processed foods, and clothing.
A lawyer named John Neely Bryan
founded Dallas in 1841. The town grew
as a center of trade. In the early 1900s
one of the world’s largest cotton markets
was in Dallas.
In 1930 a huge oil field was discovered
in eastern Texas. Dallas grew large and
wealthy as a center of the oil industry.
During the middle of the 1900s many
factories in the city made aircraft for the
military.
Dallas’ oil industry declined in the
1980s. However, the city continued to
grow. Many high-technology companies
The Dalai
Lama is the
head of the
group known
in English as
the Yellow Hat
order of Buddhists.
They
are called that
because they
wear a yellow
headdress.
8 Dallas BRITANNICA STUDENT ENCYCLOPEDIA
established themselves in Dallas in the
late 20th and early 21st centuries.
#More to explore
Houston • Texas
Dam
A dam is a barrier that blocks flowing
water or sends it in a new direction.
People and animals build dams to control
rivers and to create ponds or lakes.
A beaver’s dam works in much the same
way as the giant dams that people build.
Both must be high enough and strong
enough to keep the water from flowing
forward.
People throughout the world have built
dams since ancient times. Early Egyptians
built a dam almost 5,000 years ago.
The Chinese built stone structures to
block rivers as early as 240 BC. Two
dams in Spain that were built by ancient
Romans are still in use today.
Uses
People build dams to keep rivers from
flooding and to save water. During
heavy rains a river’s waters may rise too
high.Without a dam to contain it, the
extra water is wasted as it flows away
unused.
With a dam in place, a deep reservoir, or
pool, of water builds up behind the
dam. This extra water can be put to
many good uses. It might be used as a
lake for boating or swimming, for drinking
water, for farm irrigation, or in
manufacturing.
The force created by water rushing
through pipes in a dam is also useful.
The flowing water makes wheels called
turbines spin. The turbines run
machines called generators, which produce
electricity. This type of power is
known as hydroelectric power.
How DamsWork
Most dams operate in the same basic
way. The weight of the water pushing
against a dam creates tremendous pressure.
A dam’s shape and thickness must
work together to hold the water back,
even when rainfall increases the pressure.
The dam wall must be made of something
strong enough to keep water from
breaking through. Some dams are made
of earth and rock packed together to
form a thick barrier. However, most
dams are made of concrete. Some concrete
dams are thicker at the bottom
than they are at the top. They are built
The Hoover Dam is 726 feet (221 meters)
high. It is on the Colorado River between
Nevada and Arizona.
BRITANNICA STUDENT ENCYCLOPEDIA Dam 9
that way because water creates more
pressure as it gets deeper. The bottom of
the dam must be thicker and stronger to
hold back the deeper water. Other concrete
dams have a curved shape that
helps to lessen the pressure of the water.
The curve sends some of the pressure to
the rock walls on the sides of the dam.
A spillway is a passage over or around
the dam wall. If the water behind the
dam builds up to a dangerous level,
some of the water is released through the
spillway.
Passages called sluices are other important
parts of a dam. Some water constantly
moves through these passages to
keep the river flowing on the other side
of the dam. Gates in the passages allow
workers to control the amount of water
that flows through the dam.
#More to explore
Beaver • Electricity • Flood • Irrigation
Damascus
Population
(2004 estimate)
1,614,500
Damascus is the capital of Syria, a country
in the Middle East. It is one of the
world’s oldest cities. For thousands of
years Damascus has been an important
trading center. Today it is also Syria’s
center of education and culture.
Damascus is surrounded by desert.
However, the city gets plenty of water
from the Barada River. The only easy
route through the Anti-Lebanon Mountains
ends in Damascus.
Many people in Damascus work for the
government. Others work in education,
health care, tourism, and other businesses.
Factories in the city make processed
foods, cloth, chemicals, plastics,
and machinery.
Damascus is at least 5,000 years old. In
ancient times many groups fought to
control the city and its trade.
Muslim Arabs took over Damascus in
AD 635. It was the capital of the Muslim
world from 661 to 750. Damascus was
later an important city in a series of
Muslim empires.
People walk through the entrance
of a mosque in Damascus.
10 Damascus BRITANNICA STUDENT ENCYCLOPEDIA
The Turks ruled Damascus from the
1500s to the early 1900s. It remained a
major trade center. People traveling to
the Muslim holy cities also bought supplies
in Damascus.
France took control of Syria in 1920.
Damascus underwent many changes to
become more modern. In 1946 Syria
became an independent country with
Damascus as its capital. The city grew
greatly during the 20th century.
#More to explore
Syria
Dance
Dance involves the rhythmic movement
of the human body, usually to music. It
is one of the oldest forms of human
