症。
第3节 翻译练习9
Even if our measurements show the same velocity over different sections, it may be asserted that the motion is uniform only with the degree of accuracy that the measurements were made with. 虽然自动化机器有很多优点，但它们只能做人们吩咐它们要做的事。 The interpretation, which is not entirely convincing, is at least partly true. 这种解释虽不完全令人信服，至少有几分真实性。
Anyone who has tugged heavy hand baggage down endless airport corridors, or waited for a delayed flight in a sterile lounge will know how user-unfriendly many airports are in design terms.
任何人当他拖着沉重的随身行李，走过无止境的机场走廊，或者在枯燥乏味的休息室里等候延误的班机，都会体会到许多机场在设计上对旅客是多么不方便。 第3节 翻译练习10
The facts of science are not, as some think, dry and lifeless.
与某些人想象的相反，科学上的某些事实并不是那样的枯燥无味和没有生命力。 The patient did not have a heart attack as the doctor had feared. 与医生担心的相反，患者并没有复发心脏病。 第3节 翻译练习11
Glaxo Wellcome is developing a method of vaccine delivery where the vaccine is attached to tiny gold particles.
格莱索·威尔克姆正在开发一种疫苗释放方法，即将疫苗粘附在微金粒上。
Astronomers and scientists think that a black hole is a region of space (not a thing) into which matter has fallen and from which nothing can escape——not even light.
天文学家和科学家认为黑洞不是一种物体，而是太空的一个区域，即物质掉进去后，任何东西，甚至于光，都无法逃出。 第3节 翻译练习12
Episodes of low cardiac output which were accompanied by significant increases in PAP/AOP ratio occurred in 20 patients and were managed by alteration and administration of pulmonary vasodilators. 在20个患有低心输出血量发病的患者中，伴有肺动脉压和主动脉压比率明显增加，其处理方法是改善通气并给以肺血管扩张剂。
The ceramic will be made of oxide powders, using techniques that must be carried out only in an atmosphere of ultra-pure and inert nitrogen.
这种陶瓷用氧化物粉末制成，所采用的工艺必须在超高纯度和超高惰性的氮气中才能进行。 New evidence to be published later this month in the journal Science offers a simple explanation why two laboratories came to claim credit for the same discovery.
本月晚些时候将在《科学》杂志上公布新的证据，就为什么两个实验室对同一个发现都要求荣誉权的问题提供一个简明的解释。 第3节 翻译练习13
Putting a restriction on the depth to which a diver can descend might be helpful. 对潜水员下潜的深度实行限制也许是有益的。
Nowhere in the whole domain of the motions of the natural world have we yet found any case to which the analysis of mechanics does not apply.
在自然界的整个运动领域里，我们还没有发现力学分析不适用的任何情况。 This is an incandescent bulb, the light of which travels in all directions. 这是一盏白炽灯，它的光是向四面八方传播的。
Some elements are known, the atoms of which usually do not combine with more than tone atom of any other element.
某些元素是众所周知的，这些元素的原子通常不能同其他任何元素的一个以上的原子相结合。 Air is a mixture, the components of which are of great use in the chemical industry. 空气是一种混合物，其成分在化学工业中大有用处。 第4节 翻译练习1
As we observe objects around us, one of the most noticeable properties is their motion. 当我们观察周围的物体时，最显著的特性之一是其运动性。
When an object is suspended by a single string, it is observed that the string hangs vertically. 当一物体用一根细绳悬挂起来时，可以看到绳子垂直的挂着。
When one form of energy disappears, other forms appear in equivalent amounts. 每当一种形式的能量消失时，就会出现等量的其他形式的能量。 第4节 翻译练习2
Our whole physical universe, when reduced to the simplest terms, is made up of two things, energy and matter.
我们的整个物质世界，如果用最简单的话来说，是由两样东西组成的，即能和物质。 We are always able to know the third one when we know the magnitudes of the other two. 如果已知其中的两个变量，我们就可以求出第三个变量。 第4节 翻译练习3
A body at rest will never move till a force compels it. 精致的物体在外力迫使它运动之前决不会运动。
Heat and temperature were confused until Joseph Black cleared up the subject in 1760.
