MATHEMATICAL GEOGRAPHY. 15

 

mines the value of its latitude. If we conceive
eighty-nine equidistant parallels drawn between
the equator and either pole, they will divide all
the meridians into ninety nearly equal parts; the
value of each of these parts will be one degree
of latitude. Therefore, if the parallel running
through a place is distant from the equator forty-
five of these parts, its latitude is 45°. If more
than eighty-nine parallels be drawn, the value
of each part will be less than one degree.
Places north of the equator are in north lati-
tude; those south of it are in south latitude.
Since the distance from the equator to the poles
is one-fourth of an entire circle, and there are

only 360° in any circle, 90° is the greatest value —

of latitude a place can have. Latitude 90° N.
therefore corresponds to the north pole.

To recapitulate: Latitude is measured on the |
meridians by the parallels. :

18. Longitude is distance east or west of any
given meridian.

Places on the equator have their longitude measured
along it; everywhere else longitude is measured along the
parallels.

The meridian from which longitude is reckoned
is called the Prime Meridian. Most nations take
the meridians of their own capitals for their prime
meridian. The English reckon from the me-
ridian which runs through the observatory at
Greenwich; the French from Paris. In the
United States we reckon from Washington.

Any prime meridian circle divides all the par-
allels into two equal parts. A place situated east
of the prime meridian is in east longitude; west
of it is in west longitude.

Since there are only 180° in half a circle, the greatest
value the longitude can have is 180°; for a place 181° east
of any meridian would not fall within the eastern half of
the parallel on which it is situated, but in the western
half; and its distance, computed from the prime meridian,
would be 179° west. . 3 ¢

It is the meridians that divide the parallels
into degrees; therefore longitude is measured on
the parallels by the meridians. —

19. Value of Degrees of Latitude and Longi-
tude——As latitude is distance measured on the
are of a meridian, the value of one degree must
be the 345th part of the circumference along that
meridian, since there are only 360° in all. This
makes the value of a single degree approximately
equal to 694 miles. Near the poles the flattening
of the earth causes the value of a degree slightly
to exceed that of one near the equator.

 

 

 

The value of a degree of longitude is subject
to great variation. It is equal to the g{oth part
of the earth’s circumference, provided the place
be situated on the equator; otherwise, it is the
gigth part of the parallel passing through the
place that is taken; and as the parallels decrease
in size as we approach the poles, the value of a
degree of longitude must likewise decrease as the
latitude increases, until at either pole the longi-
tude becomes equal to zero.

The value of a single degree of longitude on the equator,

or at lat. 0°, is equal to about 694 miles.
At latitude 45° it is equal to about 49 miles.

“ 60° “ “c 35 “
“ce 80° “ , “c 12 coe
“ 90° 73 “ 0 “

Geographical Mile.—The sztypth of the equatorial
circumference, or the one-siatieth of a degree of longitude
at the equator, is called a nautical or geographical mile.
The statute mile contains 1760 yards; the geographical or
nautical mile, 2028 yards. The nautical mile is sometimes
called a knat.

20. Map Projections—The term projection as
applied to map-drawing means the various methods
adopted for representing portions of the earth’s
surface on the plane of a sheet of paper.

The projections in most common use are Merca-
tor’s, the orthographic, the stereographic, and the
conical projections. Of these the stereographic is
best adapted to ordinary geographical maps, and
Mercator’s to physical maps. All -projections
must be regarded as but approximations.

1. The Orthographic Projection is that by which the
earth’s surface is represented as it would appear to an
observer viewing it from a great distance.

2. The Stereographic Projection is that by which the
earth’s surface is represented as it would appear to an
observer whose eye is directly on the surface, if he looked
through the earth as through a globe of clear glass, and
drew the details of the surface as they appeared projected
on a transparent sheet of paper stretched in front of his
eye across the middle of the earth. There may be an
almost infinite number of such projections, according to
the position of the observer. The two stereographic pro-

jections in most common use are the Equatorial and the
Polar.

Mercator’s Projection represents the earth on
a map in which all the parallels and meridians
are straight lines.

Mercator’s charts are drawn by conceiving the
earth to have the shape of a cylinder instead of
that of a sphere, and to be unrolled from this
cylinder so as to form a flat surface. The me-

ridians, instead of meeting in points at the north
and south poles, are drawn parallel to each other.
This makes them as far apart in the polar regions