114 PHYSICAL GEOGRAPHY.

 

clination chart, the figures near the lines giving the value
of the declination in degrees. The agone in each hemi-
sphere is marked 0. In the New World it enters South
America near Rio Janeiro, curves to the eastward around
the Antilles, passes near Washington, through the western
part of Hudson Bay, and enters the magnetic pole at
Boothia Felix. The agone, in the Old World, passes
through the west of Australia, near the western coasts
of Hindostan, through Persia, the eastern part of the Cas-
pian Sea, and through the White Sea, in Europe. The
eval curves in Eastern Asia seem to indicate a secondary
magnetic pole.

In nearly all Europe, in the whole of Africa and Ayvabia,
in eastern North and South America, and in nearly all the
Atlantic and Indian Oceans, the declination is west. It is
also west along part of the eastern shores of Asia, around
the secondary magnetic pole. In the remainder of the
world the declination is east.

303. The Inclination or Dip of the Needle—
The lines of force of the earth’s magnetic field
are in most places inclined to the earth’s surface.
The position of the needle is, therefore, horizontal
in but a few localities. In most places, one of the
poles is inclined to the earth. This is called the
inclination or dip of the needle. In the Northern
Hemisphere, it is the north pole, and in the south-
ern, the south pole that is inclined.

304. Magnetic Equator—The angle of dip is
greater, the nearer we approach either magnetic
pole. At the pole, the needle points vertically
downward; midway between the poles, the needle
is horizontal; the last position is called the mag-
netic equator.

Lines connecting places which have the same angle of
dip are called isoclinal lines. They correspond in a very
remarkable manner with the isothermal lines. This seems
to show the dependence of the intensity of magnetism on
the distribution of the sun’s heat. The inclination is also
subject to secular changes, like the declination.

~ 805. Optical Phenomena are caused by changes
in the direction, intensity, or composition of sun-
light during its passage through the atmosphere.

Sunlight, when passed through a prism, is dis-
persed or separated into a great number of differ-
ent colored lights. The following seven groups of
eolors are prominent: violet, indigo, blue, green,
yellow, orange, and red. These are called the pris-
matic colors, or, collectively, a spectrum. They
differ in the ease with which they are refracted,
or turned out of their course, in passing from
one medium to another of different density. The
above prismatic colors seen in the spectrum are
named in the order of their refrangibility, begin-
ning with the violet, the most refrangible, and
ending with the red, the least refrangible.

306. Rainbows are arches of the prismatic
colors, caused by the dispersion of the light

 

 

during its passage through the falling drops of
rain. The rays entering the drop, are reflected
from the surfaces farthest from the sun, and
emerge separated into the prismatic colors.

Rainbows are seen when the observer stands
with his back toward the sun. They are largest
when the sun is nearly setting.

A secondary bow sometimes occurs outside the
primary, with the order of its colors reversed.
Tt is caused by the light which is twice reflected
from the back of the drops.

307. The Sunset Tints of the Sky are yellow,
orange, and red. The rays of the setting sun are
dispersed, during their passage through the clouds,
or through accumulations of vapor at the horizon,
and only the colors that are least turned out of
their course, the ‘yellow, the orange, and the red,
pass through and light up the western sky.

308, The Blue Color of the Sky is caused by
the diffusion through the air and their subsequent
reflection from its particles of the more refrangi-
ble rays of light: the indigo and the blue.

309. Halos and Corone are rings of prismatic
colors surrounding the sun and moon.

Halos are caused by the presence in the air
of small crystals of ice or snow. Parhelia, or
mock suns, and Paraselene, or mock moons
(bright spots which somewhat resemble suns and
moons), are frequently seen where the complicated
circles of halos intersect each other. Corone are
circles of light, seen most frequently around the
moon. They are caused by the presence of a
small quantity of condensed vapor in the air.
They generally indicate changes in the weather.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 103, Halo,

310. The Mirage is a general term applied to
the appearance which objects present when viewed
by means of rays of light that have passed through