在1760年约瑟夫·布莱克澄清了热和温度这个问题以前，人们一直将两者混为一谈。 Slide line C to the left, until it corresponds with line D. 将线C向左平移，直到与线D重合为止。 Boil the solution till it becomes free of H2S. 将该溶液一直加热到没有硫化氢为止。
Electric current does not flow in the circuit until we add a battery. 直到我们加上电池组后，电流才能在这条电路中流动。 第4节 翻译练习4
Once a satisfactory system of lubrication has been secured, the hardness of the interacting elements becomes critical.
一旦获得了满意的润滑系统，相互作用的元件的硬度就成为了关键因素。 Once airborne particles touch each other, they combine into a bunch of particles. 空气中的粒子一旦彼此相碰，就结合成一个粒子团。
If all the impending forces of gravitation and resistance could be removed, there is no reason why the ball, once it is in motion, should ever stop.
如果一切起阻碍作用的引力和阻力能被除去，就没有理由认为，球一旦处于运动中会再停下来。 第4节 翻译练习5
Since the arc burns in a closed volume, a high gas pressure will be obtained. 因为电弧在一个密闭容器中燃烧，将得到很高的压力。
Since from now on frequent reference will be made to sample and population, it is necessary to distinguish between the mean of a sample and the mean of population.
因为从现在起要常常提到样本和种群，所以有必要把样本和种群的均值区分开来。
The results of the investigation were not conclusive since the crucial experiment had been performed only once.
研究的结果还不是结论性的，因为决定性的实验只进行过一次。 第4节 翻译练习6
Where there is no need for electrical isolation an auto-transformer is sometimes employed. 在不需要电绝缘的地方，有时使用自耦变压器。
Where only small a variation in output is required, synchronous tap changing is often used. 在要求输出电压调节范围较小的电路中，常常采用同步抽头变换法。 第4节 翻译练习7
Where the bearing capacity is particularly poor or the quality of the soil varies considerably, a raft foundation is often recommended to support normal buildings.
若承载力特别低或土质差异巨大，通常用片筏基础支撑一般的建筑物。
Concrete foundations need not always be reinforced, but where the load to be carried is high and the veering capacity of the soil poor, heavy reinforcement is usually necessary.
混凝土基础不必总是用钢筋来加固。但是，若承受的载荷很大，而且地基的承载力又比较低，通常有必要用钢筋进行大力加固。 第4节 翻译练习8
Computer technology is advancing so rapidly that no book can ever be fully up to date.
计算机技术发展非常迅速，所以任何时候也不可能有一本书能及时充分的反映它的最新成就。 Gears play such an important part in machines that they have become the symbol for machinery. 齿轮在机器中起着相当重要的作用，以致它已成为机械的象征。
The bar is set next to a screw, so that it touches the screw when it is straight. 金属棒安装的位置紧靠着螺钉，这样，当金属棒伸直时就碰到螺钉。 第4节 翻译练习9
In order that electrons be accelerated in an applied field, they must be able to move to higher energy levels.
为了使电子能在外加场中加速，电子应能移到更高的能级。 The belt is adjusted so that it will fit tightly over both wheels. 为了减少摩擦，金属表面必须涂上润滑脂。
The complier must know its operation code and its parameter so that an instruction can be assembled. 为了能储存大量电荷，电容器通常用大面积的金属板制成。
Negative work should be maintained at as low a value as possible for any given pressure ratio, in order that a maximum amount of useful work may be obtained.
在给定的压力下，负功应尽可能维持在低的数值下，以便获得最大的有用功。 I am going to start up the generator in case the power goes off. 我去启动发电机，以免断电。
Set all screws so they bear evenly on leveling plates. 调整各个螺钉，使它们都均匀的压在调平板上。 第4节 翻译练习10
The pressure of a gas is inversely proportional to its volume if its temperature is kept constant. 如果温度保持不变，则气体压力和体积成反比。
As long as the current flows, the armature will keep rotating. 只要有电流流通，电枢就会不停的转动。
An observer will not see a rainbow unless there are drops of water suspended in the air. 如果没有小水滴悬在空中，观察者就看不到彩虹。
The law is true provided that the physical conditions of the circuit remain constant. 当电路的物理条件不变，这一定律就是正确的。
If the admissible gripping force is exceeded, a hydraulic overload protection is operated. 当夹紧力超过了允许值，液压超负荷防护装置就会起作用。
We can keep the steam engine at work, so long as we have means of producing heat. 我们能够使蒸汽一直运转，只要有办法产生热。 第4节 翻译练习11
Electricity flows through a wire just as water flows through a pipe. 电流通过导线，正像水流通过管道一样。
Exotic plastics, glass and ceramics will shape the future just as surely as have genetic engineering and computer science.
