Airships are huge balloons with propellers to drive them and they were once used to carry as many as...: ответ на тест 1006961 - Английский язык

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Airships are huge balloons with propellers to drive them, and they were once used to carry as many as 200 passengers.
The airships were filled with hydrogen, a light gas. However, hydrogen is also inflammable. There were several disasters in which airships caught fire, and they disappeared by the end of the 1930s.
However, airships are now flying again, but they are not common. They are lifted by helium gas, which does not catch fire.

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Airships are huge balloons with propellers to drive them, and they were once used to carry as many as 200 passengers.
The airships were filled with hydrogen, a light gas. However, hydrogen is also inflammable. There were several disasters in which airships caught fire, and they disappeared by the end of the 1930s.
However, airships are now flying again, but they are not common. They are lifted by helium gas, which does not catch fire.

Magnets can be natural and artificial. The Earth, for example, contains magnetic substances in its nucleus. These substances, which are also in our body, make our planet a big natural magnet. That’s why when you hang a bar magnet from a thread in a horizontal position, its north pole points to the geographic north pole of the Earth. A bar magnet is an artificial magnet. This type of magnets are usually made from a combination of elements that contain iron, cobalt or nickel.
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Benjamin Franklin is acknowledged to be the founder of the theory of atmospheric electricity. He was the first to prove that the lighting was an electrical phenomenon. Franklin developed a new theory of electricity that he called positive and negative. He finally invented a means of protection against the disastrous effects of lighting – the lighting rod. Franklin is known and respected all over the world not only as a scientist but also as a citizen who did as much as he could for the good of his country. Coming out in defence of the Afro-Americans, Franklin declared slavery to be not only an evil from the moral point of view, but also an obstacle to the social interests of America. He is thought to be one of the broadest as well as one of the most creative minds of his time.
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What is the density of water?
Density (specific weight) is the amount of mass in a unit volume. It is measured in kilograms per cubic metre. The density of water is 1000 kg/m3 but most materials have a higher density and sink in water. Aluminium alloys, with typical densities around 2800 kg/m3 are considerably less dense than steels, which have typical densities around 7800 kg/m3. Density is important in any application where the material must not be heavy.
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What is the density of water?

Water is not used in refrigerators because it evaporates too slowly. Ether, another refrigerant, is never used in domestic refrigerators since it is both poisonous and inflammable. However, it is sometimes used for industrial refrigeration because it evaporates very fast. Freon is usually used in domestic refrigerators as it evaporates fast, is non-toxic, and non-flammable.
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Why is water not used in refrigerators?