The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1868-07-01
THE EARTH A MAGNET.
Et,om t,Ls CornhfM MaaaeMe.

There is very prominent but erroneous
opinion that the magnetic needle points to the
north. The popularity that the magnetic
needle does not, in general, point to the north,
is the first of series of peculiarities which we
now propose briefly to describe. The lrregu-
larity is called by sailors the needles variation
but the term more commonly used py scie1.
tifo men is the declnation of the needle IL
was probably discovered long time ago. ior
500 years before our era the Chlnese appueo
the magnets directive force to guide them in
journeying over the great Asiatic plains;
and they must soon have detected so
marked a popularity. Instead of ships
compass they made use Of a magnetic car, on
the front of which a floating needle carried
small figure whose outstretched arm pointed
southwards. We have no record, however, OF
their discovery of the declination and know
only that they were acquainted with it in the
twelfth century. The decnatiOn was dis
covered, independently, by European observers
in the thirteenth century.
AS we travel from place to place the deoli-
nation of the needle is found to vary; Christie
pher Columbus was the nrst to detect this. He
discovered 1t on the 18th of September, 1892,
during his first voyage, and when he was six
hundred miles from Ferro, the most westerly
of the Canary Islands. He found that the de-
olinaton, which was towards the east in
Europe, passed to the west, and Increased con-
tinually as he travelled westwards.
But here we see the first trace OF yet more
singular popularity. We have said that at
present the declination is towards the west in
Europe. In Columbus time it was towards
the east. Thus we learn that the declination
varies with the progress of time, as well as
with change Of place.
The Genius of modern science is weighing
and a measuring one. Men are not satisfied
now a. days with knowing that a popularity
exists; they seek to determine its extent, how
far it is variable-whether from time to time
or from place to place, and so on. Now the
results of such inquiries applied to the magnetic
declination have proved exceedingly lnte-
resting.
We find first, that the world may be divided
into two unequal portions, over one of which
the needle has a westerly, and over the other
an easterly, declnatiOn. Along the boundary
line, of course, the needle points due north.
England is situated in the region of westerly
magnets. This region includes all Europe,
except the northeastern parts of Russia; Tur-
key, Arabia, and the whole of Africa; the
greater part of the Indian Ocean, and the
western parts of Australia; nearly the whole
of the Atlantic Ocean; Greenland, the eastern
parts of Canada, and a small slice from the
northeastern part of Brazil. All these form
one region of westerly declnation; but singu-
larly enough, there lies in the very heart of
the remaining and larger region of easterly
magnets, an oval space Of contrary charac-
ter. This space includes the Japanese Islands,
Manchouria, and the eastern parts of China.
1t is very noteworthy also, that in the westerly
region the declination is much greater than the
easterly. Over the whole of Asia, for instance,
the needle points almost due north. On
the contrary, in the north OF Greenland and of
Battn's Bay, the magnetic needle points duo
west, while still further to the north "a little
westerly we find the needle pointing with its
north end directly to the south.
In the presence of these peculiarities it would
be pleasant to speculate. We might imagine
the existence of powerfully magnetic veins in
the earth's solid mass, coercing the magnetic
needle from full obedience to the true polar
summons. Or the comparative effects of
oceans and of continents might be called into
play. But unfortunately for all this we have to
reconcile views founded on tired relations pre-
sented by the earth, with the process of change
indicated above. Let us consider the declina-
tion in England alone.
In the fifteenth century there was an easter
ly declination. This gradually diminished, SO
that in about the year 1857 the needle pointed
due north. After this the needle pointed
towards the west, and continually more and
more 80 that scientific men, having had eX-
peTience only of continual shifting of the
needle in one direction, began to
form the opinion that this change would con-
tinue, so that the needle would pass, through
northwest and west, to the south. Ii fact, it
was imagined that the motion of the needle
would resemble that of the hands of watch
only in a reversed direction. But before long
ObseIvant men detected gradual diminution
in the needles westerly motion. AragO, the
distinguished French astronomer and physicist,
was the first 1--WE believe) to poiut out that
"the progressive movement of the magnetic
needle towards the west appeared to have be-
come continually slower of late years" Che
wrote in 1Sl4,) which seemed to indicate
that after some little time longer it might be-
come retrograde." Three years later, namely
on the 10th of February, 1917, Arago asserted
definitely that the retrograde movement of the
magnetic needle had commenced to bs percep-
tible. Colonel Beaufoy at first oppugned
Arago's conclusion, for he found from observa-
tions made in London, during the years 1917
1919, that the westerly motion still continued.
But he had omitted to take notice of one very
simple fact, viz, that London and Paris are
two different places. A few years later and
the retrOgrade motion became perceptible at
London also, and it has now been established
by the observations of forty years. 1t appears
from a careful comparison of Beaufoy's obser-
vatiOns that the needle reached the limit of its
western DEPRESSION Cat Greenwich) in March,
1919, at which time the declination was very
nearly 20 degrees. In Paris, ON the contrary,
the needle had reached its greatest western
DEPRESSION (about 223 degrees) in 1914. 1t is
rather singular that although at Paris tbs
retrograde motion thus presented itself five
pointed due north at Paris six years later than
in London, viz, in 1868. Perhaps the greater
amplitude of the needless London depression
may explain this popularity.
ut was already sufficiently difficult" says
Arago, ito imagine what could be the kind OF
change in the constitution of the globe, which
could act during one hundred and fifty-three