The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1868-07-01

globes circuit in most singular
manner since 1000. They have varied alike in
direction and complexity. The Siberian oval |
now distinct, was, in 1857, merely loop OF |
the eastern line of no deolination. The oval |
appears now to be continually diminishing,
and will one day probably disappear.
We find here presented to us phenomenon
as mysterious, as astonishing, and as worthy
of careful study as any embraced in the wide
domains of science. But other peculiarities
await our notice.
II magnetic needle of suitable length be
carefully poised on a fne point, or better, be
suspended from a silk thread without torsion,
il will be found to exhibit each day two small
but clearly perceptible oscillations. M. Arago,
from a careful series of observations, deduced
the following results:-
At about eleven at night, the north end OF
the needle begins to move from west to east,
and having reached its greatest easterly excur-
sion at about quarter past eight in the morn
ing, returns towards the west to attain its
greatest westerly excursion at quarter past
one. 1t then moves again to the east, ana
having reached its greatest easterly excursion
at haif-past eight in the evening, returns to
the west, and attains its greatest westerly ex-
cursion at eleven, as at starting.
OF course, these excursions take place on
either side of the main position of the needle,
and as the excursions are small, never exceed
ing the fifth part of a degree, while the mean
the west of north, it. is clear that the ex-
cursions are only nominally eastern and
western, the needle pointing, throughout, far
to the west.
Now if we remember that the north end of
the needle is that farthest from the sun, it will
be easy to trace In NJ AragO's results sort OF
effort on the part of the needle to turn towards
the sun-not merely when that luminary is
above the horizon, but during his nocturnal
! path also.
We are prepared, therefore, to expect that
a variation having an annual period shall ap-
pear, on close observation of our suspended
| needle. Such a variation has been long since
recognized. 1t is found that in the summer
of both hemispheres, the daily variation is
exaggerated, while in winter it is diminished.
But besides the divergence of magnetized
needle from the north pole, there is a diver
gence from the horizontal position, which
must now claim our attention. II a non
magnetic needle be carefully suspended
! so as TO rest horizontally, and be then
magnetized, it will be found no longer to
! preserve that position. The northern end
! dips very sensibly. This happens in our
hemisphere. In the southern it is the south
ern end which dips. 1t is clear, therefore,
that if we travel from one hemisphere to the
other we must find the northern dip of the
needle gradually diminishing till at some
point near the equator the needle is horizontal,
and as we pass thence to southern regions
gradually increasing southern inclination is
presented. This has been found to be the
case, and the position of the line along
which there is no inclination (called the
magnetic e9uatOr) has been traced around
the globe. 1t is not concident with the earth
equator, but crosses that circle at an angle of
twelve degrees, passing from north to south of
! the equator in logitude three degrees west of
Greenwich, and from south to north in longi-
| tude 1S7 degrees east of Greenwich. The
form of the line is not exactly that of great
circle, but presents here and there (and esps-
! cially where it crosses the Atlantic) percep-
! tible excursions from such figure.
At two points on the earth's globe the
needle will rest in a vertical position. These
are the magnetic poles of the earth. The
northern magnetic pole was reached by Sir J.G.
Ross, and lies in seventy degrees north lat-
tude, and 263 degrees east longitude, that is,
! to the north of the American continent and
not very far from Boothia Gulf One of the
! objects with which Ross set out on his cele-
brated expedition TO the Antarctic Seas was the
discovery if possible of the southern magnetic
pole. In this he was not successful. Twice he was
in hopes of attaining his object, but each time
he was stopped by barrier OF land. He ap-
preached so near however, to the pole that
the needle was inclined at an angle of nearly
ninety degrees to the horizon, and he was able
to assign to the southern pole position in rs
degrees south latitude l54 degrees east longi-
either pole is fixed, since we shall now see that
the inclination, like the declination of the mag
netic needle, is variable from time to time, as
well as from place to place; and in particular,
the magnetic equator is apparently subjected
to a slow but uniform process of change.
ATago tells us that the inclination of the
needle at Paris has been observed to diminish
year by year since 1911. At that time the in
clinaton was no less than 75 degrees in other
words, the needle was inclined only Is degrees
to the vertical. In 179l the inclination was
less than II degrees. In 1881 it was less than
6S degrees. In like manner the inclination at
London has been observed to diminish, from
72 degrees in 1856 to 70 degrees in 1904, and
thence to 6S degrees at the present time.
1t might be anticipated from such changes
as these that the position of the magnetic
equator would be found to be changing. Nay,
we can even guess in which way it must be
changing. For, since the inclination is di-
minishing at London and Paris, the magnetic
equator must be approaching these places,
and this Gin the present position of the curve0
can only happen by gradual shifting of the
magnetic equator from east to west along the
true equator. This motion has been found to
be really taking place. 1t is supposed that
the movement is accompanied by change of
form; but more observations are necessary to
establish this interesting point.
Can it be doubted that while these changes
are taking place, the magnetic poles also are
slowly shifting round the true pole? Must
not the northern pole, for instance, be further
from Paris now that the needle is inclined
more than % degrees from the vertical, than
in 1871, when the inclination was only Is
degrees. 1t appears obvious that this must
be so, and we deduce the interesting concu-
sion that each of the mapnatic noles ia rofatino,