In an electrical storm , the discharge (i.e. the lightning) is seen first ; the noise of the discharge (i.e. the thunder) is heard some time afterwards．

在雷电交加的暴风雨时，先看到放电现象(即闪电)；过一些时间后才听到放电的响声(即打雷)。

Under modern conditions it is often difficult to make an accurate estimate in advance/beforehand of the cost of a complex production programme.

在现代条件下，对于复杂的生产计划的成本往往很难预先作出精确的估计。

Very few scientists were able to see the practical implications of the Einstein relation(E=mc2) at the time it was first formulated．

当爱因斯坦的方程式(E=mc2)最初被提出来时，很少科学家能看出它的实际用途。

In 1798 Malthus predicted that , at the current rates of growth , food supplies would eventually/in time be insufficient for the population .

马尔萨斯 1798 年曾预言，按当时(人口)增长的速度，粮食供应最终将不足以满足人口增长的需要。

The traditional methods of shaping metals and alloys have hitherto/up to the present been by means of expensive and short-lasting cutting tools． In the future , however , laser beams or electrical discharges will probably be widely used instead．

到目前为止，金属和合金的传统成形法一直是借助于昂贵而又寿命不长的切削工具。可是将来，很可能代之而广泛使用激光束或电火花放电现象。

In the past , natural resources were thought to be unlimited ; nowadays , it is realized that many items are likely to be used up in the comparatively near future.

过去，自然资源被认为是无穷无尽的；今天，人们认识到，很多资源项目在不远的将来很可能被耗尽。

Only three types of elementary particles were thought to constitute the 'building blocks' of matter until/till/up to 1947. Research since this date has revealed the existence of many more．

直到1947年以前，人们曾以为构成物质的“建筑砌块”只有三种基本粒子。自那时以来进行的研究业已证明还存在有许多基本粒子。

Before/Prior to entering the nozzle , the steam is at high pressure． (=Before it enters the nozzle ... )

在进入喷管之前蒸汽处在高压之下。

When/While/In passing through the blades , the steam is deflected． (= While it is passing through the blades ... )

蒸汽通过叶片时受到偏转。

After/On leaving the blades , the steam passes out to the atmosphere , (= After it leaves the blades ... )

离开叶片后，蒸汽被排入大气中。

On removing the impurities , the water can be passed back to the boiler . (= When we remove the impurities ...)

除去杂质后，可将水送回锅炉内。

When installing a boiler , the floor space which is available is very important． (= When we are installing a boiler ... )

安装锅炉时，可用的占地面积十分重要。

Before the piston reaches the top of its stroke , the mixture is ignited. It continues to burn until combustion is complete．By the time the piston reaches top dead centre , combustion is well under way. While any unburnt gas remains in the cylinder , combustion will continue. As combustion proceeds , the gases expand and drive the piston down. As soon as the piston reaches bottom dead

centre the exhaust valve is opened. When the piston nears the top of its stroke again , the inlet valve is opened. Once all the exhaust gases are driven out． the exhaust valve is closed. After one cycle is completed , another cycle begins．

活塞到达冲程的上端前，混合气体被点燃。它一直燃烧到完全烧尽。活塞到达上死点时之前，燃烧正充分地在进行。当汽缸中仍有未烧尽的气体时，燃烧仍将继续下去，在燃烧进行的同时，气体膨胀并将活塞压向下面。当活塞刚到达下死点时，排气阀就打开。当活塞又一次接近其冲程的上端时，进气阀打开。一旦全部废气排出，排气阀便关闭。每一循环完成后，又开始另一循环。

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