奇异的塑料、玻璃和陶瓷无疑将同遗传工程和计算机科学一样塑造着未来。 Solids expand and contract as liquids and gases do. 固体像液体和气体一样地膨胀和收缩。 第4节 翻译练习12
Although these particles are very light, their energies are considerable because of their high speeds. 虽然这些粒子很轻，但由于速度很快，因而能量很大。
Even if someone is not a programmer, he can have a great impact on piece of software by suggesting how to improve it to the development team.
即使一个人不是专业编程人员，他也可以通过向开发小组提出改进建议，而对一个软件产生极大的影响。
While the stars look like mere specks, they are in reality enormous bodies. 虽然看起来只是些光点，实际上星星都是庞然大物。
Though a little over 100 elements in nature, they combine to form thousands upon thousands of different substances in the world.
虽然在自然界中只有100多种元素，可是它们却构成世界上成千上万种物质。 No matter how small a stress is applied to a body, the body will yield a little. 作用在一物体上的应力无论多么小，该物体总会有一点变形。 第4节 翻译练习13
Virtual convergence will take the power on your desktop and make it available wherever you want it. 这种放大器无论信号变化怎样缓慢，都能放大输入信号。
The unique training of the chemical engineer becomes essential in these areas whenever processes involve the chemical or physical transformation of matter.
只要工艺过程涉及到物质的化学或物理转换，就必然要专门培训化学工程师。 第4节 翻译练习14
It is anticipated that LSI chips will show twice as high a reliability as they have now. 预计大规模集成电路芯片的可靠性比目前可提高1倍。
The electrons meet less resistance when conductor is cold than when it is hot. 导体冷的时候，电子遇到的阻力比导体热的时候小。
Electronic engineers are more interested in the application than in the development of semiconductor devices.
电子工程师对半导体器件的应用比对半导体器件的研究更感兴趣。 第5节 翻译练习1
We believe that the ratio is modest hypercholesterolemia since individuals with low or very high cholesterol concentrations are unlikely to have their risk of developing CHD (coronary heart disease) altered either by HDL-C measurement or the calculation of the ratio.
我们认为这一比率只适用于血胆固醇有些过高的病人，因为胆固醇偏低或过高的人得冠心病的可能性不大会因为他们的高密度脂蛋白胆固醇的高低或这一比率的大小而有所改变。
Now, a team of astronomers has discovered evidence that the object, whatever it may be, is spewing vast quantities of energetic particles that collide with a surrounding irregular ring of cold hydrogen gas.
现在，一组天文学家已经发现了证据，即这个天体（不管它是什么），在不断地喷出大量高能粒子，同周围的形状不规则的冷氢气环相碰撞。
However, in cases that had episodes of increased PAP/AOP ratio, the mean duration of function was increased to five days, and this was almost certainly related to the use of continuous tolazoline infusion through the catheter.
然而，在患有增加肺动脉压和主动脉压比率的病例中，平均效用时间增加到五天，这几乎肯定与通过导管继续输入抗肾上腺素有关。 第5节 翻译练习2
Just as a pace satellite, once it has got away from the pull of the earth’s atmosphere, continues to circle round and round the earth for ever, so an electric current circling a frozen ring of mercury should continue to flow for ever, if the temperature is kept below the essential four degrees above absolute zero.
如果把冷冻的水银环的温度保持在低于绝对零度以上4度的话，环绕水银环流动的电流就必定会不停的流动。正如人造卫星一旦摆脱地球大气的引力，就会永远绕地球运行一样。
It is very interesting to note the differently chosen operating mechanism by the different manufacturers, in spite of the fact that the operating mechanism has a major influence on the reliability of the circuit-breakers.
尽管操作装置对断路器的可靠性具有主要的影响，但注意不同的制造厂按不同形式选择操作装置非常有趣的。
Thus, marginal cost to the producer may fall below marginal cost from the point of view of the whole economy when extra output requires extra foreign exchange and when foreign exchange is particularly scarce.
