14

 

PHYSICAL GEOGRAPHY.

 

 

miles; its polar diameter, or the length of its
axis, is 7899 miles. The circumference is 24,899
miles. The entire surface is equal to nearly
197,000,000 square miles.

The specific gravity of the earth is about 53; that is, the
average weight of all its matter is five and two-third
times heavier than an equal volume of water.

16. Imaginary Circles—In order to locate
places on the earth, as well as to represent por-
tions of its surface on maps, we imagine the earth
to be encircled by a number of curved lines
called great and small circles.

A great circle is one which would be formed
on the earth’s surface by a plane passing through
the earth’s centre, hence dividing it into two
equal parts. All great circles, therefore, divide
the earth into hemispheres.

The formation of a great circle on a sphere by cutting
it into two equal parts is shown in Fig. 7.

 
  

 

The shortest distance between any two places on the
earth is along the arc of a great circle.

All planes passing through the earth’s centre form ap-
proximately great circles on its surface.

A small circle is one formed by a plane which

does not cut the earth into two equal parts.

The formation of a small circle by cutting a sphere into
unequal parts is shown in Fig. 8.

 

Fig, 8. Small Circle.

The great circles employed most frequently in
geography are the equator and the meridian
circles. The small circles are the parallels,

 

 

——,

If we divide the circumference of any circle, whether
great or small, into three hundred and sixty equal parts,
each part is called a degree. The one-sixtieth part of a
degree is a minute; the one-sixtieth part of a minute is a
second. These divisions are represented as follows: 34°,
12!, 38’°; which reads, thirty-four degrees twelve minutes
and thirty-eight seconds.

The Equator is that great circle of the earth
which is equidistant from the poles.

Meridian Circles are great circles of the earth
which pass through both poles.

The Meridian of any given place is that half
of the meridian circle which passes through that
place and both poles. A meridian of any place
reaches from that place to both poles, and there-
fore is equal to one-half of a great circle, and,
with the meridian directly opposite to it, forms
a great circle called a meridian circle. There
are as many meridian circles as there are places
on the equator or on any parallel.

In large cities the meridian is generally assumed to pass
through the principal observatory.

 

 

 

 

 

 

 

 

 

Fig. 9, Meridians and Parallels,

Parallels are small circles which pass around
the earth parallel to the equator.

The meridians extend due north and@ south, and are
everywhere of the same length; the parallels extend due
east and west, and decrease in length as they approach the
poles.

The Tropics are parallels which lie 23° 27’
porth and south of the equator: the northern
tropic is called the Tropic of Cancer, the south-
ern tropic is called the Tropic of Capricorn.

The Polar Circles are parallels which lie 23°
27’ from each pole. The circle in the Northern
Hemisphere is called the Arctic Circle; that in
the Southern Hemisphere, the Antarctic Cirele.

17. Latitude is distance north or south from

"the equator toward the poles, measured ‘along

the meridians. It is reckoned in degrees. .

The meridian circles are divided into nearly
equal parts by the parallels, and it is the number
of these parts that occur on the meridian of any
place between it and the equator which deter-