20 Railroad BRITANNICA STUDENT ENCYCLOPEDIA
History
Before there were trains and locomotives,
people used horses to pull carts
along tracks. Europeans began using this
early type of railroad in the 1500s. They
used it to carry heavy loads to and from
mines.
In 1803 Richard Trevithick, a British
engineer, planned and built a locomotive
that ran on steam power. Mining
companies used it. In the 1820s another
British inventor, George Stephenson,
designed and built the first steam train
to carry goods and passengers.
In 1869 the United States completed a
railroad system that stretched all the way
from the East Coast to theWest Coast.
This railroad was called the transcontinental
railroad.
Between the 1930s and the 1950s, diesel
engines replaced steam engines as the
power source in most locomotives.
Today freight trains operate throughout
the world. But in many countries trucks
carry much of the freight that trains
once did. Passenger trains are still common,
too. However, airlines have taken
passengers away from many railroads.
#More to explore
Transportation
Rain
Water has three forms. It may be a liquid,
a solid called ice, or a gas called
water vapor or steam. Rain is the liquid
form of water that falls from the sky in
drops.
Rain fills lakes, ponds, rivers, and
streams. It provides the freshwater
needed by humans, animals, and plants.
If too much rain falls, however, dangerous
flooding may happen.
How Rain Forms
Rain is a part of Earth’s endless water
cycle. At the beginning of the cycle, sunlight
heats up water on Earth’s surface.
The heat causes the water to evaporate,
George Stephenson of England built the
Rocket with his son, Robert, in 1829. It was
the fastest steam locomotive of its time, with
a speed of 36 miles (58 kilometers) per hour.
A tree frog enjoys the rain.
BRITANNICA STUDENT ENCYCLOPEDIA Rain 21
or to turn into water vapor. This water
vapor rises into the air. As the water
vapor cools, it turns back into water, in
the form of droplets. The droplets form
around dust and other particles through
a process called condensation.
Clouds form from large numbers of
these droplets. In a cloud, droplets come
together with other droplets to form
larger drops of water. Eventually the
drops become too heavy to stay in the
cloud. They fall to Earth as rain. Then
the water cycle begins again.
Sometimes drops of water freeze into ice
crystals in the clouds. Sometimes the ice
crystals melt as they fall toward the
ground. This is another way that rain
forms.
Where Rain Falls
Rain falls almost everywhere on Earth.
One of the world’s rainiest places is
MountWaialeale in Hawaii. It rains
about 350 days a year there. About 460
inches (1,170 centimeters) of rain fall
there every year. One of the driest places
on Earth is the Atacama Desert in Chile.
It receives less than 0.04 inch (0.1 centimeter)
of rain a year. It has not rained in
some parts of this desert in hundreds of
years.
Acid Rain
Rain washes dust and dirt from the air.
But rain itself is not always pure. Sometimes
polluting chemicals from cars,
factories, and power plants become
trapped in clouds. The rain from these
clouds contains those harmful chemicals.
This polluted rain, known as acid
rain, can damage plants, animals,
people, and property.
#More to explore
Acid Rain • Cloud • Flood •Water
Rainbow
A rainbow is a multicolored arc, or
curved line, in the sky. Most rainbows
form when the sun’s rays strike raindrops
falling from faraway rain clouds.
Rainbows appear in the part of the sky
opposite the sun, usually in the early
morning or late afternoon. From inside
to outside, the colors of a rainbow are
violet, indigo, blue, green, yellow,
orange, and red.
Sunlight travels through space in the
form of waves. Scientists use an idea
called wavelength to describe these
waves. Some light waves have long wavelengths,
while others have short wavelengths.
Light waves with different
wavelengths appear as different colors.
Usually all light waves blend together to
form white light. But when light waves
A rainbow arcs over a beach in Hawaii. A
faint secondary bow is visible above the
bright primary bow.
The more raindrops
a cloud
contains, the
darker it
becomes.
22 Rainbow BRITANNICA STUDENT ENCYCLOPEDIA
pass through raindrops, they separate.
This happens because the raindrops
bend light waves with different wavelengths
by a different amount. The separated
light waves appear as the colors of
a rainbow.
The brightest and most common type of
rainbow is called a primary bow. Sometimes
a fainter rainbow forms outside
the primary bow. This is called a secondary
bow or, sometimes, a double rainbow.
A secondary bow forms when the
light bends twice inside the water drops.
The first bend makes the primary bow,
and the second bend makes the secondary
bow. The colors in the secondary
bow appear in the opposite order of the
colors in the primary bow.
#More to explore
Light • Rain
Rain Forest
Thick forests found in wet areas of the
world are called rain forests. Most
people are familiar with hot, tropical
rain forests filled with trees that stay
green year-round. But there are other