以至当生产额外的产品需要额外的外汇，而外汇又很缺乏的情况下，生产者的边际成本可能低于从整个经济角度来看的边际成本。 第5节 翻译练习3
This development is in part a result of experimental studies indicating that favorable alterations in the determinants of myocardial oxygen consumption may reduce ischemic injury and that reduction in afterload may be associated with improved cardiac performance.
从某种角度上讲，这方面的进展是实验研究的结果。实验研究表明，心肌耗氧量的决定因素的改善可减轻局部缺血性损害，并且，负荷的减轻也能改善心肌功能。
Up to a point, flyovers and underpasses, being the first really decisive steps to speed up and diver traffic, have eased the situation, but a considerable contribution toward a satisfactory solution can be
16
made by underground railway networks.
在某种程度上，立交桥和地下道的建设，在加速和分流交通方面迈出了具有决定意义的第一步，从而缓解了交通不畅的局面。但要从根本上解决交通问题，仍需依赖地铁系统。
In order to understand what happens during sintering of compacts, the changes in density, in dimensions, in metallographic structure and in mechanical properties during sintering must be studied from an experimental point of view, beginning with those in compacts from a single metal powder or from homogeneous solid solution alloy powders.
为了了解用单一金属粉末或同类固体溶解合金粉制成的压坯在烧结过程中发生的变化，必须从实验的观点研究烧结过程中压坯的密度、尺寸、金属结构和机械性能的变化情况。 第5节 翻译练习4
However, in our experience some patients, in whom a high pulmonary vascular resistance was successfully lowered by the above measures, have had episodes of increased pulmonary vascular resistance post-operatively accompanied by clinical deterioration, which in some cases was extremely rapid.
然而，根据我们的经验，通过上述措施，某些病人的肺血管阻力成功的降低了，但术后由于临床恶化并在某些病例中发展很快，所以仍有肺血管阻力增高的现象。
The nutritional dilemma of persons with AIDS, or PWAS, is that they must eat abundantly yet they must also avoid foodborne illness, juggle foods and medications, and surmount the poor appetite and indigestion caused by their medications and by AIDS itself.
艾滋病患者又称PWAS， 他们所遭遇的饮食困境表现为患者一方面必须大量进食，另一方面又不得不防范因饮食而导致的疾病，拒绝零食和用药，还要克服因治疗以及艾滋病本身而引发的食欲不振和消化不良。
What is questioned is whether a country like Britain has a chance, assuming it has the will, to succeed where so many have failed and even assuming that is has, should make the tremendous effort and take substantial financial risk of trying to leapfrog into leadership in entirely new technology or whether it should take the softer option of merely catching up with the rest of the industrialized world.
问题在于像英国这样一个国家，假定它有决心，是否在那么多的国家已告失败的情况下有取得成功的可能，即使有可能，是否应该做出巨大努力并冒相当大的财政风险，试图在完全新的技术领域跃居领导地位，或者是应该做较为轻松的选择，仅仅赶上工业世界其他国家。 总复习题
In the early days of the telegraph and the telephone, men observed that signals sometimes jumped the space between wires running next to each other.
在电报及电话发展的早期，人们注意到信号有时会跳过并行导线的空间。
To understand both sides of the question, some background on how energy is obtained from a nuclear reaction must be studied.
要理解这一问题的两个方面，必须研究有关如何由核反应获得能量的一些基本情况。 Voyager’s photographs may help explain how they (Neptune’s rings) formed. 旅行者号拍摄的照片也许有助于说明这些光环是怎样形成的。
If the water is heated, the particles move more quickly, which agrees with our theory. 如果把水加热，粒子运动就更快，这一现象是和我们的理论吻合的。
Radiant heat waves pass through a vacuum, but may be absorbed by bodies whose temperature is thereby raised.
辐射热波能通过真空，但能被一些物体所吸收，因而这些物体的温度便升高了。
And there was the possibility that a small electrical spark might accidentally bypass the most carefully planned circuit.
