from great cir
Gle. 1t crosses the terrestrial equator near
the western coast of Africa, attains its
greatest southern latitude in South Ameri-
ca, where it is Is deg. south of the geo
graphical equator, crosses the equator
again near the meridian of New Zealand,
and attains north latitude of 10 deg. near
the southern part of Hindostan.
As we travel northward from the mag.
netic equator, the north end of the needle
inclines downward and the dip continual
ly increases, at the rate of about deg for
1 deg. of latitude, until we reach the north
magnetic pole, where the needle stands
vertically, in latitude 20 deg. min. N,
longitude SO deg. As min. w.
AS we travel southwark from the mag.
netic equator, the south end of the needle
inclines downward, and this dip eontinu.
ally increases until we reach. the south
magnetic pole. -
That terrestrial magnetism is not pro-
duced, in any important degree, by mag
netic forces external to the earth is proba-
Bile, because, if there were an external
cause for magnetism, it is scarcely con-
ceivable that some large part of it would
not act in planes parallel to the geographi-
Cal equator, and, if so, its effects at any
one place would undergo very great
chang yes in the earth's diurnal revolution,
every part of the earth being presented,
in the course Of the day, in different as:
poets toward forces so acting Now the
fact is that the diur1al changes are very
small, only about 1850 part of the whole
horizontal force. 1t would seem certain,
therefore, that external bodies or spaces
do not produce any sensible part of the
magnetism in the planes to which the
earth's axis is normal. ~
That terrestrial magnetism does not re.
side, in any important degree, in the
earth's surface, is probable, because of the
non-magnetic property of the materials OF
which the earth's surface is composed, and
upon the general absence of any percepti.
BILE change in magnetism depending on
the change of soil. .
Humboldt adopted the idea that the