screw produces a force that can push
the screw into wood or tighten it
against a nut. Similar threads are used
on jars and other containers so that lids
close tightly.
A pulley is a wheel with a rope or something
similar around its edge. The pulley
changes the direction of the force
applied to one end of the rope. For
example, by pulling down on one end of
the rope, a person can lift an object
attached to the other end.
Compound Machines
Simple machines can be combined to
create compound machines. Compound
machines can be as small as a mechanical
watch or as large as a construction
crane.
In a compound machine, forces and
motion are transferred from one part to
another. One way this is done is with
gears. A gear is typically a circular piece
of metal with teeth, or ridges, along its
outer edge. The teeth of one gear fit into
those of another. When one gear turns,
it also turns the other gear. Another way
of transferring forces and motion is with
a type of pulley that uses a chain or a
band of flexible material called a belt.
A bicycle is an example of a compound
machine that uses a chain to transfer
force. The chain runs around two separate
toothed wheels, which act as pulleys.
One is attached to the axle of the
rear wheel. The other is attached to the
pedals through an axle. The pedals work
like the crank of a wheel-and-axle
machine. The force used to turn the
pedals becomes a stronger turning force
on the axle and its toothed wheel. The
chain transfers the force to the rear
wheel and makes it turn. In some
bicycles the chain can be shifted between
A crane is a machine that makes lifting
heavy objects easier.
A mechanic puts together complex machinery
that will power an airplane.
BRITANNICA STUDENT ENCYCLOPEDIA Machine 7
toothed wheels of different sizes. This
changes the amount of force the rider
needs to turn the rear wheel.
#More to explore
Force • Motion
Machu Picchu
In the mountains of south-central Peru
stands Machu Picchu, a deserted city of
the Inca people. The name Machu Picchu
means “old peak” in Quechua, the
language of the Inca. The dwellings at
the site were probably built and occupied
from the mid-1400s to the early or
mid-1500s. Machu Picchu is one of the
few American Indian sites dating from
that long ago to be found nearly intact.
Machu Picchu is located about 50 miles
(80 kilometers) northwest of the city of
Cuzco, which was the capital of the Inca
Empire. It lies between two sharp peaks
in the eastern Andes Mountains, at a
height of 7,710 feet (2,350 meters).
Mountain forest surrounds the site.
Machu Picchu is divided into two sections,
one farming and one urban. The
farming section is made up of steplike
fields called terraces. The Inca used the
terraces for growing crops such as corn
and sweet potatoes. The urban section
has plazas, temples, houses, and a cemetery.
They are connected by walkways
and thousands of stone steps.
Most of the structures are very well preserved
because of the quality of Incan
engineering and stonework. One
notable structure is the Temple of the
Sun. Once a year sunlight shines
through a window onto the middle of a
large stone. This stone was a sort of calendar.
Another famous structure is a
pillar known as the Intihuatana. The
pillar was probably used to keep track of
the seasons.
The role of Machu Picchu in Incan society
is not entirely understood. Many
archaeologists believe it was the estate of
a powerful Inca emperor. They do not
know why the site was abandoned. Lack
of water may have been a reason.
Machu Picchu remained hidden from
the Spanish when they conquered the
Inca in the 1500s. Plants grew over the
site, and for hundreds of years it was
known only by a few people who lived
in the area. The world learned of Machu
Picchu when a local farmer led a U.S.
archaeologist to the ruins in 1911.
Today Machu Picchu is a popular tourist
attraction.
#More to explore
Andes • Inca • Peru
Mountains and forests surround the ruins of
Machu Picchu.
8 Machu Picchu BRITANNICA STUDENT ENCYCLOPEDIA
Madagascar
Madagascar is an island country located
off the southeastern coast of Africa. The
island is the fourth largest in the world.
Only Greenland, New Guinea, and
Borneo are larger. Madagascar is known
as the Great Red Island for its rich, red
soil. Its capital is Antananarivo.
Geography
Madagascar lies in the western Indian
Ocean. The Mozambique Channel separates
the island from the African coast.
The central part of Madagascar is a plateau,
or high, flat land. Mountains rise
above the plateau in some places.
Maromokotro Peak in the north is the
island’s highest point. It rises 9,436 feet
(2,876 meters). A narrow plain runs
along the east coast. Low plateaus and
plains cover the west. Rivers are short
and fast flowing in the east and longer in
the west.
The east coast has a hot, humid tropical
climate. The central plateau is cooler.
The southwest is dry.
Plants and Animals
Grasses cover most of the island. Some
tropical forests and mangrove swamps
survive on the coasts. Hundreds of different
kinds of orchid also grow in
Madagascar.
Madagascar has many types of animals
that live almost nowhere else in the
