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．

在雷電交加的暴風雨時，先看到放電現(xiàn)象(即閃電)；過一些時間后才聽到放電的響聲(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.

在現(xiàn)代條件下，對于復雜的生產(chǎn)計劃的成本往往很難預先作出精確的估計。

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

當愛因斯坦的方程式(E=mc2)最初被提出來時，很少科學家能看出它的實際用途。

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

馬爾薩斯 1798 年曾預言，按當時(人口)增長的速度，糧食供應最終將不足以滿足人口增長的需要。

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．

到目前為止，金屬和合金的傳統(tǒng)成形法一直是借助于昂貴而又壽命不長的切削工具。可是將來，很可能代之而廣泛使用激光束或電火花放電現(xiàn)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.

過去，自然資源被認為是無窮無盡的；今天，人們認識到，很多資源項目在不遠的將來很可能被耗盡。

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，人們曾以為構(gòu)成物質(zhì)的“建筑砌塊”只有三種基本粒子。自那時以來進行的研究業(yè)已證明還存在有許多基本粒子。

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

在進入噴管之前蒸汽處在高壓之下。

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

蒸汽通過葉片時受到偏轉(zhuǎn)。

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

離開葉片后，蒸汽被排入大氣中。

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

除去雜質(zhì)后，可將水送回鍋爐內(nèi)。

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

安裝鍋爐時，可用的占地面積十分重要。

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．

活塞到達沖程的上端前，混合氣體被點燃。它一直燃燒到完全燒盡?；钊竭_上死點時之前，燃燒正充分地在進行。當汽缸中仍有未燒盡的氣體時，燃燒仍將繼續(xù)下去，在燃燒進行的同時，氣體膨脹并將活塞壓向下面。當活塞剛到達下死點時，排氣閥就打開。當活塞又一次接近其沖程的上端時，進氣閥打開。一旦全部廢氣排出，排氣閥便關(guān)閉。每一循環(huán)完成后，又開始另一循環(huán)。

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