而且总有这种可能性——一个小小的电火花，可能会意外的绕过了最为精心设计的线路。 People see things differently in the light of the fact that they are materialists and idealists. 由于人们的观点有唯物、唯心之分，因而他们对事物的看法也就不同。 There was evidence that all atoms consists of a nucleus and electrons. 有证据表明，所有原子都由原子核和电子组成。
Generally speaking, we do not use materials in the form in which they occur in nature. 一般来说，我们并不用自然界中天然存在的那种形态的物质。
These motors are, as a rule, especially designed for the specific machines with which they are used. 总的来说，这些电动机是为使用它们的专门机器特别设计的。
A machine spindle has different speeds which the operator selects according to the work. 机床主轴有几种不同的速度，操作人员可以根据工件进行选用。
Now that we have examined several areas of natural science, perhaps it would be well to attempt to define science.
既然我们以考察了自然科学的若干方面，也许不妨可以试试给科学下一定义。
Since an ammeter must be placed in series with the circuit, the latter must be opened for insertion of the meter.
由于电流计必须与电路串联，故后者必须断开以插入电流计。
Many of them have more than one solid states, as in the case of phosphorus, which is known in three solid phase: black, more familiar red and yellow forms. 热是能的一种形式，这是大家都知道的。
A nuclear reactor gets energy from the loss in mass during the nuclear reactions, so we can consider mass a form of energy.
核反应堆在核反应期间由质量的损失而获得能量，因此我们可以把质量看作是能量的一种形式。 An automatic production line is excellent for the automotive industry, where thousands of identical parts are produced.
自动生产线很适用于汽车工业，因为那里生存成千上万个同样的零件。
If you are traveling forward in a train, surrounding objects outside appear to be moving backwards. 如果你在一列向前行驶的火车里，外面周围的东西好像是在向后移动。
Television equipment makes use of devices that can be considered optical because they depend on light rays for their operation.
电视装置所用的器件可看成光学器件，因为这些器件是靠光线来工作的。
If the laser drifts, an attempt should be made to scan the whole free spectral range as fast as possible. 如果激光漂移，则应尽快地尝试扫描整个自由谱区。
The adjustment must be repeated each time the vessel is changed. 每当更换容器时都要重新调整。
Each time the short circuit happens, the short circuit current runs the battery down. 每当短路发生时，短路电流就将电池的电能耗尽。 When pressure is applied to ice, part of it will melt. 假如对冰加压，一部分冰就会融化。
An object, once it is in motion, will persist in that motion because of its inertia. 物体一经运动，由于惯性就将继续保持其运动。
Seeing that heat can be used to drive machines, it is a form of energy. 由于热能够用来驱动机器，所以它是一种能。
The black stain usually occurs where parts are in contact with each other. 黑锈斑通常发生在零件相互接触之处。
In the subsequent years there was a trend of higher yields in tilled plots, although the increase was non-significant.
尽管在正常的情形下绝缘材料的性能很稳定，但在极端情况下，比如加压过高，高温或在某种化学物质作用下，绝缘材料的性能会遭到完全的破坏。
In general, there is a measurable difference in potential between the two electrodes whether the cell is passing a current or not.
通常，不论电池有无电流流过，两极之间均具有可测量的电位差。
It is evident that if in an oxygen converter carbon is replenished by continuous addition of coal and the exothermic combustion of carbon in the bath is balanced with the endothermic direct reduction of iron ore, and then the converter will be producing iron at a certain carbon content instead of steel.
很明显，如果在氧气转炉内碳是通过不断地添加煤来补充，而且熔池中碳的放热燃烧与铁矿吸热的直接还原相平衡，那么转炉可以生产含一定碳的铁而不是钢。
As shown by Sumitomo Metals the processes are not restricted to bottom blowing and a combination of top and bottom blowing is most likely to be chosen especially as post combustion of the CO is a most important possibility for the future.
住友金属公司的工作表明，这种工艺不仅局限于底吹，而且将来使一氧化碳产生二次燃烧成为最大可能时，组合式的顶一底吹会成为首选工艺。
The study of the chemical sensors was mainly concentrated on the subjects of selectivity, life, and cost; i.e. how to detect a specific material from the environment where many different substances co-exist (selectivity); how to maintain the sensor in the original condition (life); and how to economically achieve the targets (cost).
