1.Each of us drinks about three pints of fresh water every day，and we use a great deal of water for washing，for cooking，and for other purposes①.Where does all this water come from？In some places we get water from wells，and in other places from rivers or lakes.But from whatever source②we get the water，it is quite certain that at one time or another it must have fallen from the clouds as drops of rain.
2.Have you ever been in a cloud？Perhaps you have climbed to the top of a high hill，and have seen a white cloud come scurrying③towards you，till it has covered the hill-top and everything above，below，and around you.The cloud was no longer white，but of a dull grey colour，like the mist④which we see on a foggy⑤morning.Indeed a fog is simply a cloud resting on the ground.
3.But what are the clouds made of？They are just like the clouds of white steam，as we call it，that you see rising from a boiling kettle.The white steam and the clouds are both com-①Purposes，uses.
②Source，origin；beginning；the place where anything rises or begins.
③Scurrying，hurrying；moving rapidly.
④Mist，fog；visible moisture in the air.
⑤Foggy，misty.
posed of extremely small specks of water，so small that they have been called water-dust.When a number of these water-specks join together，they form a drop of rain，and then the weight of this rain-drop makes it come pattering quickly to the earth.
4.How did all this water which forms the clouds and the rain get into the air？It was evaporated by the heat of the sun from the surface of the oceans and seas，and rivers and lakes.Three-quarters of the surface of the great globe on which we live is covered with water.Every day the sun’s rays change thousands of tons of liquid water into water-vapour；and this water-vapour mingles with the air，and is carried along by the wind.While it remains in the form of vapour，however，we cannot see it，because it is quite colourless and transparent.
5.The air near the surface of the earth is warmer than that which is higher up，so when the water-vapour rises very high above the ground，it is cooled.We have learned that when water-vapouris cooled it is condensed.So the water-vapour is first changed into those very tiny specks of liquid water which have been called water-dust，and appears as clouds；and then these specks of water join together to form drops of rain.
6.You must at one time or another have tasted dropsof rain as they blew against your face，and so you know that therain-water is fresh water.But the water of the sea，from which that rain-water came，is quite salt.How is it that rain-drops of fresh water can come from the salt sea？
FORMATION OF CLOUDS（云的形成）
7.In last lesson you were told that salt water can be made fresh by distilling it.Now rainwater is really distilled water.If you boil salt water in a small saucepan until the water has all passed away as steam，the salt that was dissolved in it will all be left behind as a white crust on the bottom of the saucepan.In exactly the same way the heat of the sun turns the water of the sea into vapour，but not the salt that is dissolved in it.So when this vapour is changed back into water again，it forms pure fresh water.
SUMMARY
All the water of wells，rivers，and lakes falls from the clouds in drops of rain.Clouds，like steam，are formed of extremely small specks of water，which join together to form rain-drops.A fog is a cloud resting on the ground.The water-vapour which forms the clouds has been evaporated by the heat of the sun from the large surfaces of water on the face of the globe.When this water-vapour rises into the higher and colder air it becomes visible as white clouds.The water-specks unite to form a rain-drop.Rain is fresh water distilled from the salt water of the sea.
【中文阅读】
1．我们每个人每一天都会饮用3品脱左右的淡水①，而日常生活中的清洗、烹饪与其他诸多活动都离不开用水。这些水都是从哪里来的呢？在有些地方，人们从井中取水，而在另外一些地方，人们从河流或湖泊中取水。然而，无论我们从哪里取水，可以肯定的是，天上的云朵时不时就会降下雨水。
2．你看到过云朵吗？也许你曾经爬到山峰之上，看到一块白云快速朝你飘来，直到它将山顶与周围空间中的万物，包括你在内，全部包裹起来②。这时，云朵不再是白色，它变成了昏暗的灰色，就像是朦胧的清晨时分天空的那层薄雾。实际上，雾气就是落在地面上的云朵。
3．然而，云朵究竟由什么构成呢？其实，它们与那些从沸腾的水壶中冒出的白色水蒸气一样。白色蒸汽与云朵均是由很小的水粒构成，因为它们的体积实在太小，所以我们称之为水雾。当许多水粒凝聚在一起时，它们就变成了雨滴。随后，雨滴的重量使得自己迅速落到了地面。
4．这些构成云朵和雨水的水分是怎样进入到空气中的呢？它们由海洋、河流与湖泊表面的水在太阳热量的作用下蒸发而成。在我们生活的这个巨大星球中，四分之三的表面都被海水所覆盖。每天，太阳光都能够将数千吨的液态水转化为水蒸气；随后，水蒸气与空气混合在一起并随风飘动。然而，只要这些水分保持蒸汽状态，我们就无法看到，因为它是无色透明的。
5．靠近地球表面的空气温度比高空的空气温度要高一些，因此，当水蒸气漂浮到高空时就会被冷却。我们知道，水蒸气遇冷之后就会产生冷凝。因此，水蒸气首先变成了非常细小的水粒--也就是我们所说的水雾--并构成了云朵，然后这些水粒汇聚在一起成为了雨滴。
6．你一定淋过雨而且尝到过雨水的味道，因此你知道，雨水属于淡水。然而，雨水来自海水，而海水中却含有很多盐分。那么，海里的咸水怎么会变成不含盐的雨水呢③？
7．通过上一课的学习，你了解到，咸水可以通过蒸馏而变成淡水。实际上，雨水是名符其实的蒸馏水。如果你用炖锅来煮沸咸水，等到所有的水分都以蒸汽的方式蒸发之后，咸水中溶解的盐分就会变成锅底上一层白色的粉末。太阳的热量正是通过这种方法将海水变成蒸汽，而海水中溶解的盐分则留在了海洋之中。因此，当水蒸气再次变成水滴的时候，淡水应运而生。
译注
①3品脱大约是1700毫升。关于每天的最佳饮水量说法很多，但一般在1500-2000毫升之间。
②有趣的是，就算不形成雨，构成云的小水滴也是比空气重的，所以也会下降。但这种下降速度非常慢，还经常会被风再次托起来，所以我们印象里的云就总是在天上飘了。云的各种形态叫“云相”，是气象学研究的重要对象之一。
③现代雨水虽然不含盐，但可能包含空气中的污染物，落到地面就成了“酸雨”
（acid rain），是环境污染的重要来源。
THE STURDY①OAK
