when warm, moist air passes over cool
surfaces, the air cools down. The water
vapor in the air begins to condense, or
form into small drops. The drops cling
to cool surfaces such as leaves, flower
petals, and grass. The temperature at
which this condensation begins to happen
is known as the dew point.
Dew usually forms on clear nights when
the air is calm. Dew can form on any
surface that is cool and exposed to the
sky. It often forms on the ground and on
plants because these things usually cool
faster than the air. For the same reason,
dew often collects on the tops of cars.
The time of night when dew begins to
form is called dewfall.
Dew can help plants if they are dry. But
damaging fungi may form on plants if
dew makes them too wet. Dew that contains
pollution is another danger. Pollution
can get into dew when the water
vapor in polluted air condenses. Like
acid rain, this polluted dew, called acid
dew, can harm living things.
..More to explore
Rain •Water
Dhaka
Population
(2006
estimate), city,
6,479,751;
urban area,
11,813,728
Dhaka is the capital of Bangladesh, a
country of southern Asia. The city lies
on the Buriganga River. It is the largest
city in Bangladesh. Dhaka is a center of
industry, education, and culture.
The old section of Dhaka has many historic
buildings. The city has several hundred
mosques. The Star Mosque was
built in the 1700s. It is decorated with
twinkly tiles set in star shapes. Lalh Bagh
is an unfinished fort from the 1600s. It
includes a mosque, a tomb, and a
museum.
Many people in Dhaka work for the
government. Banking, education, health
Dew often forms on grass during cool nights. care, and construction also bring money
42 Dew BRITANNICA STUDENT ENCYCLOPEDIA
to the city. Factories in Dhaka make
fibers and fine cloth. Other factories
make rubber, jewelry, furniture, and
machinery.
People have lived in Dhaka since at least
the 900s. In the 1600s the Mughal
emperors of India took over the area.
They made Dhaka the capital of a province
in India. The city grew wealthy as a
center of sea trade. Many palaces and
gardens were built. In 1704 the Mughals
moved their capital to another city. This
hurt Dhaka’s economy. Great Britain
took control of the city in 1765.
Dhaka became the capital of a province
of Pakistan in 1947. In 1971 the province
became the independent country of
Bangladesh. Dhaka was made the new
country’s capital.
#More to explore
Bangladesh • Mughal Empire
Diabetes
Two different diseases are called diabetes.
Diabetes mellitus happens when
there is too much glucose in the body.
Glucose is a sugar that the body gets
from food and uses for energy. Diabetes
insipidus is a rare disease that affects the
kidneys.
Types of Diabetes
There are two major types of diabetes
mellitus. In type I the body stops making
insulin or does not make enough.
Insulin is a hormone, or chemical, that
normally controls the level of glucose in
the blood. This type of diabetes often
happens in children and teenagers. In
type II the body makes insulin but cannot
use it properly. Type II usually
occurs in adults. It is the most common
type of diabetes.
Diabetes insipidus is linked to a hormone
called vasopressin. When the body
Dhaka, Bangladesh, is one of
the world’s fastest growing
urban centers.
A woman with diabetes gives
herself a shot of insulin.
BRITANNICA STUDENT ENCYCLOPEDIA Diabetes 43
does not make vasopressin or cannot use
it, the kidneys produce too much urine.
Symptoms
Diabetes mellitus and diabetes insipidus
have similar symptoms. People with
either disease are often very thirsty. They
also urinate a lot. The loss of water
through urination can cause dry skin
and tiredness. Untreated diabetes mellitus
can lead to heart disease, kidney
problems, blindness, the loss of feet or
legs, and death. Untreated diabetes
insipidus can lead to low blood pressure
and shock.
Causes
Doctors do not know the exact cause of
diabetes mellitus. People who are overweight
and do not exercise seem to have
an increased risk of developing the disease.
Diabetes insipidus may be caused
by brain damage, kidney damage, or
certain drugs.
Treatment
People with type I diabetes mellitus
must have daily shots of insulin. People
with type II can usually control the disease
with a healthy diet and weight loss.
In some cases, however, medication is
needed. Doctors treat diabetes insipidus
with medications.
#More to explore
Hormone • Kidney
Diana
#see Artemis.
Dias, Bartolomeu
Bartolomeu Dias of Portugal was one of
the most important European explorers
before Christopher Columbus. He led
the first European expedition around the
southern tip of Africa. This opened the
way for sea trade between Europe and
Asia.
Bartolomeu Dias (also spelled Bartholomew
Diaz) was born in about 1450
near Lisbon, Portugal. In August 1487
Dias took three ships in search of the
southern tip of Africa. In January 1488
he passed the tip, but he did not see it. It
was very stormy at the time. After he
turned around and reached the southern
coast of Africa he realized what had happened.
Dias wanted to go on to India, but his
crew would go no farther. Upon rounding
the tip of Africa again, Dias named
