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  3. By the 1820’s in the United States, when steamboats were common on western waters, these boats were mostly powered by engines bu

By the 1820’s in the United States, when steamboats were common on western waters, these boats were mostly powered by engines bu

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问题      By the 1820’s in the United States, when steamboats were common on western waters, these boats were mostly powered by engines built in the West (Pittsburgh, Cincinnati, or Louisville), and of a distinctive western design specially suited to western needs. The first steam engines  【21】______   practical use in England and the United States were  【22】______   low-pressure design. This was the type first  【23】______   by James Watt, then manufactured by the firm of Boulton and Watt. Steam was  【24】______   in a large, double-acting vertical cylinder, but the steam reached only a few pounds of pressure per square inch. It was low-pressure engines of this type that 【25】______   first introduced into the United States by Robert Fulton. He 【26】______   such a Boulton and Watt engine from England to run the Clermont. But this type of engine was ,expensive and complicated,  【27】______   many precision-fitted moving parts.
      The engine that became standard on western steamboats was of a different and  【28】______   design. It was the work primarily of an unsung hero of American industrial progress, Oliver Evans (1755-1819), the  【29】______   son of a Delaware farmer. Evans early became  【30】______   by the possibilities of mechanized production and steam power. As early as 1802 he  【31】______   a stationary steam engine of high-pressure design in his mill. Engines of this type were not  【32】______  , but before Evans they were generally considered 【33】______   and dangerous. Within a decade the high-pressure engine, the new type, had become standard on western waters. Critics  【34】______   of western conditions often attacked it as wasteful and dangerous.
     But people who really knew the Ohio, the Missouri, and the Mississippi  【35】______  , with good reasons, that it was the only engine for them. In  【36】______   western rivers the weight of vessel and engine  【37】______   important; a heavy engine added to the problem of navigation. The high-pressure engine was 【38】______  lighter in proportion to horsepower, and, with less than half as many moving parts, was much easier and cheaper to repair. The main advantages of low-pressure engines were safe operation and  【39】______   of fuel consumption,  【40】______   of which meant much in the West..  
【32】
选项 A、known
B、knowing
C、unknowing
D、unknown
答案D
解析
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