六级冲刺练习阅读（45）

　　Rainmaking

　　The idea of rainmaking is almost as old as man, but it was not until 1946 that man succeeded in making rain. In ancient times, rainmakers had claimed to bring rain by many methods: dancing, singing, killing various kinds of living creatures , and blowing a stream of water into the air from a kind of pipe.

　　More recently, some rainmakers claimed to make rain by shooting guns, causing explosions, or burning chemicals, the smoke of which was supposed to cause rain to fall. These rainmakers asked for as much as 1,000 dollars to make an inch of rain. One was so effective that he was almost hanged. He was believed to have caused a twenty-inch rain in southern California that flooded the land, killed several people, and did millions of dollars worth of damage.

　　Before 1946, rainmakers were either liars or honest people who happened to have good luck. Scientific rainmaking was started in that year by Vincent J. Schaefer, a scientist at the laboratories of the General Electric Company in New York State. His success was the result of a lucky accident that changed years of failure into victory.

　　For a long time, men have understood where rain comes from. Water from the surface of oceans and lakes becomes part of the air, where it forms clouds from which rain falls. But exactly what starts the formation of raindrops was not known until quite recently. A man named John Aitken proved that drops of water gather around tiny bits of dust or other matter. The centers of the drops are so small that the human eye cannot see them. Without such centers, it seems, raindrops do not form.

　　During World War II, Dr. Irving Langmuir, a scientist, was hired by the General Electric Company to study how and why ice forms on the wings of airplanes. He and a young assistant named Schaefer went to a mountain in the state of New Hampshire, where snowstorms are common and cold winds blow.

　　While in New Hampshire, Langmuir and Schaefer were surprised to learn that often the temperature of the clouds surrounding them was far below the freezing point, and yet ice did not form in the clouds. After the War, Schaefer experimented with a machine that created cold, moist air similar to the air found in clouds. To imitate the moist air of a cloud, Schaefer would breathe into the machine. Then he would drop into the freezer a bit of powder, sugar, or some other substance. For weeks and months he tried everything he could imagine. Nothing happened. No crystals of ice were formed. None of the substances would serve as the center of a snow crystal or raindrop.

　　One July morning, Schaefer was dropping in bits of various substances and watching the unsuccessful results. Finally, a friend suggested that they go to eat lunch, and Schaefer gladly went with him. As usual, he left the cover of the freezer up, since cold air sinks and would not escape from the box.