with explosive material. This material is
usually a mixture of chemicals. A part
called a fuse ignites (sets on fire) the
explosive material. There are several
types of fuses. Some fuses cause the
bomb to blow up when it hits something.
Other fuses cause the bomb to
explode after a certain amount of time.
Bombs that fly through the air have fins
or wings to keep them steady. Some
bombs, called smart bombs, have a system
that allows people to guide them in
flight from far away.
Types of Bombs
Armies and other military forces use
several types of bombs. Demolition
bombs destroy buildings with a powerful
blast. Fragmentation bombs explode
into small, fast-moving metal fragments
that are meant to kill people. Generalpurpose
bombs combine demolition and
fragmentation. Armor-piercing bombs
are heavy and pointed. Armies use them
against protected targets. Incendiary
bombs cause intensely hot fires. Cluster
bombs contain many small bombs, or
bomblets.
Criminals and terrorists sometimes
make and use their own bombs. These
bombs may be simpler than the bombs
used by armies, but they can be very
destructive. They include car bombs,
pipe bombs, and letter bombs.
Nuclear bombs are the most powerful
bombs of all. There are two types of
nuclear bombs: atom and thermonuclear
(or hydrogen). They both get their tremendous
force from reactions that take
place in the nuclei, or centers, of atoms.
(Atoms are the tiny building blocks of
everything.) The energy released by a
nuclear bomb destroys nearly everything
for miles around.
History
Grenades were the earliest bombs. These
are small devices that are thrown by
hand. Soldiers first began using grenades
in about the 1400s. Eventually other,
more accurate weapons were developed.
But grenades are still used today.
A U.S. Air Force plane drops bombs during
an attack on Afghanistan in 2001.
104 Bomb BRITANNICA STUDENT ENCYCLOPEDIA
After airplanes were invented in the
early 1900s armies started using them
to drop bombs. During World War I
(1914–18) bomber planes could not
carry many bombs at one time. By
World War II (1939–45), however, that
changed. Airplanes were able to carry
larger bombs in much greater numbers.
The first nuclear bombs were developed
during World War II. The war ended
shortly after the United States dropped
atom bombs on two Japanese cities,
Hiroshima and Nagasaki, in August
1945. The bombs caused immense
devastation. They killed more than
100,000 people. The United States
tested the first hydrogen bomb in
November 1952. It was even more
powerful than the atom bomb. Several
other countries later developed nuclear
bombs of their own.
In the 1980s and 1990s many people
were afraid that other countries would
use nuclear weapons. Leaders of the
countries that had such weapons began
to talk about ways to control the
weapons. They agreed to limit the
production, testing, and use of nuclear
weapons. In 1996 representatives from
approximately 60 countries signed an
agreement to stop all testing. The
agreement was called the United
Nations Comprehensive Test-Ban
Treaty.
In wars today armies still use traditional
types of bombs. However, new inventions
have made it easier to deliver them
with greater accuracy. Armies therefore
do not use as many bombs as they once
did.
#More to explore
Army • Nuclear Energy • Rocket
Bombay
#see Mumbai.
Bonaparte,
Napoleon
#see Napoleon.
Bone
Humans and many other animals have a
skeletal system made up of hard pieces
called bones. Bones help to support the
body and to protect its organs. Bones
store important nutrients and minerals,
too. Bones also make blood cells. The
human body has more than 200 bones.
A huge cloud forms after a test of an atom
bomb. The bomb causes so much damage
that it has been used only twice.
BRITANNICA STUDENT ENCYCLOPEDIA Bone 105
Bone Structure
There are two basic types of bone: compact
bone and spongy bone. Both types
are made of calcium and other minerals,
fibers of protein, and water.
Compact bone is solid and very hard. It
makes up the outer layer of all bones. It
also makes up the shaft, or long tube, of
arm and leg bones.
Spongy bone lies beneath most compact
bone. It looks like a honeycomb or a
sponge, but it is not soft. The many
spaces within spongy bone let it absorb
shock, or sudden pressure on the bone.
Bone marrow fills the hollow spaces in
spongy bone. It also fills the hollow
tubes of long bones. Marrow is a soft,
jellylike substance that can be red or
yellow. Red marrow makes blood cells.
Yellow marrow stores fats.
A thin membrane, or skin, covers each
bone. The membrane contains nerves
and blood vessels that are attached to the
bone. The blood vessels carry nutrients
to the bone. They also carry blood cells
from the bone to the rest of the body.
How Bone Is Made
Special cells start making new bone by
building a structure of protein. Blood
then brings in calcium, which sticks to
the structure. When the calcium hardens,
the new bone is complete. The
body can also rebuild bone that wears
away or breaks.
Broken Bones
When bones break, the injury is called a
fracture. Most fractures are simple,
which means they are beneath the skin.
