46．　If an engineer applies aformula or a tabulation or a curve blindly，　without knowing its history，its accuracy and the limits of its sapplicability，the results that he obtains will be neither intelligent nor practical．

如果一个工程师盲目地应用一个公式、一张图表或一条曲线，而对它的来历、精确度和应用范围却一无所知，那么他所得到的结果既不明智，也不实际。(without．．．applicability介词短语作谓语applies的条件状语。that引导定语从句，在句中作obtains的宾语。)

　47．The neutron carries no electrical charge and in some respects acts as though it were composed of a proton and electron combined，with the positive charge of the proton neutralized by the　negative charge of the electron．

中子不带电荷，并且有点像是由质子和电子结合而成的，质子的正电荷被电子的负电荷中和了。(in somerespects，“在某些方面”，“有点”。act as though，“表现得似乎”。as though引导方式方法状语从句，其谓语常用虚拟语气。句末with引出介词复合结构，对前句的谓语作补充说明。)

48．We know that the rate of cooling from above the critical range will greatly change the structure and therefore the properties of any steel．

我们知道，从临界温度以上冷却的速率会大大改变任何一种钢的结构，从而会改变其性能。(from above．．，“从……以上"，是双重介词。)

49．Increasing problems of all kinds with fossil fuel power plants are turning power and utility companiesmoreandmore tOtheuseOfnuclearpoweredelectricgeneratingplants．

矿物燃料发电厂所产生的问题日益增多，这就促使电力公司和公用事业公司越来越倾向于采用核动力发电厂。(problem．．．with，“在……方面产生的问题”，with有表示“范围”。)

50．Ever since，people have had difficulty in deciding whether the hovercraft should be ranged　among ships,planes，or land vehicles—for it is something in between a boat and anaircraft．

从那以后，人们感到很难确定把这种气垫船归为哪一类：船、飞机还是车辆，因为它是一种介于船与飞机之间的交通工具。(in between，“介于两者之间”，作定语修饰something。)

51．The electric resistance of a wire is the ratio of the potential difference between it stwo ends to the current in thewire．

导线的电阻等于该导线两端之间的电位差与导线中电流的比值。(tothecurrent．．。是ratio要求的。the ratio of A to B，“A与B之比”。)

52．Pollution is caused either by man's release of completely new and often artificial substances into the environment，or by releasing greatly increased amounts of a natural substance，such as oil from oil tankers into the sea．

引起污染有两种途径：一是人们经常把全新的人造物质释放到环境中，或是把大量的天然物质譬如油轮溢出的石油释放到海里。(此句为简单句，主要是由两个by引出的短语把这个句变得复杂了。第一个into是名词~ease要求的。第二个into是动名词releasing要求的。From oil tankers作定语修饰oil。)

53．Hardening of grey cast iron may be carried out by heating the casting to above the critical　range and then cooling it，usually in oil．

灰口铸铁的淬火过程是：先把铸铁加热到临界温度以上，然后加以冷却，通常在油里冷却。(heating．．．to above，“把……加热到……之上”。to above是双重介词。)

54．Motion in the most general sense，conceived as the mode of existance，the inherent attribute，of matter，comprehends a11 changes and processes occurring in the universe，from mere　change of place right up to thinking．

从最普遍的意义上来看，可以认为运动是物质存在的形式，是物质的固有属性，运动包括宇宙中所发生的一切变化和过程，从单纯位置的变化直到思维。(主语是motion，谓语是comprehends，被一个介词短语inthemostgeneralsense和一个分词短语conceived as...隔开了。of　matter既说明existance，也说明attribute。介词短语from．．．用来说明a11 changes and processes。)

55．The air is heated by the flame，expands，and thus becomes less dense．　Denser air flows in　from below and the result is that irregular layers of air of different densities are formed above the　flame．

空气受火焰加热而膨胀，于是变得比较稀薄。较密的空气从底下流人，其结果是，火焰上 方形成了密度不同的不规则的空气层。　(flow in，　“流人”。from below，　“自下”，　“从下面”。 above the flame作地点状语。)