A clock is a device used to tell time.
Moving hands on the face of a clock
point to the current hour, minute, and
second. A clock can be big enough to be
in a giant tower. A clock can also be
small enough to fit on a person’s wrist,
where it is called a watch. Many clocks
are made to be beautiful objects as well
as useful devices.
How ClocksWork
There are three main types of clocks:
mechanical, electrical, and atomic. All
three have the same basic parts. They
must have a source of power, a device to
keep the parts moving at regular intervals,
and a way to display the time.
Mechanical clocks get their power from
moving weights or springs. These parts
are attached to gears, or toothed wheels.
The gears are attached to the hands of
the clock. As the gears move they move
Hillary
Rodham Clinton
was the
first wife of a
sitting president
to be
elected to public
office.
The Houses of Parliament (a group of government
buildings in London) are famous
for their large clock tower. Big Ben is the
name of the bell inside the tower.
140 Clock BRITANNICA STUDENT ENCYCLOPEDIA
the hands. Another device attached to
the gears keeps them moving at a regular
pace. Mechanical clocks must be wound
up to work.
Electrical clocks get their power from
electricity instead of weights or springs.
The electricity can come from a battery
or from an electrical socket in a wall.
Some electrical clocks have hands, as
mechanical clocks do. Others have a
digital display, where the hours, minutes,
and seconds are shown as numbers.
Electrical clocks are the most common
type of clock used today.
Atomic clocks measure time using the
waves of energy given off by atoms—the
tiny bits of matter that make up the
universe. Atomic clocks normally have
digital displays. Because atomic clocks
are the most accurate type of clock, scientists
use them in their work.
History
In ancient times people measured time
by watching the position of the sun in
the sky. As the sun passed over a column
or other device it would create a shadow.
The length of the shadow would change
depending on where the sun was in the
sky. The length of a column’s shadow
therefore showed the basic time of day.
Eventually people wanted to tell time
more accurately. The first mechanical
clock was invented in the mid-1300s.
Clocks powered by coiled springs and
swinging weights were introduced in the
1500s. Clocks powered by electricity
appeared in the late 1800s and early
1900s. Atomic clocks were developed in
the 1950s.
#More to explore
Sundial • Time
Clothing
Many animals have fur, feathers, or
other natural coverings to protect them
from the weather. Humans have almost
no natural protection from the weather.
Even so, they live in some of the hottest
and coldest places on Earth. People also
have modesty, or the desire to cover
their body. In addition, each person has
a personality, a culture, and a place in
society to show to other people. For all
these reasons, people wear clothing.
Clothing Materials
Early humans made clothing from the
skins and fur of animals. When humans
Women wore long dresses and big hats in
about 1900.
BRITANNICA STUDENT ENCYCLOPEDIA Clothing 141
began to settle in villages and to grow
crops, they started making cloth from
natural fibers. Such fibers as silk and
wool came from animals. Cotton and
flax were common plant fibers.
Late in the 1800s a French chemist created
the first artificial fabric. He made
an artificial silk out of a plant fiber that
had been turned into liquid form. This
is now called rayon. In the 1930s scientists
made an even more useful material
from petroleum, or oil. It is called
nylon. Rayon, nylon, and other materials
made from chemicals may be stronger
and less likely to wrinkle or shrink
than natural cloth.
Clothing Styles
Clothing styles depend on the climate of
a region. People in a tropical rain forest
need little or no clothing. In the hot
deserts of the Middle East and North
Africa, people wear loose white robes for
protection against the sun. In the Arctic,
where it is very cold, the Inuit wear
warm furs and waterproof sealskin. In
many other regions the weather may be
hot or cold, depending on the season.
People in these regions must have both
light and heavy clothing.
People also wear clothing to tell others
about themselves. Uniforms show which
people are soldiers and police officers.
Women sometimes wear different colors
and styles than men do. Young people
may wear unusual clothes to rebel
against their parents. Rich people wear
expensive clothes made by famous
designers. Religion and culture affect
clothing styles as well. Despite these
differences, international business,
travel, movies, and television have
helped to spread similar styles around
the globe. Jeans, for example, are popular
almost everywhere.
#More to explore
Climate • Culture • Fibers
Cloud
On a sunny day a wispy cloud can be a
beautiful sight, but at other times a
cloud can be a sign of storms to come. A
Young people of the late 1980s and early
1990s wore flannel shirts and ripped jeans.
Wispy cirrus clouds drift high in the sky.
142 Cloud BRITANNICA STUDENT ENCYCLOPEDIA
cloud is made up of millions of tiny
