Barbed wire, first patented in the United States in 1867, played an important part in the development of American farming, as it enabled the settlers to make effective fencing to enclose their land and keep cattle away from their crops. This had a considerable effect on cattle ranching, since the herds no longer had unrestricted use of the plans for grazing, and the fencing led to conflict between the farmers and the cattle ranchers.

　　Before barbed wire camesintosgeneral use, fencing was often made from serrated wire, which was unsatisfactory because it broke easily when under strain, and could snap in cold weather due to contraction. The first practical machine for producing barbed wire was invented in 1874 by an Illinois farmer, and between then and the end of the century about 400 types of barbed wire were devised, of which only about a dozen were ever put to practical use.

　　Modern barbed wire is made from mild steel high-tensile steel, or aluminum. Mild steel and aluminum barbed wire have two strands twisted together to form a cable which is stronger than single-strand wire and less affected by temperature changes. Single-strand wire, round or oval, is made from high-tensile steel with the barbs crimped or welded on. The steel wires used are galvanized - coated with zinc to make them rustproof. The two wires that make up the line wire or cable are fed separatelysintosa machine at one end. They leave it at the other end twisted-together and barbed. The wire to make the barbs is fedsintosthe machine from the sides and cut to length by knives that cut diagonally through the wire to produce a sharp point. This process continues automatically, and the finished barbed wire is wound onto reels, usually made of wire in length of 400 meters or in weights of up to 50 kilograms.

　　A variation of barbed wire is also used for military purposes. It is formedsintoslong conies or entanglements called concertina wire.

　　中心 思想 题

　　中心思想题是 托福 考试中唯一带有主观色彩的题型，它考察你对文章大意和总体结构的把握能力。前面我们说过，托福阅读涉及的内容非常广泛，天文，地理，文学和人物传记几乎无所不包。那么，是否就没有技巧可谈了吗？绝对不是。文章内容可能千差万别，但文章的写作方式却只有寥寥的两种，即记叙文和议论文。记叙文是对某一件事，某一个现象或者某一个人的描述，即对客观事物的纪录和叙述，其中没有或者几乎没有作者自己的观点。记叙文有时间顺序和空间顺序两种组织形式。而议论文是作者根据某一客观事物或观点发表自己的看法，并用例子或者理由来证明作者自己的观点。根据文章的内容，议论文可以分成立论和驳论两种。(这部分内容我们将在GMAT和GRE中详细阐述)而根据文章的组织形式，议论文可以有3种组织结构，分别为：从具体到概括模式，从概括到具体模式以及递进模式。这部分内容我们将在后面的作文种具体描述。

　　在托福阅读中，没有真正的议论文。也就是说，阅读文章基本上都是记叙文，作者并没有试图让你接受他的某一个观点。有的也只是其中某一部分略带有作者的态度或者语气而已。所以中心思想题的正确答案肯定是一件事，一个物，一个人，一个现象或者一个动作，而不会是一个带有观点的结论性的句子。

　　掌握中心思想的技巧在于文章每个段落的第一句。标准化的考试决定了文章组织结构的严谨性，即条理清晰和结构完整。条理清晰表现为写作模式不是时间顺序就是空间顺序，而且承上启下的过渡 词汇 都出现在每个段落的第一句；结构完整表现为内容的完整性和单一性，即每一篇文章都能向你传达一个而且只有一个有意义的现象或者事件，不会让你读后感觉到不知文章所云为何物。下面是一篇结构不完整的“文章”。读完你就知道什么样的结构是不完整的，而什么样的结构才是完整的。

　　中心思想题的问法千篇一律，即不是“What is the main topic of the passage?”，就是“What does the passage mainly discuss?”。好，现在我们开始看一道中心思想题。

　　10.What is the main topic of the passage?

　　(A) Cattle ranching in the United States.

　　(B) A type of fencing

　　(C) Industrial uses of wire

　　(D) A controversy over land use.

　　我们刚才说过只需解读每段的首句你就能掌握文章的大意。到底是不是真的这么神呢？我们回到原文。

　　第一段首句的大意是，某种带刺的金属线自引进美国后，对农业起着重要的作用，因为它有效地包圈了 土地 而避免遭受牛畜的糟踏。

共4页: 1 [2] [3] [4] 下一页