Harold Michell.

An introduction to the geography of Sierra Leone online

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516
M582i
1918





THE LIBRARY

OF

THE UNIVERSITY

OF CALIFORNIA
LOS ANGELES



AN INTRODUCTIOiN



O THE



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10 Longitude West Loneitude East 10



20



These lines or parallels are of the utmost assistance
in denoting the position of a place referred to. They
are numbered from the equator in degrees, north and
south. Thus the equator is degrees, the North Pole is
90 deg. North, the South Pole is 90 deg. S., Freetown is
8 J deg. N (approximately) and so on.



10 All Intrudnction to the

The vertical lines on the map are called " Lines of
Longitude.' These longitudinal lines represent dis-
tances East or West of some given line, usually that
which passes through London (Greenwich). Thus the
longitude of London is degrees, New York is 74 deg.
West Longitude, Cairo is 31J deg. East Long., Free-
town is 13 deg. W^est Long.

By n^eans of these two sets of lines it is possible to fix
the position of any place on a map ; and in the same way
if the latitude and longitude of a place are known it is
easy to find its position on a map.

The captain of a ship on the wide ocean knows his
position by means of these lines. He has instruments
which enable him to determine his latitude and longi-
tude, and then by reference to his large map or chart he
can locate the position of his ship and shape his course
accordingly.

A knowledge of the latitude of a place will within
certain limits act as a guide in inquiring into the
climate experienced.

Later on it will be seen that there are other factors
which play an important part in determining the
climatic conditions, but as the distance north or south
of the equator is of prime importance it will be well to
consider how this comes about.

Climate. — The climate of a country usually means the
average weather conditions w^hich prevail. The
weather is an absorbing topic of interest ; so m.uch
depends on it that people are accustomed when meeting
to comment on the weather.

The weather is really the condition of the atmos-
phere with respect to temperature, air pressure, wind
and rainfall.

Of these conditions the one chiefly affected by lati-
tude is temperature, and a brief consideration of tem-
perature and how it is measured may be of assistance.

Temperature is a measure of heat. The terms, hot
and cold, are really relative. An ordinary fire made by
burning a few sticks is considered hot, yet it is possible
to have much hotter fires such as the blast furnace. Ice



Geogra'phy of Sierra Leone.



17



is said to be cold, yet there are liquids so much colder
that they will boil when poured on ice.

For ordinary purposes the usual standard is the heat
of the body. Things which are hotter than the body
are said to be hot, those which are colder are considered
cold.

For more accurate comparisons a much more careful
measure of heat is necessary, and the instrument used to
register the temperature is called a Thermometer.



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There are various kinds of thermometers, but the one
most commonly used consists of a glass tube with a very
fine bore. One end of the tube is blown into a small
bulb and this bulb with a part of the stem is filled with
a liquid, usually mercury. After expelling the air in
the remainder of the stem the other end of the tube is
sealed up.

Its action depends on the fact that liquids, like solids
and gases, expand when heated and contract when
cooled. There are certain modifications of this state-
ment, howevei", which need not be considered here. The
stem of the thermometer is then carefully graduated ;



18



An Introduction to the



aud the rise and lull of the mercury in the stem depend
on the beating and cooling of the mercury with its
consequent expansion and contraction.

The graduations on thermometers largely in use in
England and the Colonies are those of Fahrenheit.

Some relative temperatures may be helpful in tbi&
connection, for example : — Water when freezing or
changing into ice is at a temperature of 32 deg. F., when
boiling under normal conditions it is at a temperature
of 212 deg. F. ; the temperature of the body or blood
under healthy conditions is approximately 98 deg. F.

Thus if the temperature of the atmosphere rises much
above the blood temperature there is a sensation of hot-
ness, whilst if the temperature of the surrounding air
falls much below that of the body there is a feeling of
coldness. Consequently people living in cold countries
require more and warmer clothing than do people living
in warmer climes.

The earth receives its heat from the Sun, but because
of the shape of the earth this heat is not equally
distributed.

A glance at the diagram will explain how this
happens. Where the sun is overhead the rays of heat
are concentrated, elsewhere these rays have to be spread
over a much larq-pr area.




S U N'S RAYS



The straight lines represent a portion of the rays
(which travel in straight lines), and the circle repre-
sents the earth. The concentrated rays impinging on



Geography of Sierra Leone. 19

an area indicated by A, B, are spread over a much
larger area indicated by a, b, where the rays fall
obliquely.

During the whole of its annual journey round the
sun the only parts of the earth which come under the
vertical rays of the sun are those within certain limits
of latitude. There is no need to enter into the causes
which determine these limits now; it will be sufficient to
know that all places in latitudes between 23J deg.
north and 23 J deg. south are subject to the vertical rays
of the sun twice in each year.


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Online LibraryHarold MichellAn introduction to the geography of Sierra Leone → online text (page 1 of 10)