does not seem cold enough to freeze water. But they must have 

 lieen froezn somewhere before rcacliing the eartli, and so wo learn tluit 

 there must be somewhere above us on a summer day places where the 

 air is cold enough to produce ice. You can often see such a place, 

 even on a hot summer day, if you wish. Look f..)r a cloud that seems 

 very high up, one that resembles hairs or feathers with their fibers 

 curled. These clouds are sometimes called horse-tail clouds. They 

 consist really of small pieces of ice instead of drops of water. When 

 you see many of these clouds you may expect wind soon, or at least 

 a change of weather. They are very high up, sometimes as much as 

 ten miles. You must not, however, suppose that the hail actually 

 comes from these clouds. It really is formed much nearer the surface 

 of the earth by very cold currents of air. If the drops are first frozen 

 into hail by cold air, and these frozen drops before reaching the earth 

 have to pass through a thick layer of warm air, they are partly melted 

 again, and we get what is called sleet. 



We have said a good deal about water vapor shrinking as it is 

 cooled until it becomes drops of water. What happens to water as it is 

 cooled? Does it shrink? Try it for yourself in this way: Take a 

 large glass bottle with a narrow neck. Fill it -first with pretty warm 

 water, merely to warm it up. Then pour this out and fill it with 

 water that is nearly boiling. Tie a string around the neck just where 

 the water stands, cork the bottle and put it where it can cool. Look 

 at it as it cools, and notice that the level of the water falls steadily. 



We thus see that water shrinks as it cools. Notice which part of 

 the bottle is the colder and see if you can explain why. If the day 

 is cold enough you may at last get the water down to the freezing 

 point. You will find, however, that it does not go on shrinking right 

 down to where it begins to freeze. A little expansion actually occurs 

 before the freezing point is reached, but unless your bottle be very 

 large and have a very narrow neck, you may not be able to see this. 

 If you can get a small glass tube and pass it through the cork and 

 then shove the cork hard into the bottle full of water you may make a 

 very narrow neck for the bottle, and then you may be able to see 

 this slight expansion of the water before it is quite down to the freez- 

 ing point. 



But what happens when the water actually begins to freeze? Leave 

 the bottle of water out on a very cold night and see what has hap- 

 pened to it in the morning. You will find that the bottle has cracked, 

 showing that when the water began to freeze it must have expanded in 

 volume, and as the bottle couldn't expand also, the latter was shat- 

 tered. So we see that while water vapor and water both shrink steadily 



