The first person to use the sun's energy on a large scale was the Greek, Archimedes. He used it to set fire to an attacking Roman fleet at Syracuse in 212 B. C.. He did this by means of a burning glass composed of small square mirrors made to move in all directions on hinges (轉軸). In the early twentieth century, solar energy was used to power water distillation plants in Chile and irrigation pumps in Egypt.

　　Today over ninety-eight per cent of our energy comes from fossil fuels; coal, oil and natural gas. But coal and oil are not only fuels. They are also important raw materials for our chemical industries. People now think we should limit their use as fuel, otherwise the world's supply will soon run out. Consequently, much research is being done into the practical use of the sun for heating (and cooling) buildings. The less fossil fuel we use now, the more we will have for the future.

　　Solar energy is transmitted from the sun—through space—to earth by electromagnetic radiation. It must be converted to heat before it can be used. Various types of solar energy collectors (also called absorbers or absorber plates) are used to convert the sun's radiation to heat. Solar-heated hot water provides not only the building's hot-water requirements, but space-heating as well. This solar system, based on water-heating, works like this. Each of its collectors, placed on the roof, consists of a shallow fiberglass tray that holds a number of copper tubes under a special glass cover. The tubes are, in turn, fixed to a blackened metal surface which absorbs energy from the sun. The sun's rays, passing through the glass, heat the water in the tubes.

　　Since the energy is available only during the day and only then when there is not a great deal of cloud cover, the heat must be stored. This can be done in wa¬ter, or in rock or pebble beds. The energy is stored in the hot water in the pipes.

　　21. Solar energy was first used_______.

　　A. to start a fire B. to cook food

　　C. to power a heating-system D. in a war

　　22. What must be done to solar energy before it can be used?

　　A. It must be stored in rock beds. B. It must be distilled.

　　C. It must be converted to heat. D. It must be purified.

　　23. There is so much research into the use of solar energy because '_______.

　　A. more irrigation projects are needed B. the construction industry is expanding C. fossil fuels are running out D. it transmits electromagnetic radiation

　　24. Solar energy must be stored because_______.

　　A. it is limited B. there is not much of it

　　C. it is not always available D. the passage does not tell us why

　　25. How many uses has solar energy been put to according to the passage? A. 2. B. 3.

　　C. 4. D. More than 4.

　　21. D 22. C 23. C 24. C 25. B

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