化学传感器的研究主要集中在选择性、寿命和成本上，即如何在很多不同物质共存的环境中测定某一特定物质（选择性）；怎样把传感器维持在初始状态（寿命）；怎样经济地实现目标（成本）。
The understanding of the mechanisms involved in the selectivity and reversibility of the sensors would be an interesting subject because it relates to a phenomenon of life, like crown ether and Valinomycin which are the simulation of the biologically selective membrane, the selection of the enzyme, microorganisms, anti-body, and organelle immobilized.
有关传感器的选择性喝可逆性机理的研究与了解将会是一个有趣的课题，因为它与生命现象有关：像王冠以太和结氨霉素，它们就是生物选择膜的模拟，也就是酶、微生物、抗体和细胞器官固定膜的选择。
Deformation is affected by the stress inherent in the metal, the microstructural characteristics of the starting material, the temperature at which the deformation occurs, the rate at which the deformation occurs, and the frictional restraint between the material being forged and the die surface.
变形会受到金属的内在应力、原始材料的微观结构特征、变形时的温度、变形的速度以及被锻造的材料与模具表面间的摩擦阻力的影响。 第11章 翻译练习1
The root systems of terrestrial plants perform two primary functions: one is the acquisition of soil-based resources (principally water and dissolved ions) and the other is anchorage. Other root system functions, such as storage, synthesis of growth regulators, propagation, and dispersal, can be seen as secondary. Little is known of the early evolution of the nonaerial parts of plants, but it is certain that most of the first land plants had poorly developed root systems. Such fragments as are preserved are of large diameter and sparingly and often dichotomously branched. Since early land plants were small and lived in very wet environments neither anchorage nor acquisition of water is likely to have been a serious problem. It is quite possible that the most difficult function for early root systems to perform was the acquisition of poorly mobile resources, especially photosphate.
陆生植物的根系有两个主要功能。一个是获得土壤养分的作用（主要是水分和溶解在水中的离子）。另一个是固定作用。根系的其他功能如贮存、生长调节剂的合成、繁殖和扩散则被看作是次要功能。我们对植物地下部分早期进化了解得比较少。但可以肯定的是，大多的早期陆生植物根系发育低。现存下来的早期陆生植物的根，直径大而分支较少，常是两叉的。由于早期陆生植物小且生活在非常潮湿的环境中，因此植物的固定和吸收水分都不可能成为严重的问题。很可能早期植物根系最难起到的作用是吸收养分，因为养分的移动性差，特别是吸收磷酸盐。 翻译练习2
Dies are usually made of cast iron. The dies should be split so that the cores and the casting can be readily removed, the cores preferably upwards so as not to interfere with gas heating of the die assembly from below.
Where the die will not be tilted, sprues should be inclined, narrow, constricted at the bottom in front of the gate and arranged to fan out into the latter at an angle of 45 so as to avoid sharp corners. Several gates connected by a horizontal runner may be useful.
模具通常由铸铁制成。模具应制成可拆卸型，以确保方便地取出型芯和铸件。型芯最好朝上，以保证其不受在模具组建下方进行气体加热的影响。
模具在使用过程中不倾斜时，应在浇口前方底部设置倾斜，狭窄且缢缩的注入口，并使注入口以45度角展入模具底部，以避免出现急弯。有时可能用到由一条横浇道连起来的几个浇口。 翻译练习3
The suspensor used to be regarded merely as an anchorage device for the embryo, but it is now regarded as having an important role in the transfer of nutrients and plant growth regulators from the maternal tissues to the embryo.
过去人们将胚柄仅仅看作胚的固着器，但现在人们认为胚柄在把营养物质和植物生长调节因子从母体组织转移到胚中起着重要作用。当然在实验中对胚柄造成的破坏，影响胚的生长。在双子叶植物中，胚柄细胞经历了DNA内部复制，也就是DNA复制而核不分裂。尽管在植物界这是很普遍的现象，但胚柄代表了极端情况。例如，在红花菜豆的胚柄细胞中含有8192倍的DNA（即单倍体细胞中DNA的8192倍）。假定这些细胞最初是处于二倍体状态，这么大量的DNA意味着在核不分裂的情况下DNA复制12个周期。DNA内部复制的作用我们还完全不清楚。 翻译练习4
It was many hundreds of years before any further significant observations were made about the phenomenon of static electricity. Then it was discovered that many other materials besides amber could be charged by rubbing, which produced friction. A more important discovery was that there were two kinds of electrical charges.
