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The historian Frederick J. Turner wrote in the 1890’s that the agrarian discontent that had been developing　steadily in the United States since about 1870 had been precipitated by the　　closing of the internal frontier——that is，the depletion of available new land　needed for further expansion of the American farming system.

=The historian Frederick J. Turner wrote in the 1890’s that the agrarian discontent that had been developing　steadily in the United States since about 1870 had been precipitated by the closing of the internal frontier——that is，the depletion of available new land　needed for further expansion of the American farming system.

史学家Frederick J. Turner写道于十九世纪九十年代，农民的不满一直在不断发展的在美国自1870年以来已经加剧由于封闭国内边疆——即，枯竭可利用的新土地进一步扩大所需的美国农业系统的（扩大）。

The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passes，beginning as atmospheric water vapor，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation and transpiration.

水文循环，一个主要课题该学科中的，就是指整个循环过程水经历的，开始是大气中的水蒸气，进而变成液体和固体形态作为降水，由此流经地表或进入地表，最终又变回大气水蒸气形态通过蒸发和气化作用。

延伸阅读：

托福阅读专项讲解——长难句（一）

托福阅读专项讲解——长难句（二）

托福阅读专项讲解——长难句（三）

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