You may see the same thing on a much larger scale if you look at 

 the puffs that come from a locomotive drawing a train. Sometimes 

 there is very little smoke mixed with tlie steam, and the cloud that 

 rises is like the cloud from the spout of the kettle. Look carefully 

 and you will probably find that there is a space between the top of 

 the smoke-stack and the cloud where nothing can be seen. Just above 

 it is cooled by the air, and it shrinlcs into separate drops of water. If 

 you could see inside the boiler you would find that the steam above 

 the water is quite invisible water-vapor, and the same thing can be 

 shown by boiling water in a thin glass flask such as chemists use. 

 Notice the clouds thus formed by a locomotive, and you will observe 

 that on some days they hang lazily in the air for quite a long time, 

 and on other days they fade away qiiite rapidly. Observe what the 

 days are like on which they disappear rapidly, and what the other days 

 are like on which they disappear slowly. 



It will help you to understand the matter if you will try some ex- 

 periments to find how fast water will turn into invisible vapor. Turn 

 a saucer upside down and you will see a shallow depression with a rim 

 around it. Put the saucer with the bottom up somewhere out of doors 

 where it will not be disturbed, and pour enough water in the hollow 

 to fill it. Then find how long it takes the water to dry up. Do this 

 on several days, and put down in a little note book the length of time 

 it takes in each case. Note also in your book whether the day is 

 cloudy or sunny, warm or cold, windy or still, and see if you can find 

 any connection between these things. If you wish, you may use a wet 

 rag hung from a clothes-line instead of the saucer. Always use the 

 same rag and hang it up without wringing. Find as before how long 

 it takes to dry. 



We can now understand how real clouds are formed. Vapor is con- 

 tinually rising from the water on the surface of the earth, the process 

 being greatly aided by the heat of the sun's rays. It is quite invisible 

 until it rises high in the air. Now, people who have climbed high 

 mountains and men who have gone up in balloons tell us that the air 

 is much colder higher up than it is at the surface of the earth. We 

 can understand, then, what happens to the invisible vapor when it 

 rises high in the air. It is cooled and turned again into water-drops. 

 These make up the cloud that we see far above us. If you watch the 

 different clouds on a fine daj^, you will probably see some that look 

 like large masses of cotton- wool. They are wavy or irregular above, 

 but bounded by a sharp line below. The line marks the place at 

 which the air becomes cold enough to turn the vapor into drops. If 

 now a cold wind comes along and chills these clouds still more, the 



