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134
In addition, many fuels contain sulfur; this bums to produce sulfur oxides. Also, in
the heat of combustion, some of the nitrogen in the air combines with oxygen to form
nitrogen oxides (NO). Some of the fuel may not bum completely, so that smoke and
ash are formed. Smoke is singly particles of unburned fuel and soot, called
particulates, mixed with air.
Altogether, it is estimated that 200 million tons of man-made pollutants enter
the air every year in the United States alone. This is about a ton for every man,
woman, and child in the country! This man-made pollution is what clean-air laws are
aimed at. Consider Los Angeles, a large city set in a basin, with about 7,000,006
inhabitants. It is surrounded on three sides by mountains, and on the fourth by the
Pacific Ocean. When the wind blows out over the ocean, it sweeps away pollutants.
But at other times, the air is stagnant. Smoke and other pollutants from industry and
automobiles do not blow away. They just build up into a thick, smelly, foggy layer of
smog. The location of Los Angeles plus all the people and industry there, make it one
of the biggest "smog, centers" in the country. And it is Los Angeles which has led in
measures to reduce smog.
Los Angeles has banned unrestricted burning, for example, burning trash.
Incinerators without pollution controls were outlawed. Industry was forced to change
combustion processes and add controls to reduce pollutants coming from their
chimneys. Laws were passed that required the addition of emission controls on
automobiles. All these measures significantly reduced atmospheric pollution in the
Los Angeles area.
If not controlled, the automobile can give off pollutants from four places.
Pollutants can come from the fuel tank, the carburetor, the crankcase, and the tail
pipe. Pollutants from the fuel tank and carburetor consist of gasoline vapors.
Pollutants from the crankcase consist of partly burned air-fuel mixture that has blown
by the piston rings. Pollutants from the tail pipe consist of partly burned gasoline
(HC), carbon monoxide (CO) nitrogen oxide (NO
x
), and -if there is sulfur in the
gasoline - sulfur oxider (SO
x
).
Текст 7
WATER TRANSPORT
One of the most important things about water transport is the small effort
needed to move floating craft. A heavy boat or a barge weighing several tons can be
moved through the water, slowly but steadily, by one man. An aeroplane of the same
weight as the barge needs engines of 1,000 horsepower or more in order to fly.
The raft made of logs of wood is supposed to be the earliest type of boat.
Rafts seem to be clumsy vessels, .although the Norwegian scientist Thor Heyerdahl
and his five companions in 1947 made a voyage on the raft Коп-Tiki from Peru to
Tuamotu Islands - a distance of 4,500 miles.
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