The oldest known glass is more than
4,000 years old. Ancient Egyptians
made glass beads and jars in about 2500
BC. Glassblowing probably began in
Syria more than 2,000 years ago.
Glassmaking became important in
Europe after the 1100s. Many European
churches were decorated with stainedglass
windows. In stained-glass windows,
strips of lead hold pieces of colored glass
together.
People in the American colonies started
making glass in 1608. In 1825 a company
in Pittsburgh, Pennsylvania,
invented a glass-pressing machine. The
machine helped to make glass in huge
amounts.
In the 1900s glass became easier to
make, less expensive, and stronger. Glass
windows and containers became everyday
features of most homes.
#More to explore
Fiber Optics • Lens • Mirror • Sand
Glider
A glider is an aircraft that soars through
the air without the help of a motor.
Some gliders look very much like airplanes.
They have wings, a tail, a cockpit,
an instrument panel, and a fuselage,
or body. Some are even called sailplanes.
How GlidersWork
Gliders and airplanes with engines work
in a similar way. A pilot controls both
kinds of aircraft. Unlike airplanes with
engines, however, gliders must be towed,
or pulled, into the air to begin their
flight. People may tow gliders using an
automobile or a winch, which is a reel
attached to a motor. A rope called a tow-
Glassblowing was invented more than
2,000 years ago. Today most people who
blow glass are artists.
Sailplanes are the most advanced type of
glider. They can soar for hours.
BRITANNICA STUDENT ENCYCLOPEDIA Glider 119
line connects the glider to the automobile
or winch. The automobile or winch
pulls the glider forward along the
ground and into the wind. As the glider’s
speed increases, air flows over its
wings at a faster and faster rate. This
produces an upward force, called lift, on
the wings. When this force becomes
greater than the downward force of gravity,
the glider can stay in the air. Then
the glider pilot releases the towline. An
airplane with an engine may also tow a
glider into the air.
Once the glider is flying, its pilot uses
controls to move it higher, lower, and to
the right or left. Glider pilots search for
upward-moving air currents, called
updrafts. The updrafts push the glider
higher into the sky.
History
Gliders were developed in the late 1800s
in Germany. By experimenting with
gliders theWright brothers were able to
develop the first airplane in the early
1900s. Gliders were later used in warfare.
For example, gliders carried many
soldiers into enemy lands duringWorld
War II. Today gliders are used mostly for
sport.
#More to explore
Airplane
GlobalWarming
The average surface temperature on
Earth is slowly increasing. This trend is
known as global warming.
Greenhouse Effect
To understand global warming, it helps
to understand the greenhouse effect. A
greenhouse is a glass house where plants
grow. Glass lets light in and keeps heat
from escaping. This trapped heat keeps
the plants warm even when it is cold
outside.
Likewise, Earth’s atmosphere traps
energy from the sun. Carbon dioxide
and other gases—called greenhouse
gases—in the air do this trapping.Without
these gases too much heat would go
back into space, and living things could
not survive. However, as more greenhouse
gases get into the air, they also
trap more heat. This leads to global
warming.
Causes
For much of Earth’s history, greenhouse
gases were not a problem. This situation
changed as people came to depend on
fossil fuels (oil, gas, and coal). People
burn fossil fuels to power factories, run
cars, produce electricity, and heat
A glider’s
wings are long
and narrow.
The shape
helps the
glider soar
through the
air.
Factories that burn fossil fuels help to cause
global warming.
120 Global Warming BRITANNICA STUDENT ENCYCLOPEDIA
houses. As fossil fuels burn, they release
carbon dioxide into the atmosphere.
Also, people have cut down many forests.
Trees use carbon dioxide when they
make their own food. Fewer trees mean
that less carbon dioxide is being taken
out of the atmosphere.
Reasons for Concern
Scientists cannot tell how warm Earth
may get over time. Some guess an
increase between 2.5° F and 10.4° F
(1.4° C and 5.8° C) by the year 2100.
The warmer weather could harm living
things. It also could cause polar ice caps
to melt. This would cause sea levels to
rise. Plants, animals, and buildings along
coastlines would be in danger.
Stopping GlobalWarming
Global warming is a worldwide concern.
Governments are trying to find ways to
limit the amount of greenhouse gases
put into the air. Individual people can
help by driving less. They also can save
energy by turning off unneeded lights
and other electrical devices.
#More to explore
Atmosphere • Fossil Fuel • Greenhouse
Effect
Globe
#see Map and Globe.
Goat
Goats are horned mammals that can be
either wild or tame. People keep goats
for their meat, milk, and wool. They are
hardy animals that can live on coarse,
thin grass. Goats can be raised on land
that does not have enough plants for
cows or sheep.
Goats are closely related to sheep, but
they are not as solidly built. A domestic
