1.Do you want to know what steam is？Let us get a lump of ice，or a snow-ball，and place it in a saucer in a warm room.It soon melts，and then the saucer is half full of water.If we leave the saucer in the warm room for a few hours，we shall find that it has become quite empty.
2.The ice or snow was solid water.The heat changed it intoliquid①water，such as we drink.And then，by more heat，the liquid water was changed into water-vapour②，which mixed with the air and was lost to our sight.
3.This shows that water can exist③in three forms-first，as thesolid we call ice or snow；second，as the liquid water we drink；and third，as the gas we call water-vapour.
4.If we put the liquid water into a little sauce-pan on the fire，the heat of the fire will change it into vapour in a few minutes.Vapour made by the boiling of water is called steam.
5.Now hold a tea-spoon，or something cold，in the steam as it rises.In a moment the spoon will be covered with little beads of①Liquid，fluid；anything in a melted state.
②Vapour，steam；small particles of water floating in the air.
③Exist，be.
water.The cold spoon has turned the steam back into water.Heat turns water into steam；cold turns steam into water again.
6.Suppose we were to put some water into a kettle，and then close up its spout，fasten down its lid，and put it over a hot fire，what do you think would happen ？
7.There would soon be an explosion①；the kettle would beburst open.It is in this way that boiler explosions are often caused.When there is too much steam in a boiler，and it cannot get out，it presses against the sides of the boiler，until at last the boiler bursts.
8.To prevent these accidents，every boiler has a safety-valve，or an opening through which the steam can escape when its pressure②begins to get too great.It is only when something goeswrong with the safety-valve that an explosion occurs③.
9.Steam requires seventeen hundred times more room than the water from which it is formed.One kettle full of water would make enough steam to fill seventeen hundred kettles.
10.It is heat that makes steam take up so much more room than water.The heat drives the tiny pieces of which water is made further and further away from each other.
11.This power of steam is a very good thing when it is properly④used.It is this pressure that does the work in all steam-engines，and that sets their parts in motion.
12.The first really good steam-engines were made by James Watt，about a hundred years ago.It was by noticing the steam making the lid of a kettle dance and rattle，that he first began to think of the great power of steam.
①Explosion，a sudden bursting out or flying to pieces with noise；report.
②Pressure，weight；state of being pressed.
③Occurs，takes place；happens.
④Properly，rightly.
LITTLE JAMES WATT（小时候的詹姆斯·瓦特）SUMMARYSteam is vapour made by the boiling of water.Heat turns water into steam，cold turns steam back to water.Steam needs much more room than the water it came from.This makes it press on the sides of the boiler，and a safety-valve is needed to prevent the boiler from bursting.The pressure of the steam causes the engine to move.The first good steam-engine was made by James Watt.
【中文阅读】
1．小朋友，你想知道蒸汽是什么东西吗？咱们拿一块冰或者一个雪球，放在一个碟子里，放到温暖的房间里。冰雪很快就会熔化，碟子里会盛上半碟子水。要是把碟子留在温暖的房间里几个小时，里面就会空了。①2．冰和雪都是“固态”水。热量让固态水变成了“液态”水，就是我们喝的水。然后，如果再加热，液态水就会变成“水蒸气”（vapour）②，水蒸气和空气混在一起，我们就看不见了。
3．这说明，水的存在形态有三种--第一种，我们叫做冰雪的“固态”；第二种，我们喝的“液态”；第三种，我们叫水蒸气的“气态”。
4．要是我们把液态水倒进汤锅，放在火上煮，火焰的热量，只要几分钟，就能把液态水变成水蒸气。水开了散发出来的水蒸气，就是三点水的“蒸汽”（steam）。③5．拿一把茶匙或者别的冷东西，放在往上冒的蒸汽里面。马上，茶匙就会盖上一层小水珠。冷的茶匙又把蒸汽变回了水。热可以把水变成蒸汽，冷又能把蒸汽变回水。④6．假如，我们把水倒进水壶，堵住壶嘴，封死了壶盖，再放在火上烧，会发生什么事？⑤7．很快，就会发生爆炸，将水壶炸开。锅炉经常发生爆炸，就是这个原因。锅炉里蒸汽太多，又出不来的时候，蒸汽就会紧紧压在锅炉壁上，直到最后把锅炉炸开。
8．为了避免这种事故，所有的锅炉都装着安全阀，这是一个开口，当蒸汽的压力太大的时候，就能通过这个开口逃出去。只有在安全阀出问题的时候，才会发生爆炸。⑥9．一份水变成的蒸汽，需要1700份这么多水的空间才能盛下。一把盛满水的水壶，里面的水变成的蒸汽，可以盛满1700个水壶呢！
10．是什么让蒸汽占的地方，比水多了这么多呢？是热量。热量让水分子之间的距离越来越远。
11．如果用得合适，蒸汽的力量是一件非常好的事情。所有的蒸汽机，利用的都是这种力量。它们要靠蒸汽的力量，来推动部件运动。
12．最早一批实用的蒸汽机，是詹姆斯·瓦特在大约100年前制造的。瓦特当初看到了蒸汽推动水壶的盖子跳舞，发出响声，才开始思考蒸汽的强大力量。⑦译注①读者看见这句话，会不会说“不对”呀？是的！我们在前面课文里读到过，碟子里面盛满了空气，但是这只是从严格的科学意义上说，或者在强调空气的时候才说。我们平时，也可以说除了空气什么都没有的容器是“空的”，对吗？
②这是英式拼写，美式拼写少一个u，是vapor。
③物理学上，“蒸汽”指平时是液体或固体的一切物质的气态，“水蒸气”是“蒸汽”的一种，专指气态的水。平时在语境清楚的情况下，也可以把“水蒸气”叫做“蒸汽”。但是现实中的水蒸气往往带着一些没有气化的水分子，属于“湿蒸汽”。纯粹的水蒸气是看不见的。
④严格地说，“冷”只是热量的缺乏。热量是物质运动的表现形式，不是具体的物质。
⑤读者请千万不要真的这样做，非常危险！
⑥人们家里用的高压锅，也装着这种安全阀。
⑦瓦特的英语名字是James Watt（1736-1819），其实，课文中提到的情节是一个传说，他不是蒸汽机最早的发明者。但是，瓦特之前的“纽可门”
（Newcomen）式蒸汽机很笨重，效率不高，而且只能用来从矿井里抽水。瓦特经过反复思考和实践，在1766年、1781年、1782年对纽可门蒸汽机作出了三次重大改进，让它成为能带动一切工作机的原动力，获得了“瓦特蒸汽机”的专利，从此，人类在大自然面前，第一次真正感觉到了自己的力量，在蒸汽机的基础上发明了蒸汽船、蒸汽火车等改变世界的机器。就这样，瓦特蒸汽机极大地推动了第一次工业革命，改变了世界的面貌。后人为了纪念他，把功率的单位定为“瓦特”，你现在听说的某个电器是“多少瓦”，就是他的名字！从前，几乎所有工业部门，都要使用蒸汽机，后来蒸汽机才把霸主地位让给了效率更高的内燃机和电动机。现在，蒸汽机一般在高山上的火车上使用。还有一种经过改进的蒸汽机，叫做“汽轮机”，用在核电站里。
THE USEFUL SHEEP