数百年过去了，对静电现象的进一步观察才取得突破性成果。人们发现，除了琥珀之外还有许多其他材料也能摩擦起电。更为重要的发现了两种电荷的存在。
这两种电荷被称为正电荷与负电荷。正电荷用（+）号表示，负电荷用（—）号表示。这些符号今天仍然通用。人们还发现，同种电荷——同正或同负——相互排斥，而异种电荷——一正一负——则相互吸引。
人们更晚才知道，是一种叫做电子的物质微粒的移动引起电。电子三组成原子的一种微粒，而原子又是组成化学物质的基本单位。原子的中心是原子核，几乎包含原子的全部重量或者物质。原子核本身由两种不同的微粒，即质子和中子组成。 总复习题
Precision Forging
Precision forging is normally taken to mean close-to-final form or close tolerance forging. In the final analysis it is not a special technology of its own, but a refinement of existing technologies to a point where the forged part can be used with little or no subsequent machining. The improvements cover not only the forging method proper, but extend to preheating, descaling, lubrication, and temperature control practices. 精密锻造
精密锻造通常指近终型锻造或小公差锻造。归根到底，它本身不是一项特殊的技术，而是对现有技术的改进。它生产的锻件很少经过或不经过后续加工就可以直接使用。这种改进不仅包括锻造方法本身，而且扩展到预热、除鳞、润滑和温度控制等。
是否使用精密锻造技术主要取决于附加锻造操作以及用刀具和取消机械加工之间的相对经济性问题。由于刀具开发的成本较高，精密锻造通常仅限于要求产品质量很高的生产中。
一个使用精密锻造的例子是涡轮叶片，现在它是用不锈钢和其他难以锻造的合金制造的。因为轻的叶片连接在相对重的基体上，涡轮叶片的形状不是特别适合于锻造。必须进行延长预变形，为实现此目标可以采用不同的生产方法。有的制造者通过镦粗其一端的方法生产，还有的挤压足够的材料形成叶片，但是在每种情况下，最后的模槽都是用于提高表面质量和保证较高的尺寸精度。
Antifriction Bearings
Antifriction Bearings operate with rolling elements (either balls or rollers), and hence rolling resistance, rather than slicing friction, predominates. The cause of rolling resistance is the deformation of mating surfaces of the rolling element and the raceway on which it rolls. The three basic types of antifriction bearings are ball bearings, roller bearings, needle bearings. Ball and roller bearings can be designed to absorb thrust loads in addition to radial loads, whereas needle bearings are limited to radial load applications only. 滚动轴承
滚动轴承利用滚动体（滚珠或滚柱）来工作，因此占主导的是滚动阻力而非滑动摩擦力。滚动阻力的起因在于滚动体的配合面和它滚动的滚道的变形。滚动轴承的三种基本类型是：滚珠轴承、滚柱轴承和滚针轴承。滚珠和滚柱轴承在承受径向荷载之外还可承受轴向荷载，而滚针轴承仅限于承受径向荷载。
滚动（滚动接触）轴承的一个主要优点在于几乎完全消除了摩擦。因此，滚动轴承润滑剂的主要功能是帮助防腐及污垢污染。由于确实存在一定的摩擦，尤其是在滚动体与轴承座之间，润滑剂也必须带走这一滑动作用所产生的热，因此虽然有很小的摩擦，但是润滑剂仍然是必要的。 引起正常润滑的滚动轴承失效的原因是疲劳，它是发生在滚道的接触面或滚动体上的一种应力反向现象。这样，滚动轴承的寿命是以疲劳失效前轴滚动的转数来衡量的。轴承的载荷越大，其以转数为单位的寿命就越短。而且，轴的速度越快，轴承的寿命就越短。由于统计中要考虑到疲劳因素，所以也必须考虑失效的百分比概率。
了解各种类型的轴承的优点与缺点及它们的突出特点很有必要。例如，滚珠和滚柱轴承可以是自定位型以便调整轴的对中不准和偏移。此外，挡板及密封用于保护轴承以防外来污染物的污
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染。一些滚动轴承内装有密封好的润滑剂，这些润滑剂是由制造商提供的，使得轴承在整个使用过程中得到润滑。 In Situ Desulphurisation
Partial removal of the sulfur values in fossil hydrocarbon fuels can be accomplished in place during the application of so called “in situ” recovery techniques. Perhaps the earliest of these techniques to be explored was in situ coal gasification. During the 1940’s and 1950’s, both the Russians and the British carried out extensive field trials with in situ coal gasification and demonstrated the practical feasibility of obtaining a low BTU gas product from the process. More recently in situ coal gasification trials have been conducted in Alberta and several projects in the Colorado oil shales have focused on in situ retorting of the shale to liberate the hydrocarbon. In situ recovery techniques using fire, steam flooding, or a combination of both, also appear to be one of the most practical routes for fluidizing the heavy bitumens of deeply buried oil sands thus enabling their recovery by drilled wells rather than mining. 现场脱硫
化石碳氢燃料所含硫的部分脱除可以在实施所谓的“就地”开采技术的过程中完成，这方面最早探索的技术大概是现场煤气化。在20世纪40和50年代，苏联和英国都进行了广泛的煤气化的现场试验，证明了此过程获得低BTU煤气产品的实际可行性。后来在亚伯达进行了现场煤气化实验。另外还有几项对科罗拉多页岩油的开发项目则集中研究对页岩油的现场提纯技术以产生碳氢化合物。采用燃烧、蒸汽注入或两者结合的就地开采技术也已经成为深层油砂中的重沥青流体化的最实际的技术途径，从而可以使用钻井法而不使用开矿法来开采这些沥青。
就脱硫而言，这种原地加工技术的最重要的优点大概在于可以达到需要的化学反应的温度和压力。如果在地面进行，则该温度和压力需要负责而昂贵的加工设备方能实现。这样可以就地实现化学反应状态，而如果将化石燃料开采出来以后再用传统的方法进行提炼精制，这样做既不切实际又不经济。在脱硫反应中这种高温高压条件特别重要。地面上的炼焦设备也许可以模拟原地注入燃烧过程的氢转移反应，产生低分子量的碳氢化合物和硫化氢。因此，可以达到从有机硫化合物的母体中就地除硫的目标，而且可以出去更易于处理的气态H2S。 Seed Health
Many pests and diseases are seed-transmitted; for example, the stem and bulb eelworm may be passed to the progeny via the see of Allium spp., while celery leaf spot can be transmitted by the infected seed. Many economically important plant viruses——for example, lettuce mosaic virus and tomato mosaic virus——are seed-transmitted. Virus in the seed or in the seed coat can lead to early initial field infection which spreads to other plants within the crop. 种子的健康
许多虫害和病害是通过种子传播的：例如茎线虫和球线虫可能通过葱属植物的种子传播，而芹菜叶斑病能通过感病的种子传播。从经济角度讲，许多造成很大经济损失的植物病毒，例如莴苣花叶病毒和番茄花叶病毒是通过种子传播的。种子内和种皮内的病毒能导致田间的初期感染，并在同种作物的其他植株间蔓延。
发展中国家的许多小规模种植者种植他们自己作物留的种子。他们常常忍不住从没有销路的作物中采集种子。此后导致下一代遗传特性差。小规模种植者不使用例如后代检测这样的技术。事实上，在一些国家许多大规模生产者也不使用这些相对简单的技术，因此种子改良的可能性，甚至维持原种水平的可能性有时也丧失了。 Nucleus
The nucleus is usually more or less spherical, though nuclei with other shapes have also been observed. The nucleus is surrounded by and an envelope and contains the nuclear matrix and one or more nucleoli. 细胞核
尽管也观察到过其他形状的细胞核，但细胞核一般都是接近球形的。细胞核被膜包围着而且内含核基质和一个或多个核仁。在核质中有由脱氧核糖核酸和蛋白质组成的染色体。在染色体中对碱性染料有一定的亲和性的DNA和蛋白质的复合体称染色质。在分裂期间（核分裂之间）染色体解螺旋而且不能被分清。在这个阶段，在核质中常常可见到浓缩的染色深的染色质团。这种染色质称异染色质。比异染色质染色浅的染色质称常染色质。在分裂期间DNA合成，为染色体的复制过程做准备。核膜由两层单位膜组成。核膜之间狭窄的空间称核周腔。核膜和内质网相连。核膜上有一定规律地分布着的核孔。
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